draft-ietf-smime-rfc3369bis-04.txt   rfc3852.txt 
S/MIME Working Group R. Housley Network Working Group R. Housley
Internet-Draft Vigil Security Request for Comments: 3852 Vigil Security
When Approved, Obsoletes: 3369 May 2004 Obsoletes: 3369 July 2004
Category: Standards Track
Cryptographic Message Syntax (CMS) Cryptographic Message Syntax (CMS)
<draft-ietf-smime-rfc3369bis-04.txt>
Status of this Memo Status of this Memo
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Abstract Abstract
This document describes the Cryptographic Message Syntax (CMS). This This document describes the Cryptographic Message Syntax (CMS). This
syntax is used to digitally sign, digest, authenticate, or encrypt syntax is used to digitally sign, digest, authenticate, or encrypt
arbitrary message content. arbitrary message content.
Table of Contents Table of Contents
1 Introduction ............................................. ?? 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Evolution of the CMS ..................................... ?? 1.1. Evolution of the CMS . . . . . . . . . . . . . . . . . 3
1.1.1 Changes Since PKCS #7 Version 1.5 ........................ ?? 1.1.1. Changes Since PKCS #7 Version 1.5. . . . . . . 3
1.1.2 Changes Since RFC 2630 ................................... ?? 1.1.2. Changes Since RFC 2630 . . . . . . . . . . . . 4
1.1.3 Changes Since RFC 3369 ................................... ?? 1.1.3. Changes Since RFC 3369 . . . . . . . . . . . . 4
1.2 Terminology .............................................. ?? 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Version Numbers .......................................... ?? 1.3. Version Numbers . . . . . . . . . . . . . . . . . . . . 5
2 General Overview ......................................... ?? 2. General Overview. . . . . . . . . . . . . . . . . . . . . . . 5
3 General Syntax ........................................... ?? 3. General Syntax . . . . . . . . . . . . . . . . . . . . . . . 6
4 Data Content Type ........................................ ?? 4. Data Content Type . . . . . . . . . . . . . . . . . . . . . . 6
5 Signed-data Content Type ................................. ?? 5. Signed-data Content Type. . . . . . . . . . . . . . . . . . . 7
5.1 SignedData Type .......................................... ?? 5.1. SignedData Type. . . . . . . . . . . . . . . . . . . . 8
5.2 EncapsulatedContentInfo Type ............................. ?? 5.2. EncapsulatedContentInfo Type . . . . . . . . . . . . . 10
5.2.1 Compatibility with PKCS #7 ............................... ?? 5.2.1. Compatibility with PKCS #7. . . . . . . . . . 11
5.3 SignerInfo Type .......................................... ?? 5.3. SignerInfo Type. . . . . . . . . . . . . . . . . . . . 12
5.4 Message Digest Calculation Process ....................... ?? 5.4. Message Digest Calculation Process . . . . . . . . . . 14
5.5 Signature Generation Process ............................. ?? 5.5. Signature Generation Process . . . . . . . . . . . . . 15
5.6 Signature Verification Process ........................... ?? 5.6. Signature Verification Process . . . . . . . . . . . . 15
6 Enveloped-data Content Type .............................. ?? 6. Enveloped-data Content Type . . . . . . . . . . . . . . . . . 16
6.1 EnvelopedData Type ....................................... ?? 6.1. EnvelopedData Type . . . . . . . . . . . . . . . . . . 17
6.2 RecipientInfo Type ....................................... ?? 6.2. RecipientInfo Type . . . . . . . . . . . . . . . . . . 19
6.2.1 KeyTransRecipientInfo Type ............................... ?? 6.2.1. KeyTransRecipientInfo Type. . . . . . . . . . 20
6.2.2 KeyAgreeRecipientInfo Type ............................... ?? 6.2.2. KeyAgreeRecipientInfo Type. . . . . . . . . . 21
6.2.3 KEKRecipientInfo Type .................................... ?? 6.2.3. KEKRecipientInfo Type . . . . . . . . . . . . 24
6.2.4 PasswordRecipientInfo Type ............................... ?? 6.2.4. PasswordRecipientInfo Type. . . . . . . . . . 25
6.2.5 OtherRecipientInfo Type .................................. ?? 6.2.5. OtherRecipientInfo Type . . . . . . . . . . . 26
6.3 Content-encryption Process ............................... ?? 6.3. Content-encryption Process . . . . . . . . . . . . . . 26
6.4 Key-encryption Process ................................... ?? 6.4. Key-encryption Process . . . . . . . . . . . . . . . . 27
7 Digested-data Content Type ............................... ?? 7. Digested-data Content Type. . . . . . . . . . . . . . . . . . 27
8 Encrypted-data Content Type .............................. ?? 8. Encrypted-data Content Type . . . . . . . . . . . . . . . . . 28
9 Authenticated-data Content Type .......................... ?? 9. Authenticated-data Content Type . . . . . . . . . . . . . . . 29
9.1 AuthenticatedData Type ................................... ?? 9.1. AuthenticatedData Type . . . . . . . . . . . . . . . . 30
9.2 MAC Generation ........................................... ?? 9.2. MAC Generation . . . . . . . . . . . . . . . . . . . . 32
9.3 MAC Verification ......................................... ?? 9.3. MAC Verification . . . . . . . . . . . . . . . . . . . 33
10 Useful Types ............................................. ?? 10. Useful Types. . . . . . . . . . . . . . . . . . . . . . . . . 33
10.1 Algorithm Identifier Types ............................... ?? 10.1. Algorithm Identifier Types . . . . . . . . . . . . . . 33
10.1.1 DigestAlgorithmIdentifier ................................ ?? 10.1.1. DigestAlgorithmIdentifier . . . . . . . . . . 34
10.1.2 SignatureAlgorithmIdentifier ............................. ?? 10.1.2. SignatureAlgorithmIdentifier. . . . . . . . . 34
10.1.3 KeyEncryptionAlgorithmIdentifier ......................... ?? 10.1.3. KeyEncryptionAlgorithmIdentifier. . . . . . . 34
10.1.4 ContentEncryptionAlgorithmIdentifier ..................... ?? 10.1.4. ContentEncryptionAlgorithmIdentifier. . . . . 34
10.1.5 MessageAuthenticationCodeAlgorithm ....................... ?? 10.1.5. MessageAuthenticationCodeAlgorithm. . . . . . 35
10.1.6 KeyDerivationAlgorithmIdentifier ......................... ?? 10.1.6. KeyDerivationAlgorithmIdentifier. . . . . . . 35
10.2 Other Useful Types ....................................... ?? 10.2. Other Useful Types . . . . . . . . . . . . . . . . . . 35
10.2.1 RevocationInfoChoices .................................... ?? 10.2.1. RevocationInfoChoices . . . . . . . . . . . . 35
10.2.2 CertificateChoices ....................................... ?? 10.2.2. CertificateChoices. . . . . . . . . . . . . . 36
10.2.3 CertificateSet ........................................... ?? 10.2.3. CertificateSet. . . . . . . . . . . . . . . . 37
10.2.4 IssuerAndSerialNumber .................................... ?? 10.2.4. IssuerAndSerialNumber . . . . . . . . . . . . 37
10.2.5 CMSVersion ............................................... ?? 10.2.5. CMSVersion. . . . . . . . . . . . . . . . . . 38
10.2.6 UserKeyingMaterial ....................................... ?? 10.2.6. UserKeyingMaterial. . . . . . . . . . . . . . 38
10.2.7 OtherKeyAttribute ........................................ ?? 10.2.7. OtherKeyAttribute . . . . . . . . . . . . . . 38
11 Useful Attributes ........................................ ?? 11. Useful Attributes . . . . . . . . . . . . . . . . . . . . . . 38
11.1 Content Type ............................................. ?? 11.1. Content Type . . . . . . . . . . . . . . . . . . . . . 39
11.2 Message Digest ........................................... ?? 11.2. Message Digest . . . . . . . . . . . . . . . . . . . . 39
11.3 Signing Time ............................................. ?? 11.3. Signing Time . . . . . . . . . . . . . . . . . . . . . 40
11.4 Countersignature ......................................... ?? 11.4. Countersignature . . . . . . . . . . . . . . . . . . . 41
12 ASN.1 Modules ............................................ ?? 12. ASN.1 Modules . . . . . . . . . . . . . . . . . . . . . . . . 42
12.1 CMS ASN.1 Module ......................................... ?? 12.1. CMS ASN.1 Module . . . . . . . . . . . . . . . . . . . 43
12.2 Version 1 Attribute Certificate ASN.1 Module ............. ?? 12.2. Version 1 Attribute Certificate ASN.1 Module . . . . . 50
13 Normative References ..................................... ?? 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 51
14 Informative References ................................... ?? 13.1. Normative References . . . . . . . . . . . . . . . . . 51
15 Security Considerations .................................. ?? 13.2. Informative References . . . . . . . . . . . . . . . . 52
16 Acknowledgments .......................................... ?? 14. Security Considerations . . . . . . . . . . . . . . . . . . . 53
17 Author Address ........................................... ?? 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 55
18 Full Copyright Statement ................................. ?? 16. Author's Address. . . . . . . . . . . . . . . . . . . . . . . 55
17. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 56
1. Introduction 1. Introduction
This document describes the Cryptographic Message Syntax (CMS). This This document describes the Cryptographic Message Syntax (CMS). This
syntax is used to digitally sign, digest, authenticate, or encrypt syntax is used to digitally sign, digest, authenticate, or encrypt
arbitrary message content. arbitrary message content.
The CMS describes an encapsulation syntax for data protection. It The CMS describes an encapsulation syntax for data protection. It
supports digital signatures and encryption. The syntax allows supports digital signatures and encryption. The syntax allows
multiple encapsulations; one encapsulation envelope can be nested multiple encapsulations; one encapsulation envelope can be nested
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[PROFILE]. [PROFILE].
The CMS values are generated using ASN.1 [X.208-88], using BER- The CMS values are generated using ASN.1 [X.208-88], using BER-
encoding [X.209-88]. Values are typically represented as octet encoding [X.209-88]. Values are typically represented as octet
strings. While many systems are capable of transmitting arbitrary strings. While many systems are capable of transmitting arbitrary
octet strings reliably, it is well known that many electronic mail octet strings reliably, it is well known that many electronic mail
systems are not. This document does not address mechanisms for systems are not. This document does not address mechanisms for
encoding octet strings for reliable transmission in such encoding octet strings for reliable transmission in such
environments. environments.
1.1 Evolution of the CMS 1.1. Evolution of the CMS
The CMS is derived from PKCS #7 version 1.5, which is documented in The CMS is derived from PKCS #7 version 1.5, which is documented in
RFC 2315 [PKCS#7]. PKCS #7 version 1.5 was developed outside of the RFC 2315 [PKCS#7]. PKCS #7 version 1.5 was developed outside of the
IETF; it was originally published as an RSA Laboratories Technical IETF; it was originally published as an RSA Laboratories Technical
Note in November 1993. Since that time, the IETF has taken Note in November 1993. Since that time, the IETF has taken
responsibility for the development and maintenance of the CMS. responsibility for the development and maintenance of the CMS.
Today, several important IETF standards-track protocols make use of Today, several important IETF standards-track protocols make use of
the CMS. the CMS.
This section describes that changes that the IETF has made to the CMS This section describes the changes that the IETF has made to the CMS
in each of the published versions. in each of the published versions.
1.1.1 Changes Since PKCS #7 Version 1.5 1.1.1. Changes Since PKCS #7 Version 1.5
RFC 2630 [CMS1] was the first version of the CMS on the IETF RFC 2630 [CMS1] was the first version of the CMS on the IETF
standards track. Wherever possible, backward compatibility with PKCS standards track. Wherever possible, backward compatibility with PKCS
#7 version 1.5 is preserved; however, changes were made to #7 version 1.5 is preserved; however, changes were made to
accommodate version 1 attribute certificate transfer and to support accommodate version 1 attribute certificate transfer and to support
algorithm independent key management. PKCS #7 version 1.5 included algorithm independent key management. PKCS #7 version 1.5 included
support only for key transport. RFC 2630 adds support for key support only for key transport. RFC 2630 adds support for key
agreement and previously distributed symmetric key-encryption key agreement and previously distributed symmetric key-encryption key
techniques. techniques.
1.1.2 Changes Since RFC 2630 1.1.2. Changes Since RFC 2630
RFC 3369 [CMS2] obsoletes RFC 2630 [CMS1] and RFC 3211 [PWRI]. RFC 3369 [CMS2] obsoletes RFC 2630 [CMS1] and RFC 3211 [PWRI].
Password-based key management is included in the CMS specification, Password-based key management is included in the CMS specification,
and an extension mechanism to support new key management schemes and an extension mechanism to support new key management schemes
without further changes to the CMS is specified. Backward without further changes to the CMS is specified. Backward
compatibility with RFC 2630 and RFC 3211 is preserved; however, compatibility with RFC 2630 and RFC 3211 is preserved; however,
version 2 attribute certificate transfer is added, and the use of version 2 attribute certificate transfer is added, and the use of
version 1 attribute certificates is deprecated. version 1 attribute certificates is deprecated.
S/MIME v2 signatures [OLDMSG], which are based on PKCS#7 version 1.5, S/MIME v2 signatures [OLDMSG], which are based on PKCS#7 version 1.5,
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Specific cryptographic algorithms are not discussed in this document, Specific cryptographic algorithms are not discussed in this document,
but they were discussed in RFC 2630. The discussion of specific but they were discussed in RFC 2630. The discussion of specific
cryptographic algorithms has been moved to a separate document cryptographic algorithms has been moved to a separate document
[CMSALG]. Separation of the protocol and algorithm specifications [CMSALG]. Separation of the protocol and algorithm specifications
allows the IETF to update each document independently. This allows the IETF to update each document independently. This
specification does not require the implementation of any particular specification does not require the implementation of any particular
algorithms. Rather, protocols that rely on the CMS are expected to algorithms. Rather, protocols that rely on the CMS are expected to
choose appropriate algorithms for their environment. The algorithms choose appropriate algorithms for their environment. The algorithms
may be selected from [CMSALG] or elsewhere. may be selected from [CMSALG] or elsewhere.
1.1.3 Changes Since RFC 3369 1.1.3. Changes Since RFC 3369
This document obsoletes RFC 3369 [CMS2]. As discussed in the This document obsoletes RFC 3369 [CMS2]. As discussed in the
previous section, RFC 3369 introduced an extension mechanism to previous section, RFC 3369 introduced an extension mechanism to
support new key management schemes without further changes to the support new key management schemes without further changes to the
CMS. This document introduces a similar extension mechanism to CMS. This document introduces a similar extension mechanism to
support additional certificate formats and revocation status support additional certificate formats and revocation status
information formats without further changes to the CMS. These information formats without further changes to the CMS. These
extensions are primarily documented in section 10.2.1 and section extensions are primarily documented in section 10.2.1 and section
10.2.2. Backward compatibility with earlier versions of the CMS is 10.2.2. Backward compatibility with earlier versions of the CMS is
preserved. preserved.
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Since the publication of RFC 3369, a few errata have been noted. Since the publication of RFC 3369, a few errata have been noted.
These errata are posted on the RFC Editor web site. These errors These errata are posted on the RFC Editor web site. These errors
have been corrected in this document. have been corrected in this document.
The text in section 11.4 that describes the counter signature The text in section 11.4 that describes the counter signature
unsigned attribute is clarified. Hopefully the revised text is unsigned attribute is clarified. Hopefully the revised text is
clearer about the portion of the SignerInfo signature that is covered clearer about the portion of the SignerInfo signature that is covered
by a countersignature. by a countersignature.
1.2 Terminology 1.2. Terminology
In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD, In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as
described in [STDWORDS]. described in [STDWORDS].
1.3 Version Numbers 1.3. Version Numbers
Each of the major data structures includes a version number as the Each of the major data structures includes a version number as the
first item in the data structure. The version numbers are intended first item in the data structure. The version numbers are intended
to avoid ASN.1 decode errors. Some implementations do not check the to avoid ASN.1 decode errors. Some implementations do not check the
version number prior to attempting a decode, and if a decode error version number prior to attempting a decode, and if a decode error
occurs, then the version number is checked as part of the error occurs, then the version number is checked as part of the error
handling routine. This is a reasonable approach; it places error handling routine. This is a reasonable approach; it places error
processing outside of the fast path. This approach is also forgiving processing outside of the fast path. This approach is also forgiving
when an incorrect version number is used by the sender. when an incorrect version number is used by the sender.
Most of the initial version numbers were assigned in PKCS #7 version Most of the initial version numbers were assigned in PKCS #7 version
1.5. Others were assigned when the structure was initially created. 1.5. Others were assigned when the structure was initially created.
Whenever a structure is updated, a higher version number is assigned. Whenever a structure is updated, a higher version number is assigned.
However, to ensure maximum interoperability the higher version number However, to ensure maximum interoperability the higher version number
is only used when the new syntax feature is employed. That is, the is only used when the new syntax feature is employed. That is, the
lowest version number that supports the generated syntax is used. lowest version number that supports the generated syntax is used.
2 General Overview 2. General Overview
The CMS is general enough to support many different content types. The CMS is general enough to support many different content types.
This document defines one protection content, ContentInfo. This document defines one protection content, ContentInfo.
ContentInfo encapsulates a single identified content type, and the ContentInfo encapsulates a single identified content type, and the
identified type may provide further encapsulation. This document identified type may provide further encapsulation. This document
defines six content types: data, signed-data, enveloped-data, defines six content types: data, signed-data, enveloped-data,
digested-data, encrypted-data, and authenticated-data. Additional digested-data, encrypted-data, and authenticated-data. Additional
content types can be defined outside this document. content types can be defined outside this document.
An implementation that conforms to this specification MUST implement An implementation that conforms to this specification MUST implement
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perform encode operations using the Distinguished Encoding Rules perform encode operations using the Distinguished Encoding Rules
(DER) [X.509-88] encoding in a single pass since the lengths of the (DER) [X.509-88] encoding in a single pass since the lengths of the
various components may not be known in advance. However, signed various components may not be known in advance. However, signed
attributes within the signed-data content type and authenticated attributes within the signed-data content type and authenticated
attributes within the authenticated-data content type need to be attributes within the authenticated-data content type need to be
transmitted in DER form to ensure that recipients can verify a transmitted in DER form to ensure that recipients can verify a
content that contains one or more unrecognized attributes. Signed content that contains one or more unrecognized attributes. Signed
attributes and authenticated attributes are the only data types used attributes and authenticated attributes are the only data types used
in the CMS that require DER encoding. in the CMS that require DER encoding.
3 General Syntax 3. General Syntax
The following object identifier identifies the content information The following object identifier identifies the content information
type: type:
id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2) id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 6 } us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 6 }
The CMS associates a content type identifier with a content. The The CMS associates a content type identifier with a content. The
syntax MUST have ASN.1 type ContentInfo: syntax MUST have ASN.1 type ContentInfo:
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an object identifier; it is a unique string of integers assigned an object identifier; it is a unique string of integers assigned
by an authority that defines the content type. by an authority that defines the content type.
content is the associated content. The type of content can be content is the associated content. The type of content can be
determined uniquely by contentType. Content types for data, determined uniquely by contentType. Content types for data,
signed-data, enveloped-data, digested-data, encrypted-data, and signed-data, enveloped-data, digested-data, encrypted-data, and
authenticated-data are defined in this document. If additional authenticated-data are defined in this document. If additional
content types are defined in other documents, the ASN.1 type content types are defined in other documents, the ASN.1 type
defined SHOULD NOT be a CHOICE type. defined SHOULD NOT be a CHOICE type.
4 Data Content Type 4. Data Content Type
The following object identifier identifies the data content type: The following object identifier identifies the data content type:
id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2) id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 } us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }
The data content type is intended to refer to arbitrary octet The data content type is intended to refer to arbitrary octet
strings, such as ASCII text files; the interpretation is left to the strings, such as ASCII text files; the interpretation is left to the
application. Such strings need not have any internal structure application. Such strings need not have any internal structure
(although they could have their own ASN.1 definition or other (although they could have their own ASN.1 definition or other
structure). structure).
S/MIME uses id-data to identify MIME encoded content. The use of S/MIME uses id-data to identify MIME encoded content. The use of
this content identifier is specified in RFC 2311 for S/MIME v2 this content identifier is specified in RFC 2311 for S/MIME v2
[OLDMSG] and RFC 2633 for S/MIME v3 [MSG]. [OLDMSG] and RFC 3851 for S/MIME v3.1 [MSG].
The data content type is generally encapsulated in the signed-data, The data content type is generally encapsulated in the signed-data,
enveloped-data, digested-data, encrypted-data, or authenticated-data enveloped-data, digested-data, encrypted-data, or authenticated-data
content type. content type.
5. Signed-data Content Type 5. Signed-data Content Type
The signed-data content type consists of a content of any type and The signed-data content type consists of a content of any type and
zero or more signature values. Any number of signers in parallel can zero or more signature values. Any number of signers in parallel can
sign any type of content. sign any type of content.
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The typical application of the signed-data content type represents The typical application of the signed-data content type represents
one signer's digital signature on content of the data content type. one signer's digital signature on content of the data content type.
Another typical application disseminates certificates and certificate Another typical application disseminates certificates and certificate
revocation lists (CRLs). revocation lists (CRLs).
The process by which signed-data is constructed involves the The process by which signed-data is constructed involves the
following steps: following steps:
1. For each signer, a message digest, or hash value, is computed 1. For each signer, a message digest, or hash value, is computed
on the content with a signer-specific message-digest algorithm. on the content with a signer-specific message-digest algorithm.
If the signer is signing any information other than the content, If the signer is signing any information other than the
the message digest of the content and the other information are content, the message digest of the content and the other
digested with the signer's message digest algorithm (see Section information are digested with the signer's message digest
5.4), and the result becomes the "message digest." algorithm (see Section 5.4), and the result becomes the
"message digest."
2. For each signer, the message digest is digitally signed using 2. For each signer, the message digest is digitally signed using
the signer's private key. the signer's private key.
3. For each signer, the signature value and other signer-specific 3. For each signer, the signature value and other signer-specific
information are collected into a SignerInfo value, as defined in information are collected into a SignerInfo value, as defined
Section 5.3. Certificates and CRLs for each signer, and those not in Section 5.3. Certificates and CRLs for each signer, and
corresponding to any signer, are collected in this step. those not corresponding to any signer, are collected in this
step.
4. The message digest algorithms for all the signers and the 4. The message digest algorithms for all the signers and the
SignerInfo values for all the signers are collected together with SignerInfo values for all the signers are collected together
the content into a SignedData value, as defined in Section 5.1. with the content into a SignedData value, as defined in Section
5.1.
A recipient independently computes the message digest. This message A recipient independently computes the message digest. This message
digest and the signer's public key are used to verify the signature digest and the signer's public key are used to verify the signature
value. The signer's public key is referenced either by an issuer value. The signer's public key is referenced either by an issuer
distinguished name along with an issuer-specific serial number or by distinguished name along with an issuer-specific serial number or by
a subject key identifier that uniquely identifies the certificate a subject key identifier that uniquely identifies the certificate
containing the public key. The signer's certificate can be included containing the public key. The signer's certificate can be included
in the SignedData certificates field. in the SignedData certificates field.
This section is divided into six parts. The first part describes the This section is divided into six parts. The first part describes the
top-level type SignedData, the second part describes top-level type SignedData, the second part describes
EncapsulatedContentInfo, the third part describes the per-signer EncapsulatedContentInfo, the third part describes the per-signer
information type SignerInfo, and the fourth, fifth, and sixth parts information type SignerInfo, and the fourth, fifth, and sixth parts
describe the message digest calculation, signature generation, and describe the message digest calculation, signature generation, and
signature verification processes, respectively. signature verification processes, respectively.
5.1 SignedData Type 5.1. SignedData Type
The following object identifier identifies the signed-data content The following object identifier identifies the signed-data content
type: type:
id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 } us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
The signed-data content type shall have ASN.1 type SignedData: The signed-data content type shall have ASN.1 type SignedData:
SignedData ::= SEQUENCE { SignedData ::= SEQUENCE {
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signerInfos is a collection of per-signer information. There MAY signerInfos is a collection of per-signer information. There MAY
be any number of elements in the collection, including zero. The be any number of elements in the collection, including zero. The
details of the SignerInfo type are discussed in section 5.3. details of the SignerInfo type are discussed in section 5.3.
Since each signer can employ a digital signature technique and Since each signer can employ a digital signature technique and
future specifications could update the syntax, all implementations future specifications could update the syntax, all implementations
MUST gracefully handle unimplemented versions of SignerInfo. MUST gracefully handle unimplemented versions of SignerInfo.
Further, since all implementations will not support every possible Further, since all implementations will not support every possible
signature algorithm, all implementations MUST gracefully handle signature algorithm, all implementations MUST gracefully handle
unimplemented signature algorithms when they are encountered. unimplemented signature algorithms when they are encountered.
5.2 EncapsulatedContentInfo Type 5.2. EncapsulatedContentInfo Type
The content is represented in the type EncapsulatedContentInfo: The content is represented in the type EncapsulatedContentInfo:
EncapsulatedContentInfo ::= SEQUENCE { EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType, eContentType ContentType,
eContent [0] EXPLICIT OCTET STRING OPTIONAL } eContent [0] EXPLICIT OCTET STRING OPTIONAL }
ContentType ::= OBJECT IDENTIFIER ContentType ::= OBJECT IDENTIFIER
The fields of type EncapsulatedContentInfo have the following The fields of type EncapsulatedContentInfo have the following
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within EncapsulatedContentInfo is absent, then the signatureValue is within EncapsulatedContentInfo is absent, then the signatureValue is
calculated and the eContentType is assigned as though the eContent calculated and the eContentType is assigned as though the eContent
value was present. value was present.
In the degenerate case where there are no signers, the In the degenerate case where there are no signers, the
EncapsulatedContentInfo value being "signed" is irrelevant. In this EncapsulatedContentInfo value being "signed" is irrelevant. In this
case, the content type within the EncapsulatedContentInfo value being case, the content type within the EncapsulatedContentInfo value being
"signed" MUST be id-data (as defined in section 4), and the content "signed" MUST be id-data (as defined in section 4), and the content
field of the EncapsulatedContentInfo value MUST be omitted. field of the EncapsulatedContentInfo value MUST be omitted.
5.2.1 Compatibility with PKCS #7 5.2.1. Compatibility with PKCS #7
This section contains a word of warning to implementers that wish to This section contains a word of warning to implementers that wish to
support both the CMS and PKCS #7 [PKCS#7] SignedData content types. support both the CMS and PKCS #7 [PKCS#7] SignedData content types.
Both the CMS and PKCS #7 identify the type of the encapsulated Both the CMS and PKCS #7 identify the type of the encapsulated
content with an object identifier, but the ASN.1 type of the content content with an object identifier, but the ASN.1 type of the content
itself is variable in PKCS #7 SignedData content type. itself is variable in PKCS #7 SignedData content type.
PKCS #7 defines content as: PKCS #7 defines content as:
content [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL content [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL
The CMS defines eContent as: The CMS defines eContent as:
eContent [0] EXPLICIT OCTET STRING OPTIONAL eContent [0] EXPLICIT OCTET STRING OPTIONAL
The CMS definition is much easier to use in most applications, and it The CMS definition is much easier to use in most applications, and it
is compatible with both S/MIME v2 and S/MIME v3. S/MIME signed is compatible with both S/MIME v2 and S/MIME v3. S/MIME signed
messages using the CMS and PKCS #7 are compatible because identical messages using the CMS and PKCS #7 are compatible because identical
signed message formats are specified in RFC 2311 for S/MIME v2 signed message formats are specified in RFC 2311 for S/MIME v2
[OLDMSG] and RFC 2633 for S/MIME v3 [MSG]. S/MIME v2 encapsulates [OLDMSG] and RFC 3851 for S/MIME v3.1 [MSG]. S/MIME v2 encapsulates
the MIME content in a Data type (that is, an OCTET STRING) carried in the MIME content in a Data type (that is, an OCTET STRING) carried in
the SignedData contentInfo content ANY field, and S/MIME v3 carries the SignedData contentInfo content ANY field, and S/MIME v3 carries
the MIME content in the SignedData encapContentInfo eContent OCTET the MIME content in the SignedData encapContentInfo eContent OCTET
STRING. Therefore, in both S/MIME v2 and S/MIME v3, the MIME content STRING. Therefore, in both S/MIME v2 and S/MIME v3, the MIME content
is placed in an OCTET STRING and the message digest is computed over is placed in an OCTET STRING and the message digest is computed over
the identical portions of the content. That is, the message digest the identical portions of the content. That is, the message digest
is computed over the octets comprising the value of the OCTET STRING, is computed over the octets comprising the value of the OCTET STRING,
neither the tag nor length octets are included. neither the tag nor length octets are included.
There are incompatibilities between the CMS and PKCS #7 SignedData There are incompatibilities between the CMS and PKCS #7 SignedData
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encapsulated content is not formatted using the Data type. encapsulated content is not formatted using the Data type.
Implementations MAY examine the value of the eContentType, and then Implementations MAY examine the value of the eContentType, and then
adjust the expected DER encoding of eContent based on the object adjust the expected DER encoding of eContent based on the object
identifier value. For example, to support Microsoft Authenticode identifier value. For example, to support Microsoft Authenticode
[MSAC], the following information MAY be included: [MSAC], the following information MAY be included:
eContentType Object Identifier is set to { 1 3 6 1 4 1 311 2 1 4 } eContentType Object Identifier is set to { 1 3 6 1 4 1 311 2 1 4 }
eContent contains DER encoded Authenticode signing information eContent contains DER encoded Authenticode signing information
5.3 SignerInfo Type 5.3. SignerInfo Type
Per-signer information is represented in the type SignerInfo: Per-signer information is represented in the type SignerInfo:
SignerInfo ::= SEQUENCE { SignerInfo ::= SEQUENCE {
version CMSVersion, version CMSVersion,
sid SignerIdentifier, sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier, digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL, signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier, signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue, signature SignatureValue,
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signedAttrs is a collection of attributes that are signed. The signedAttrs is a collection of attributes that are signed. The
field is optional, but it MUST be present if the content type of field is optional, but it MUST be present if the content type of
the EncapsulatedContentInfo value being signed is not id-data. the EncapsulatedContentInfo value being signed is not id-data.
SignedAttributes MUST be DER encoded, even if the rest of the SignedAttributes MUST be DER encoded, even if the rest of the
structure is BER encoded. Useful attribute types, such as signing structure is BER encoded. Useful attribute types, such as signing
time, are defined in Section 11. If the field is present, it MUST time, are defined in Section 11. If the field is present, it MUST
contain, at a minimum, the following two attributes: contain, at a minimum, the following two attributes:
A content-type attribute having as its value the content type A content-type attribute having as its value the content type
of the EncapsulatedContentInfo value being signed. Section of the EncapsulatedContentInfo value being signed. Section
11.1 defines the content-type attribute. However, the content- 11.1 defines the content-type attribute. However, the
type attribute MUST NOT be used as part of a countersignature content-type attribute MUST NOT be used as part of a
unsigned attribute as defined in section 11.4. countersignature unsigned attribute as defined in section 11.4.
A message-digest attribute, having as its value the message A message-digest attribute, having as its value the message
digest of the content. Section 11.2 defines the message-digest digest of the content. Section 11.2 defines the message-digest
attribute. attribute.
signatureAlgorithm identifies the signature algorithm, and any signatureAlgorithm identifies the signature algorithm, and any
associated parameters, used by the signer to generate the digital associated parameters, used by the signer to generate the digital
signature. signature.
signature is the result of digital signature generation, using the signature is the result of digital signature generation, using the
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following meanings: following meanings:
attrType indicates the type of attribute. It is an object attrType indicates the type of attribute. It is an object
identifier. identifier.
attrValues is a set of values that comprise the attribute. The attrValues is a set of values that comprise the attribute. The
type of each value in the set can be determined uniquely by type of each value in the set can be determined uniquely by
attrType. The attrType can impose restrictions on the number of attrType. The attrType can impose restrictions on the number of
items in the set. items in the set.
5.4 Message Digest Calculation Process 5.4. Message Digest Calculation Process
The message digest calculation process computes a message digest on The message digest calculation process computes a message digest on
either the content being signed or the content together with the either the content being signed or the content together with the
signed attributes. In either case, the initial input to the message signed attributes. In either case, the initial input to the message
digest calculation process is the "value" of the encapsulated content digest calculation process is the "value" of the encapsulated content
being signed. Specifically, the initial input is the being signed. Specifically, the initial input is the
encapContentInfo eContent OCTET STRING to which the signing process encapContentInfo eContent OCTET STRING to which the signing process
is applied. Only the octets comprising the value of the eContent is applied. Only the octets comprising the value of the eContent
OCTET STRING are input to the message digest algorithm, not the tag OCTET STRING are input to the message digest algorithm, not the tag
or the length octets. or the length octets.
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This has the advantage that the length of the content being signed This has the advantage that the length of the content being signed
need not be known in advance of the signature generation process. need not be known in advance of the signature generation process.
Although the encapContentInfo eContent OCTET STRING tag and length Although the encapContentInfo eContent OCTET STRING tag and length
octets are not included in the message digest calculation, they are octets are not included in the message digest calculation, they are
protected by other means. The length octets are protected by the protected by other means. The length octets are protected by the
nature of the message digest algorithm since it is computationally nature of the message digest algorithm since it is computationally
infeasible to find any two distinct message contents of any length infeasible to find any two distinct message contents of any length
that have the same message digest. that have the same message digest.
5.5 Signature Generation Process 5.5. Signature Generation Process
The input to the signature generation process includes the result of The input to the signature generation process includes the result of
the message digest calculation process and the signer's private key. the message digest calculation process and the signer's private key.
The details of the signature generation depend on the signature The details of the signature generation depend on the signature
algorithm employed. The object identifier, along with any algorithm employed. The object identifier, along with any
parameters, that specifies the signature algorithm employed by the parameters, that specifies the signature algorithm employed by the
signer is carried in the signatureAlgorithm field. The signature signer is carried in the signatureAlgorithm field. The signature
value generated by the signer MUST be encoded as an OCTET STRING and value generated by the signer MUST be encoded as an OCTET STRING and
carried in the signature field. carried in the signature field.
5.6 Signature Verification Process 5.6. Signature Verification Process
The input to the signature verification process includes the result The input to the signature verification process includes the result
of the message digest calculation process and the signer's public of the message digest calculation process and the signer's public
key. The recipient MAY obtain the correct public key for the signer key. The recipient MAY obtain the correct public key for the signer
by any means, but the preferred method is from a certificate obtained by any means, but the preferred method is from a certificate obtained
from the SignedData certificates field. The selection and validation from the SignedData certificates field. The selection and validation
of the signer's public key MAY be based on certification path of the signer's public key MAY be based on certification path
validation (see [PROFILE]) as well as other external context, but is validation (see [PROFILE]) as well as other external context, but is
beyond the scope of this document. The details of the signature beyond the scope of this document. The details of the signature
verification depend on the signature algorithm employed. verification depend on the signature algorithm employed.
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3. For each recipient, the encrypted content-encryption key and 3. For each recipient, the encrypted content-encryption key and
other recipient-specific information are collected into a other recipient-specific information are collected into a
RecipientInfo value, defined in Section 6.2. RecipientInfo value, defined in Section 6.2.
4. The content is encrypted with the content-encryption key. 4. The content is encrypted with the content-encryption key.
Content encryption may require that the content be padded to a Content encryption may require that the content be padded to a
multiple of some block size; see Section 6.3. multiple of some block size; see Section 6.3.
5. The RecipientInfo values for all the recipients are collected 5. The RecipientInfo values for all the recipients are collected
together with the encrypted content to form an EnvelopedData value together with the encrypted content to form an EnvelopedData
as defined in Section 6.1. value as defined in Section 6.1.
A recipient opens the digital envelope by decrypting one of the A recipient opens the digital envelope by decrypting one of the
encrypted content-encryption keys and then decrypting the encrypted encrypted content-encryption keys and then decrypting the
content with the recovered content-encryption key. encrypted content with the recovered content-encryption key.
This section is divided into four parts. The first part describes This section is divided into four parts. The first part describes
the top-level type EnvelopedData, the second part describes the per- the top-level type EnvelopedData, the second part describes the
recipient information type RecipientInfo, and the third and fourth per-recipient information type RecipientInfo, and the third and
parts describe the content-encryption and key-encryption processes. fourth parts describe the content-encryption and key-encryption
processes.
6.1 EnvelopedData Type 6.1. EnvelopedData Type
The following object identifier identifies the enveloped-data content The following object identifier identifies the enveloped-data content
type: type:
id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2) id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 } us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }
The enveloped-data content type shall have ASN.1 type EnvelopedData: The enveloped-data content type shall have ASN.1 type EnvelopedData:
EnvelopedData ::= SEQUENCE { EnvelopedData ::= SEQUENCE {
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6.3. The same content-encryption algorithm and content-encryption 6.3. The same content-encryption algorithm and content-encryption
key are used for all recipients. key are used for all recipients.
encryptedContent is the result of encrypting the content. The encryptedContent is the result of encrypting the content. The
field is optional, and if the field is not present, its intended field is optional, and if the field is not present, its intended
value must be supplied by other means. value must be supplied by other means.
The recipientInfos field comes before the encryptedContentInfo field The recipientInfos field comes before the encryptedContentInfo field
so that an EnvelopedData value may be processed in a single pass. so that an EnvelopedData value may be processed in a single pass.
6.2 RecipientInfo Type 6.2. RecipientInfo Type
Per-recipient information is represented in the type RecipientInfo. Per-recipient information is represented in the type RecipientInfo.
RecipientInfo has a different format for each of the supported key RecipientInfo has a different format for each of the supported key
management techniques. Any of the key management techniques can be management techniques. Any of the key management techniques can be
used for each recipient of the same encrypted content. In all cases, used for each recipient of the same encrypted content. In all cases,
the encrypted content-encryption key is transferred to one or more the encrypted content-encryption key is transferred to one or more
recipients. recipients.
Since all implementations will not support every possible key Since all implementations will not support every possible key
management algorithm, all implementations MUST gracefully handle management algorithm, all implementations MUST gracefully handle
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RecipientInfo ::= CHOICE { RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo, ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo, kari [1] KeyAgreeRecipientInfo,
kekri [2] KEKRecipientInfo, kekri [2] KEKRecipientInfo,
pwri [3] PasswordRecipientinfo, pwri [3] PasswordRecipientinfo,
ori [4] OtherRecipientInfo } ori [4] OtherRecipientInfo }
EncryptedKey ::= OCTET STRING EncryptedKey ::= OCTET STRING
6.2.1 KeyTransRecipientInfo Type 6.2.1. KeyTransRecipientInfo Type
Per-recipient information using key transport is represented in the Per-recipient information using key transport is represented in the
type KeyTransRecipientInfo. Each instance of KeyTransRecipientInfo type KeyTransRecipientInfo. Each instance of KeyTransRecipientInfo
transfers the content-encryption key to one recipient. transfers the content-encryption key to one recipient.
KeyTransRecipientInfo ::= SEQUENCE { KeyTransRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 0 or 2 version CMSVersion, -- always set to 0 or 2
rid RecipientIdentifier, rid RecipientIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier, keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey } encryptedKey EncryptedKey }
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these alternatives. these alternatives.
keyEncryptionAlgorithm identifies the key-encryption algorithm, keyEncryptionAlgorithm identifies the key-encryption algorithm,
and any associated parameters, used to encrypt the content- and any associated parameters, used to encrypt the content-
encryption key for the recipient. The key-encryption process is encryption key for the recipient. The key-encryption process is
described in Section 6.4. described in Section 6.4.
encryptedKey is the result of encrypting the content-encryption encryptedKey is the result of encrypting the content-encryption
key for the recipient. key for the recipient.
6.2.2 KeyAgreeRecipientInfo Type 6.2.2. KeyAgreeRecipientInfo Type
Recipient information using key agreement is represented in the type Recipient information using key agreement is represented in the type
KeyAgreeRecipientInfo. Each instance of KeyAgreeRecipientInfo will KeyAgreeRecipientInfo. Each instance of KeyAgreeRecipientInfo will
transfer the content-encryption key to one or more recipients that transfer the content-encryption key to one or more recipients that
use the same key agreement algorithm and domain parameters for that use the same key agreement algorithm and domain parameters for that
algorithm. algorithm.
KeyAgreeRecipientInfo ::= SEQUENCE { KeyAgreeRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 3 version CMSVersion, -- always set to 3
originator [0] EXPLICIT OriginatorIdentifierOrKey, originator [0] EXPLICIT OriginatorIdentifierOrKey,
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generate a pairwise key. The content-encryption key is encrypted generate a pairwise key. The content-encryption key is encrypted
in the pairwise key. The issuerAndSerialNumber alternative in the pairwise key. The issuerAndSerialNumber alternative
identifies the sender's certificate, and thereby the sender's identifies the sender's certificate, and thereby the sender's
public key, by the issuer's distinguished name and the certificate public key, by the issuer's distinguished name and the certificate
serial number. The subjectKeyIdentifier alternative identifies serial number. The subjectKeyIdentifier alternative identifies
the sender's certificate, and thereby the sender's public key, by the sender's certificate, and thereby the sender's public key, by
a key identifier. When an X.509 certificate is referenced, the a key identifier. When an X.509 certificate is referenced, the
key identifier matches the X.509 subjectKeyIdentifier extension key identifier matches the X.509 subjectKeyIdentifier extension
value. When other certificate formats are referenced, the value. When other certificate formats are referenced, the
documents that specify the certificate format and their use with documents that specify the certificate format and their use with
the CMS must must include details on matching the key identifier the CMS must include details on matching the key identifier to the
to the appropriate certificate field. The originatorKey appropriate certificate field. The originatorKey alternative
alternative includes the algorithm identifier and sender's key includes the algorithm identifier and sender's key agreement
agreement public key. This alternative permits originator public key. This alternative permits originator anonymity since
anonymity since the public key is not certified. Implementations the public key is not certified. Implementations MUST support all
MUST support all three alternatives for specifying the sender's three alternatives for specifying the sender's public key.
public key.
ukm is optional. With some key agreement algorithms, the sender ukm is optional. With some key agreement algorithms, the sender
provides a User Keying Material (UKM) to ensure that a different provides a User Keying Material (UKM) to ensure that a different
key is generated each time the same two parties generate a key is generated each time the same two parties generate a
pairwise key. Implementations MUST accept a KeyAgreeRecipientInfo pairwise key. Implementations MUST accept a KeyAgreeRecipientInfo
SEQUENCE that includes a ukm field. Implementations that do not SEQUENCE that includes a ukm field. Implementations that do not
support key agreement algorithms that make use of UKMs MUST support key agreement algorithms that make use of UKMs MUST
gracefully handle the presence of UKMs. gracefully handle the presence of UKMs.
keyEncryptionAlgorithm identifies the key-encryption algorithm, keyEncryptionAlgorithm identifies the key-encryption algorithm,
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include details on matching the key identifier to the appropriate include details on matching the key identifier to the appropriate
certificate field. certificate field.
date is optional. When present, the date specifies which of the date is optional. When present, the date specifies which of the
recipient's previously distributed UKMs was used by the sender. recipient's previously distributed UKMs was used by the sender.
other is optional. When present, this field contains additional other is optional. When present, this field contains additional
information used by the recipient to locate the public keying information used by the recipient to locate the public keying
material used by the sender. material used by the sender.
6.2.3 KEKRecipientInfo Type 6.2.3. KEKRecipientInfo Type
Recipient information using previously distributed symmetric keys is Recipient information using previously distributed symmetric keys is
represented in the type KEKRecipientInfo. Each instance of represented in the type KEKRecipientInfo. Each instance of
KEKRecipientInfo will transfer the content-encryption key to one or KEKRecipientInfo will transfer the content-encryption key to one or
more recipients who have the previously distributed key-encryption more recipients who have the previously distributed key-encryption
key. key.
KEKRecipientInfo ::= SEQUENCE { KEKRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 4 version CMSVersion, -- always set to 4
kekid KEKIdentifier, kekid KEKIdentifier,
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keyIdentifier identifies the key-encryption key that was keyIdentifier identifies the key-encryption key that was
previously distributed to the sender and one or more recipients. previously distributed to the sender and one or more recipients.
date is optional. When present, the date specifies a single key- date is optional. When present, the date specifies a single key-
encryption key from a set that was previously distributed. encryption key from a set that was previously distributed.
other is optional. When present, this field contains additional other is optional. When present, this field contains additional
information used by the recipient to determine the key-encryption information used by the recipient to determine the key-encryption
key used by the sender. key used by the sender.
6.2.4 PasswordRecipientInfo Type 6.2.4. PasswordRecipientInfo Type
Recipient information using a password or shared secret value is Recipient information using a password or shared secret value is
represented in the type PasswordRecipientInfo. Each instance of represented in the type PasswordRecipientInfo. Each instance of
PasswordRecipientInfo will transfer the content-encryption key to one PasswordRecipientInfo will transfer the content-encryption key to one
or more recipients who possess the password or shared secret value. or more recipients who possess the password or shared secret value.
The PasswordRecipientInfo Type is specified in RFC 3211 [PWRI]. The The PasswordRecipientInfo Type is specified in RFC 3211 [PWRI]. The
PasswordRecipientInfo structure is repeated here for completeness. PasswordRecipientInfo structure is repeated here for completeness.
PasswordRecipientInfo ::= SEQUENCE { PasswordRecipientInfo ::= SEQUENCE {
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absent, the key-encryption key is supplied from an external absent, the key-encryption key is supplied from an external
source, for example a hardware crypto token such as a smart card. source, for example a hardware crypto token such as a smart card.
keyEncryptionAlgorithm identifies the encryption algorithm, and keyEncryptionAlgorithm identifies the encryption algorithm, and
any associated parameters, used to encrypt the content-encryption any associated parameters, used to encrypt the content-encryption
key with the key-encryption key. key with the key-encryption key.
encryptedKey is the result of encrypting the content-encryption encryptedKey is the result of encrypting the content-encryption
key with the key-encryption key. key with the key-encryption key.
6.2.5 OtherRecipientInfo Type 6.2.5. OtherRecipientInfo Type
Recipient information for additional key management techniques are Recipient information for additional key management techniques are
represented in the type OtherRecipientInfo. The OtherRecipientInfo represented in the type OtherRecipientInfo. The OtherRecipientInfo
type allows key management techniques beyond key transport, key type allows key management techniques beyond key transport, key
agreement, previously distributed symmetric key-encryption keys, and agreement, previously distributed symmetric key-encryption keys, and
password-based key management to be specified in future documents. password-based key management to be specified in future documents.
An object identifier uniquely identifies such key management An object identifier uniquely identifies such key management
techniques. techniques.
OtherRecipientInfo ::= SEQUENCE { OtherRecipientInfo ::= SEQUENCE {
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represented in the type OtherRecipientInfo. The OtherRecipientInfo represented in the type OtherRecipientInfo. The OtherRecipientInfo
type allows key management techniques beyond key transport, key type allows key management techniques beyond key transport, key
agreement, previously distributed symmetric key-encryption keys, and agreement, previously distributed symmetric key-encryption keys, and
password-based key management to be specified in future documents. password-based key management to be specified in future documents.
An object identifier uniquely identifies such key management An object identifier uniquely identifies such key management
techniques. techniques.
OtherRecipientInfo ::= SEQUENCE { OtherRecipientInfo ::= SEQUENCE {
oriType OBJECT IDENTIFIER, oriType OBJECT IDENTIFIER,
oriValue ANY DEFINED BY oriType } oriValue ANY DEFINED BY oriType }
The fields of type OtherRecipientInfo have the following meanings: The fields of type OtherRecipientInfo have the following meanings:
oriType identifies the key management technique. oriType identifies the key management technique.
oriValue contains the protocol data elements needed by a recipient oriValue contains the protocol data elements needed by a recipient
using the identified key management technique. using the identified key management technique.
6.3 Content-encryption Process 6.3. Content-encryption Process
The content-encryption key for the desired content-encryption The content-encryption key for the desired content-encryption
algorithm is randomly generated. The data to be protected is padded algorithm is randomly generated. The data to be protected is padded
as described below, then the padded data is encrypted using the as described below, then the padded data is encrypted using the
content-encryption key. The encryption operation maps an arbitrary content-encryption key. The encryption operation maps an arbitrary
string of octets (the data) to another string of octets (the string of octets (the data) to another string of octets (the
ciphertext) under control of a content-encryption key. The encrypted ciphertext) under control of a content-encryption key. The encrypted
data is included in the EnvelopedData encryptedContentInfo data is included in the EnvelopedData encryptedContentInfo
encryptedContent OCTET STRING. encryptedContent OCTET STRING.
Some content-encryption algorithms assume the input length is a Some content-encryption algorithms assume the input length is a
multiple of k octets, where k is greater than one. For such multiple of k octets, where k is greater than one. For such
algorithms, the input shall be padded at the trailing end with algorithms, the input shall be padded at the trailing end with k-(lth
k-(lth mod k) octets all having value k-(lth mod k), where lth is mod k) octets all having value k-(lth mod k), where lth is the length
the length of the input. In other words, the input is padded at of the input. In other words, the input is padded at the trailing
the trailing end with one of the following strings: end with one of the following strings:
01 -- if lth mod k = k-1 01 -- if lth mod k = k-1
02 02 -- if lth mod k = k-2 02 02 -- if lth mod k = k-2
. .
. .
. .
k k ... k k -- if lth mod k = 0 k k ... k k -- if lth mod k = 0
The padding can be removed unambiguously since all input is padded, The padding can be removed unambiguously since all input is padded,
including input values that are already a multiple of the block size, including input values that are already a multiple of the block size,
and no padding string is a suffix of another. This padding method is and no padding string is a suffix of another. This padding method is
well defined if and only if k is less than 256. well defined if and only if k is less than 256.
6.4 Key-encryption Process 6.4. Key-encryption Process
The input to the key-encryption process -- the value supplied to the The input to the key-encryption process -- the value supplied to the
recipient's key-encryption algorithm -- is just the "value" of the recipient's key-encryption algorithm -- is just the "value" of the
content-encryption key. content-encryption key.
Any of the aforementioned key management techniques can be used for Any of the aforementioned key management techniques can be used for
each recipient of the same encrypted content. each recipient of the same encrypted content.
7. Digested-data Content Type 7. Digested-data Content Type
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recipient to verify the integrity of the content. Any type of recipient to verify the integrity of the content. Any type of
content can be integrity protected for an arbitrary number of content can be integrity protected for an arbitrary number of
recipients. recipients.
The process by which authenticated-data is constructed involves the The process by which authenticated-data is constructed involves the
following steps: following steps:
1. A message-authentication key for a particular message- 1. A message-authentication key for a particular message-
authentication algorithm is generated at random. authentication algorithm is generated at random.
2. The message-authentication key is encrypted for each 2. The message-authentication key is encrypted for each recipient.
recipient. The details of this encryption depend on the key The details of this encryption depend on the key management
management algorithm used. algorithm used.
3. For each recipient, the encrypted message-authentication key 3. For each recipient, the encrypted message-authentication key
and other recipient-specific information are collected into a and other recipient-specific information are collected into a
RecipientInfo value, defined in Section 6.2. RecipientInfo value, defined in Section 6.2.
4. Using the message-authentication key, the originator computes 4. Using the message-authentication key, the originator computes a
a MAC value on the content. If the originator is authenticating MAC value on the content. If the originator is authenticating
any information in addition to the content (see Section 9.2), a any information in addition to the content (see Section 9.2), a
message digest is calculated on the content, the message digest of message digest is calculated on the content, the message digest
the content and the other information are authenticated using the of the content and the other information are authenticated
message-authentication key, and the result becomes the "MAC using the message-authentication key, and the result becomes
value." the "MAC value."
9.1 AuthenticatedData Type 9.1. AuthenticatedData Type
The following object identifier identifies the authenticated-data The following object identifier identifies the authenticated-data
content type: content type:
id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2) id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
ct(1) 2 } ct(1) 2 }
The authenticated-data content type shall have ASN.1 type The authenticated-data content type shall have ASN.1 type
AuthenticatedData: AuthenticatedData:
AuthenticatedData ::= SEQUENCE { AuthenticatedData ::= SEQUENCE {
version CMSVersion, version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL, originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos, recipientInfos RecipientInfos,
macAlgorithm MessageAuthenticationCodeAlgorithm, macAlgorithm MessageAuthenticationCodeAlgorithm,
digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL, digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,
encapContentInfo EncapsulatedContentInfo, encapContentInfo EncapsulatedContentInfo,
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digest of the content. Section 11.2 defines the message-digest digest of the content. Section 11.2 defines the message-digest
attribute. attribute.
mac is the message authentication code. mac is the message authentication code.
unauthAttrs is a collection of attributes that are not unauthAttrs is a collection of attributes that are not
authenticated. The field is optional. To date, no attributes authenticated. The field is optional. To date, no attributes
have been defined for use as unauthenticated attributes, but other have been defined for use as unauthenticated attributes, but other
useful attribute types are defined in Section 11. useful attribute types are defined in Section 11.
9.2 MAC Generation 9.2. MAC Generation
The MAC calculation process computes a message authentication code The MAC calculation process computes a message authentication code
(MAC) on either the content being authenticated or a message digest (MAC) on either the content being authenticated or a message digest
of content being authenticated together with the originator's of content being authenticated together with the originator's
authenticated attributes. authenticated attributes.
If authAttrs field is absent, the input to the MAC calculation If authAttrs field is absent, the input to the MAC calculation
process is the value of the encapContentInfo eContent OCTET STRING. process is the value of the encapContentInfo eContent OCTET STRING.
Only the octets comprising the value of the eContent OCTET STRING are Only the octets comprising the value of the eContent OCTET STRING are
input to the MAC algorithm; the tag and the length octets are input to the MAC algorithm; the tag and the length octets are
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The input to the MAC calculation process includes the MAC input data, The input to the MAC calculation process includes the MAC input data,
defined above, and an authentication key conveyed in a recipientInfo defined above, and an authentication key conveyed in a recipientInfo
structure. The details of MAC calculation depend on the MAC structure. The details of MAC calculation depend on the MAC
algorithm employed (e.g., HMAC). The object identifier, along with algorithm employed (e.g., HMAC). The object identifier, along with
any parameters, that specifies the MAC algorithm employed by the any parameters, that specifies the MAC algorithm employed by the
originator is carried in the macAlgorithm field. The MAC value originator is carried in the macAlgorithm field. The MAC value
generated by the originator is encoded as an OCTET STRING and carried generated by the originator is encoded as an OCTET STRING and carried
in the mac field. in the mac field.
9.3 MAC Verification 9.3. MAC Verification
The input to the MAC verification process includes the input data The input to the MAC verification process includes the input data
(determined based on the presence or absence of the authAttrs field, (determined based on the presence or absence of the authAttrs field,
as defined in 9.2), and the authentication key conveyed in as defined in 9.2), and the authentication key conveyed in
recipientInfo. The details of the MAC verification process depend on recipientInfo. The details of the MAC verification process depend on
the MAC algorithm employed. the MAC algorithm employed.
The recipient MUST NOT rely on any MAC values or message digest The recipient MUST NOT rely on any MAC values or message digest
values computed by the originator. The content is authenticated as values computed by the originator. The content is authenticated as
described in section 9.2. If the originator includes authenticated described in section 9.2. If the originator includes authenticated
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If the AuthenticatedData includes authAttrs, then the content-type If the AuthenticatedData includes authAttrs, then the content-type
attribute value MUST match the AuthenticatedData encapContentInfo attribute value MUST match the AuthenticatedData encapContentInfo
eContentType value. eContentType value.
10. Useful Types 10. Useful Types
This section is divided into two parts. The first part defines This section is divided into two parts. The first part defines
algorithm identifiers, and the second part defines other useful algorithm identifiers, and the second part defines other useful
types. types.
10.1 Algorithm Identifier Types 10.1. Algorithm Identifier Types
All of the algorithm identifiers have the same type: All of the algorithm identifiers have the same type:
AlgorithmIdentifier. The definition of AlgorithmIdentifier is taken AlgorithmIdentifier. The definition of AlgorithmIdentifier is taken
from X.509 [X.509-88]. from X.509 [X.509-88].
There are many alternatives for each algorithm type. There are many alternatives for each algorithm type.
10.1.1 DigestAlgorithmIdentifier 10.1.1. DigestAlgorithmIdentifier
The DigestAlgorithmIdentifier type identifies a message-digest The DigestAlgorithmIdentifier type identifies a message-digest
algorithm. Examples include SHA-1, MD2, and MD5. A message-digest algorithm. Examples include SHA-1, MD2, and MD5. A message-digest
algorithm maps an octet string (the content) to another octet string algorithm maps an octet string (the content) to another octet string
(the message digest). (the message digest).
DigestAlgorithmIdentifier ::= AlgorithmIdentifier DigestAlgorithmIdentifier ::= AlgorithmIdentifier
10.1.2 SignatureAlgorithmIdentifier 10.1.2. SignatureAlgorithmIdentifier
The SignatureAlgorithmIdentifier type identifies a signature The SignatureAlgorithmIdentifier type identifies a signature
algorithm. Examples include RSA, DSA, and ECDSA. A signature algorithm. Examples include RSA, DSA, and ECDSA. A signature
algorithm supports signature generation and verification operations. algorithm supports signature generation and verification operations.
The signature generation operation uses the message digest and the The signature generation operation uses the message digest and the
signer's private key to generate a signature value. The signature signer's private key to generate a signature value. The signature
verification operation uses the message digest and the signer's verification operation uses the message digest and the signer's
public key to determine whether or not a signature value is valid. public key to determine whether or not a signature value is valid.
Context determines which operation is intended. Context determines which operation is intended.
SignatureAlgorithmIdentifier ::= AlgorithmIdentifier SignatureAlgorithmIdentifier ::= AlgorithmIdentifier
10.1.3 KeyEncryptionAlgorithmIdentifier 10.1.3. KeyEncryptionAlgorithmIdentifier
The KeyEncryptionAlgorithmIdentifier type identifies a key-encryption The KeyEncryptionAlgorithmIdentifier type identifies a key-encryption
algorithm used to encrypt a content-encryption key. The encryption algorithm used to encrypt a content-encryption key. The encryption
operation maps an octet string (the key) to another octet string (the operation maps an octet string (the key) to another octet string (the
encrypted key) under control of a key-encryption key. The decryption encrypted key) under control of a key-encryption key. The decryption
operation is the inverse of the encryption operation. Context operation is the inverse of the encryption operation. Context
determines which operation is intended. determines which operation is intended.
The details of encryption and decryption depend on the key management The details of encryption and decryption depend on the key management
algorithm used. Key transport, key agreement, previously distributed algorithm used. Key transport, key agreement, previously distributed
symmetric key-encrypting keys, and symmetric key-encrypting keys symmetric key-encrypting keys, and symmetric key-encrypting keys
derived from passwords are supported. derived from passwords are supported.
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
10.1.4 ContentEncryptionAlgorithmIdentifier 10.1.4. ContentEncryptionAlgorithmIdentifier
The ContentEncryptionAlgorithmIdentifier type identifies a content- The ContentEncryptionAlgorithmIdentifier type identifies a content-
encryption algorithm. Examples include Triple-DES and RC2. A encryption algorithm. Examples include Triple-DES and RC2. A
content-encryption algorithm supports encryption and decryption content-encryption algorithm supports encryption and decryption
operations. The encryption operation maps an octet string (the operations. The encryption operation maps an octet string (the
plaintext) to another octet string (the ciphertext) under control of plaintext) to another octet string (the ciphertext) under control of
a content-encryption key. The decryption operation is the inverse of a content-encryption key. The decryption operation is the inverse of
the encryption operation. Context determines which operation is the encryption operation. Context determines which operation is
intended. intended.
ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
10.1.5 MessageAuthenticationCodeAlgorithm 10.1.5. MessageAuthenticationCodeAlgorithm
The MessageAuthenticationCodeAlgorithm type identifies a message The MessageAuthenticationCodeAlgorithm type identifies a message
authentication code (MAC) algorithm. Examples include DES-MAC and authentication code (MAC) algorithm. Examples include DES-MAC and
HMAC-SHA-1. A MAC algorithm supports generation and verification HMAC-SHA-1. A MAC algorithm supports generation and verification
operations. The MAC generation and verification operations use the operations. The MAC generation and verification operations use the
same symmetric key. Context determines which operation is intended. same symmetric key. Context determines which operation is intended.
MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier
10.1.6 KeyDerivationAlgorithmIdentifier 10.1.6. KeyDerivationAlgorithmIdentifier
The KeyDerivationAlgorithmIdentifier type is specified in RFC 3211 The KeyDerivationAlgorithmIdentifier type is specified in RFC 3211
[PWRI]. The KeyDerivationAlgorithmIdentifier definition is repeated [PWRI]. The KeyDerivationAlgorithmIdentifier definition is repeated
here for completeness. here for completeness.
Key derivation algorithms convert a password or shared secret value Key derivation algorithms convert a password or shared secret value
into a key-encryption key. into a key-encryption key.
KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier
10.2 Other Useful Types 10.2. Other Useful Types
This section defines types that are used other places in the This section defines types that are used other places in the
document. The types are not listed in any particular order. document. The types are not listed in any particular order.
10.2.1 RevocationInfoChoices 10.2.1. RevocationInfoChoices
The RevocationInfoChoices type gives a set of revocation status The RevocationInfoChoices type gives a set of revocation status
information alternatives. It is intended that the set contain information alternatives. It is intended that the set contain
information sufficient to determine whether the certificates and information sufficient to determine whether the certificates and
attribute certificates with which the set is associated are revoked. attribute certificates with which the set is associated are revoked.
However, there MAY be more revocation status information than However, there MAY be more revocation status information than
necessary or there MAY be less revocation status information than necessary or there MAY be less revocation status information than
necessary. X.509 Certificate revocation lists (CRLs) [X.509-97] are necessary. X.509 Certificate revocation lists (CRLs) [X.509-97] are
the primary source of revocation status information, but any other the primary source of revocation status information, but any other
revocation information format can be supported. The revocation information format can be supported. The
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CRLs are specified in X.509 [X.509-97], and they are profiled for use CRLs are specified in X.509 [X.509-97], and they are profiled for use
in the Internet in RFC 3280 [PROFILE]. in the Internet in RFC 3280 [PROFILE].
The definition of CertificateList is taken from X.509. The definition of CertificateList is taken from X.509.
RevocationInfoChoices ::= SET OF RevocationInfoChoice RevocationInfoChoices ::= SET OF RevocationInfoChoice
RevocationInfoChoice ::= CHOICE { RevocationInfoChoice ::= CHOICE {
crl CertificateList, crl CertificateList,
other [1] IMPLICIT OtherRevocationInfoFormat } other [1] IMPLICIT OtherRevocationInfoFormat }
OtherRevocationInfoFormat ::= SEQUENCE { OtherRevocationInfoFormat ::= SEQUENCE {
otherRevInfoFormat OBJECT IDENTIFIER, otherRevInfoFormat OBJECT IDENTIFIER,
otherRevInfo ANY DEFINED BY otherRevInfoFormat } otherRevInfo ANY DEFINED BY otherRevInfoFormat }
10.2.2 CertificateChoices 10.2.2. CertificateChoices
The CertificateChoices type gives either a PKCS #6 extended The CertificateChoices type gives either a PKCS #6 extended
certificate [PKCS#6], an X.509 certificate, a version 1 X.509 certificate [PKCS#6], an X.509 certificate, a version 1 X.509
attribute certificate (ACv1) [X.509-97], a version 2 X.509 attribute attribute certificate (ACv1) [X.509-97], a version 2 X.509 attribute
certificate (ACv2) [X.509-00], or any other certificate format. The certificate (ACv2) [X.509-00], or any other certificate format. The
PKCS #6 extended certificate is obsolete. The PKCS #6 certificate is PKCS #6 extended certificate is obsolete. The PKCS #6 certificate is
included for backward compatibility, and PKCS #6 certificates SHOULD included for backward compatibility, and PKCS #6 certificates SHOULD
NOT be used. The ACv1 is also obsolete. ACv1 is included for NOT be used. The ACv1 is also obsolete. ACv1 is included for
backward compatibility, and ACv1 SHOULD NOT be used. The Internet backward compatibility, and ACv1 SHOULD NOT be used. The Internet
profile of X.509 certificates is specified in the "Internet X.509 profile of X.509 certificates is specified in the "Internet X.509
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certificate Certificate, certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
v1AttrCert [1] IMPLICIT AttributeCertificateV1, -- Obsolete v1AttrCert [1] IMPLICIT AttributeCertificateV1, -- Obsolete
v2AttrCert [2] IMPLICIT AttributeCertificateV2, v2AttrCert [2] IMPLICIT AttributeCertificateV2,
other [3] IMPLICIT OtherCertificateFormat } other [3] IMPLICIT OtherCertificateFormat }
OtherCertificateFormat ::= SEQUENCE { OtherCertificateFormat ::= SEQUENCE {
otherCertFormat OBJECT IDENTIFIER, otherCertFormat OBJECT IDENTIFIER,
otherCert ANY DEFINED BY otherCertFormat } otherCert ANY DEFINED BY otherCertFormat }
10.2.3 CertificateSet 10.2.3. CertificateSet
The CertificateSet type provides a set of certificates. It is The CertificateSet type provides a set of certificates. It is
intended that the set be sufficient to contain certification paths intended that the set be sufficient to contain certification paths
from a recognized "root" or "top-level certification authority" to from a recognized "root" or "top-level certification authority" to
all of the sender certificates with which the set is associated. all of the sender certificates with which the set is associated.
However, there may be more certificates than necessary, or there MAY However, there may be more certificates than necessary, or there MAY
be fewer than necessary. be fewer than necessary.
The precise meaning of a "certification path" is outside the scope of The precise meaning of a "certification path" is outside the scope of
this document. However, [PROFILE] provides a definition for X.509 this document. However, [PROFILE] provides a definition for X.509
certificates. Some applications may impose upper limits on the certificates. Some applications may impose upper limits on the
length of a certification path; others may enforce certain length of a certification path; others may enforce certain
relationships between the subjects and issuers of certificates within relationships between the subjects and issuers of certificates within
a certification path. a certification path.
CertificateSet ::= SET OF CertificateChoices CertificateSet ::= SET OF CertificateChoices
10.2.4 IssuerAndSerialNumber 10.2.4. IssuerAndSerialNumber
The IssuerAndSerialNumber type identifies a certificate, and thereby The IssuerAndSerialNumber type identifies a certificate, and thereby
an entity and a public key, by the distinguished name of the an entity and a public key, by the distinguished name of the
certificate issuer and an issuer-specific certificate serial number. certificate issuer and an issuer-specific certificate serial number.
The definition of Name is taken from X.501 [X.501-88], and the The definition of Name is taken from X.501 [X.501-88], and the
definition of CertificateSerialNumber is taken from X.509 [X.509-97]. definition of CertificateSerialNumber is taken from X.509 [X.509-97].
IssuerAndSerialNumber ::= SEQUENCE { IssuerAndSerialNumber ::= SEQUENCE {
issuer Name, issuer Name,
serialNumber CertificateSerialNumber } serialNumber CertificateSerialNumber }
CertificateSerialNumber ::= INTEGER CertificateSerialNumber ::= INTEGER
10.2.5 CMSVersion 10.2.5. CMSVersion
The CMSVersion type gives a syntax version number, for compatibility The CMSVersion type gives a syntax version number, for compatibility
with future revisions of this specification. with future revisions of this specification.
CMSVersion ::= INTEGER CMSVersion ::= INTEGER
{ v0(0), v1(1), v2(2), v3(3), v4(4), v5(5) } { v0(0), v1(1), v2(2), v3(3), v4(4), v5(5) }
10.2.6 UserKeyingMaterial 10.2.6. UserKeyingMaterial
The UserKeyingMaterial type gives a syntax for user keying material The UserKeyingMaterial type gives a syntax for user keying material
(UKM). Some key agreement algorithms require UKMs to ensure that a (UKM). Some key agreement algorithms require UKMs to ensure that a
different key is generated each time the same two parties generate a different key is generated each time the same two parties generate a
pairwise key. The sender provides a UKM for use with a specific key pairwise key. The sender provides a UKM for use with a specific key
agreement algorithm. agreement algorithm.
UserKeyingMaterial ::= OCTET STRING UserKeyingMaterial ::= OCTET STRING
10.2.7 OtherKeyAttribute 10.2.7. OtherKeyAttribute
The OtherKeyAttribute type gives a syntax for the inclusion of other The OtherKeyAttribute type gives a syntax for the inclusion of other
key attributes that permit the recipient to select the key used by key attributes that permit the recipient to select the key used by
the sender. The attribute object identifier must be registered along the sender. The attribute object identifier must be registered along
with the syntax of the attribute itself. Use of this structure with the syntax of the attribute itself. Use of this structure
should be avoided since it might impede interoperability. should be avoided since it might impede interoperability.
OtherKeyAttribute ::= SEQUENCE { OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER, keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL } keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
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Some of the attributes defined in this section were originally Some of the attributes defined in this section were originally
defined in PKCS #9 [PKCS#9]; others were originally defined in a defined in PKCS #9 [PKCS#9]; others were originally defined in a
previous version of this specification [CMS1]. The attributes are previous version of this specification [CMS1]. The attributes are
not listed in any particular order. not listed in any particular order.
Additional attributes are defined in many places, notably the S/MIME Additional attributes are defined in many places, notably the S/MIME
Version 3 Message Specification [MSG] and the Enhanced Security Version 3 Message Specification [MSG] and the Enhanced Security
Services for S/MIME [ESS], which also include recommendations on the Services for S/MIME [ESS], which also include recommendations on the
placement of these attributes. placement of these attributes.
11.1 Content Type 11.1. Content Type
The content-type attribute type specifies the content type of the The content-type attribute type specifies the content type of the
ContentInfo within signed-data or authenticated-data. The content- ContentInfo within signed-data or authenticated-data. The content-
type attribute type MUST be present whenever signed attributes are type attribute type MUST be present whenever signed attributes are
present in signed-data or authenticated attributes present in present in signed-data or authenticated attributes present in
authenticated-data. The content-type attribute value MUST match the authenticated-data. The content-type attribute value MUST match the
encapContentInfo eContentType value in the signed-data or encapContentInfo eContentType value in the signed-data or
authenticated-data. authenticated-data.
The content-type attribute MUST be a signed attribute or an The content-type attribute MUST be a signed attribute or an
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Even though the syntax is defined as a SET OF AttributeValue, a Even though the syntax is defined as a SET OF AttributeValue, a
content-type attribute MUST have a single attribute value; zero or content-type attribute MUST have a single attribute value; zero or
multiple instances of AttributeValue are not permitted. multiple instances of AttributeValue are not permitted.
The SignedAttributes and AuthAttributes syntaxes are each defined as The SignedAttributes and AuthAttributes syntaxes are each defined as
a SET OF Attributes. The SignedAttributes in a signerInfo MUST NOT a SET OF Attributes. The SignedAttributes in a signerInfo MUST NOT
include multiple instances of the content-type attribute. Similarly, include multiple instances of the content-type attribute. Similarly,
the AuthAttributes in an AuthenticatedData MUST NOT include multiple the AuthAttributes in an AuthenticatedData MUST NOT include multiple
instances of the content-type attribute. instances of the content-type attribute.
11.2 Message Digest 11.2. Message Digest
The message-digest attribute type specifies the message digest of the The message-digest attribute type specifies the message digest of the
encapContentInfo eContent OCTET STRING being signed in signed-data encapContentInfo eContent OCTET STRING being signed in signed-data
(see section 5.4) or authenticated in authenticated-data (see section (see section 5.4) or authenticated in authenticated-data (see section
9.2). For signed-data, the message digest is computed using the 9.2). For signed-data, the message digest is computed using the
signer's message digest algorithm. For authenticated-data, the signer's message digest algorithm. For authenticated-data, the
message digest is computed using the originator's message digest message digest is computed using the originator's message digest
algorithm. algorithm.
Within signed-data, the message-digest signed attribute type MUST be Within signed-data, the message-digest signed attribute type MUST be
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A message-digest attribute MUST have a single attribute value, even A message-digest attribute MUST have a single attribute value, even
though the syntax is defined as a SET OF AttributeValue. There MUST though the syntax is defined as a SET OF AttributeValue. There MUST
NOT be zero or multiple instances of AttributeValue present. NOT be zero or multiple instances of AttributeValue present.
The SignedAttributes syntax and AuthAttributes syntax are each The SignedAttributes syntax and AuthAttributes syntax are each
defined as a SET OF Attributes. The SignedAttributes in a signerInfo defined as a SET OF Attributes. The SignedAttributes in a signerInfo
MUST include only one instance of the message-digest attribute. MUST include only one instance of the message-digest attribute.
Similarly, the AuthAttributes in an AuthenticatedData MUST include Similarly, the AuthAttributes in an AuthenticatedData MUST include
only one instance of the message-digest attribute. only one instance of the message-digest attribute.
11.3 Signing Time 11.3. Signing Time
The signing-time attribute type specifies the time at which the The signing-time attribute type specifies the time at which the
signer (purportedly) performed the signing process. The signing-time signer (purportedly) performed the signing process. The signing-time
attribute type is intended for use in signed-data. attribute type is intended for use in signed-data.
The signing-time attribute MUST be a signed attribute or an The signing-time attribute MUST be a signed attribute or an
authenticated attribute; it MUST NOT be an unsigned attribute, authenticated attribute; it MUST NOT be an unsigned attribute,
unauthenticated attribute, or unprotected attribute. unauthenticated attribute, or unprotected attribute.
The following object identifier identifies the signing-time The following object identifier identifies the signing-time
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defined as a SET OF Attributes. The SignedAttributes in a signerInfo defined as a SET OF Attributes. The SignedAttributes in a signerInfo
MUST NOT include multiple instances of the signing-time attribute. MUST NOT include multiple instances of the signing-time attribute.
Similarly, the AuthAttributes in an AuthenticatedData MUST NOT Similarly, the AuthAttributes in an AuthenticatedData MUST NOT
include multiple instances of the signing-time attribute. include multiple instances of the signing-time attribute.
No requirement is imposed concerning the correctness of the signing No requirement is imposed concerning the correctness of the signing
time, and acceptance of a purported signing time is a matter of a time, and acceptance of a purported signing time is a matter of a
recipient's discretion. It is expected, however, that some signers, recipient's discretion. It is expected, however, that some signers,
such as time-stamp servers, will be trusted implicitly. such as time-stamp servers, will be trusted implicitly.
11.4 Countersignature 11.4. Countersignature
The countersignature attribute type specifies one or more signatures The countersignature attribute type specifies one or more signatures
on the contents octets of the signature OCTET STRING in a SignerInfo on the contents octets of the signature OCTET STRING in a SignerInfo
value of the signed-data. That is, the message digest is computed value of the signed-data. That is, the message digest is computed
over the octets comprising the value of the OCTET STRING, neither the over the octets comprising the value of the OCTET STRING, neither the
tag nor length octets are included. Thus, the countersignature tag nor length octets are included. Thus, the countersignature
attribute type countersigns (signs in serial) another signature. attribute type countersigns (signs in serial) another signature.
The countersignature attribute MUST be an unsigned attribute; it MUST The countersignature attribute MUST be an unsigned attribute; it MUST
NOT be a signed attribute, an authenticated attribute, an NOT be a signed attribute, an authenticated attribute, an
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A countersignature, since it has type SignerInfo, can itself contain A countersignature, since it has type SignerInfo, can itself contain
a countersignature attribute. Thus, it is possible to construct an a countersignature attribute. Thus, it is possible to construct an
arbitrarily long series of countersignatures. arbitrarily long series of countersignatures.
12. ASN.1 Modules 12. ASN.1 Modules
Section 12.1 contains the ASN.1 module for the CMS, and section 12.2 Section 12.1 contains the ASN.1 module for the CMS, and section 12.2
contains the ASN.1 module for the Version 1 Attribute Certificate. contains the ASN.1 module for the Version 1 Attribute Certificate.
12.1 CMS ASN.1 Module 12.1. CMS ASN.1 Module
CryptographicMessageSyntax2004 CryptographicMessageSyntax2004
{ iso(1) member-body(2) us(840) rsadsi(113549) { iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) } pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) }
DEFINITIONS IMPLICIT TAGS ::= DEFINITIONS IMPLICIT TAGS ::=
BEGIN BEGIN
-- EXPORTS All -- EXPORTS All
-- The types and values defined in this module are exported for use -- The types and values defined in this module are exported for use
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-- Obsolete Extended Certificate syntax from PKCS#6 -- Obsolete Extended Certificate syntax from PKCS#6
ExtendedCertificateOrCertificate ::= CHOICE { ExtendedCertificateOrCertificate ::= CHOICE {
certificate Certificate, certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate } extendedCertificate [0] IMPLICIT ExtendedCertificate }
ExtendedCertificate ::= SEQUENCE { ExtendedCertificate ::= SEQUENCE {
extendedCertificateInfo ExtendedCertificateInfo, extendedCertificateInfo ExtendedCertificateInfo,
signatureAlgorithm SignatureAlgorithmIdentifier, signatureAlgorithm SignatureAlgorithmIdentifier,
signature Signature } signature Signature }
ExtendedCertificateInfo ::= SEQUENCE { ExtendedCertificateInfo ::= SEQUENCE {
version CMSVersion, version CMSVersion,
certificate Certificate, certificate Certificate,
attributes UnauthAttributes } attributes UnauthAttributes }
Signature ::= BIT STRING Signature ::= BIT STRING
END -- of CryptographicMessageSyntax2004 END -- of CryptographicMessageSyntax2004
12.2 Version 1 Attribute Certificate ASN.1 Module 12.2. Version 1 Attribute Certificate ASN.1 Module
AttributeCertificateVersion1 AttributeCertificateVersion1
{ iso(1) member-body(2) us(840) rsadsi(113549) { iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) v1AttrCert(15) } pkcs(1) pkcs-9(9) smime(16) modules(0) v1AttrCert(15) }
DEFINITIONS EXPLICIT TAGS ::= DEFINITIONS EXPLICIT TAGS ::=
BEGIN BEGIN
-- EXPORTS All -- EXPORTS All
skipping to change at page 51, line 38 skipping to change at page 51, line 29
serialNumber CertificateSerialNumber, serialNumber CertificateSerialNumber,
attCertValidityPeriod AttCertValidityPeriod, attCertValidityPeriod AttCertValidityPeriod,
attributes SEQUENCE OF Attribute, attributes SEQUENCE OF Attribute,
issuerUniqueID UniqueIdentifier OPTIONAL, issuerUniqueID UniqueIdentifier OPTIONAL,
extensions Extensions OPTIONAL } extensions Extensions OPTIONAL }
AttCertVersionV1 ::= INTEGER { v1(0) } AttCertVersionV1 ::= INTEGER { v1(0) }
END -- of AttributeCertificateVersion1 END -- of AttributeCertificateVersion1
13. Normative References 13. References
13.1. Normative References
[ACPROFILE] Farrell, S. and R. Housley, "An Internet Attribute [ACPROFILE] Farrell, S. and R. Housley, "An Internet Attribute
Certificate Profile for Authorization", RFC 3281, Certificate Profile for Authorization", RFC 3281, April
April 2002. 2002.
[PROFILE] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet [PROFILE] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure: Certificate and CRL X.509 Public Key Infrastructure Certificate and
Profile", RFC 3280, April 2002. Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[STDWORDS] Bradner, S., "Key Words for Use in RFCs to Indicate [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[X.208-88] CCITT. Recommendation X.208: Specification of Abstract [X.208-88] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988. Syntax Notation One (ASN.1). 1988.
[X.209-88] CCITT. Recommendation X.209: Specification of Basic [X.209-88] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One (ASN.1). Encoding Rules for Abstract Syntax Notation One (ASN.1).
1988. 1988.
[X.501-88] CCITT. Recommendation X.501: The Directory - Models. [X.501-88] CCITT. Recommendation X.501: The Directory - Models.
skipping to change at page 52, line 24 skipping to change at page 52, line 17
[X.509-88] CCITT. Recommendation X.509: The Directory - [X.509-88] CCITT. Recommendation X.509: The Directory -
Authentication Framework. 1988. Authentication Framework. 1988.
[X.509-97] ITU-T. Recommendation X.509: The Directory - [X.509-97] ITU-T. Recommendation X.509: The Directory -
Authentication Framework. 1997. Authentication Framework. 1997.
[X.509-00] ITU-T. Recommendation X.509: The Directory - [X.509-00] ITU-T. Recommendation X.509: The Directory -
Authentication Framework. 2000. Authentication Framework. 2000.
14. Informative References 13.2. Informative References
[CMS1] Housley, R., "Cryptographic Message Syntax", [CMS1] Housley, R., "Cryptographic Message Syntax", RFC 2630,
RFC 2630, June 1999. June 1999.
[CMS2] Housley, R., "Cryptographic Message Syntax", [CMS2] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
RFC 3369, August 2002. 3369, August 2002.
[CMSALG] Housley, R., "Cryptographic Message Syntax (CMS) [CMSALG] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002. Algorithms", RFC 3370, August 2002.
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME", [ESS] Hoffman, P., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999. RFC 2634, June 1999.
[MSAC] Microsoft Development Network (MSDN) Library, [MSAC] Microsoft Development Network (MSDN) Library,
"Authenticode", April 2004 Release. "Authenticode", April 2004 Release.
[MSG] Ramsdell, B., "S/MIME Version 3 Message Specification", [MSG] Ramsdell, B., "S/MIME Version 3.1 Message
RFC 2633, June 1999. Specification", RFC 3851, July 2004.
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and [OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S. and C.
C. Adams, "X.509 Internet Public Key Infrastructure Adams, "X.509 Internet Public Key Infrastructure Online
Online Certificate Status Protocol - OCSP", RFC 2560, Certificate Status Protocol - OCSP", RFC 2560, June
June 1999. 1999.
[OLDMSG] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and [OLDMSG] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
L. Repka, "S/MIME Version 2 Message Specification", L. Repka, "S/MIME Version 2 Message Specification", RFC
RFC 2311, March 1998. 2311, March 1998.
[PKCS#6] RSA Laboratories. PKCS #6: Extended-Certificate Syntax [PKCS#6] RSA Laboratories. PKCS #6: Extended-Certificate Syntax
Standard, Version 1.5. November 1993. Standard, Version 1.5. November 1993.
[PKCS#7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax, [PKCS#7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5.", RFC 2315, March 1998. Version 1.5", RFC 2315, March 1998.
[PKCS#9] RSA Laboratories. PKCS #9: Selected Attribute Types, [PKCS#9] RSA Laboratories. PKCS #9: Selected Attribute Types,
Version 1.1. November 1993. Version 1.1. November 1993.
[PWRI] Gutmann, P., "Password-based Encryption for S/MIME", [PWRI] Gutmann, P., "Password-based Encryption for CMS", RFC
RFC 3211, December 2001. 3211, December 2001.
[RANDOM] Eastlake, D., Crocker, S. and J. Schiller, "Randomness [RANDOM] Eastlake 3rd, D., Crocker, S., and J. Schiller,
Recommendations for Security", RFC 1750, December 1994. "Randomness Recommendations for Security", RFC 1750,
December 1994.
15. Security Considerations 14. Security Considerations
The Cryptographic Message Syntax provides a method for digitally The Cryptographic Message Syntax provides a method for digitally
signing data, digesting data, encrypting data, and authenticating signing data, digesting data, encrypting data, and authenticating
data. data.
Implementations must protect the signer's private key. Compromise of Implementations must protect the signer's private key. Compromise of
the signer's private key permits masquerade. the signer's private key permits masquerade.
Implementations must protect the key management private key, the key- Implementations must protect the key management private key, the
encryption key, and the content-encryption key. Compromise of the key-encryption key, and the content-encryption key. Compromise of
key management private key or the key-encryption key may result in the key management private key or the key-encryption key may result
the disclosure of all contents protected with that key. Similarly, in the disclosure of all contents protected with that key.
compromise of the content-encryption key may result in disclosure of Similarly, compromise of the content-encryption key may result in
the associated encrypted content. disclosure of the associated encrypted content.
Implementations must protect the key management private key and the Implementations must protect the key management private key and the
message-authentication key. Compromise of the key management private message-authentication key. Compromise of the key management private
key permits masquerade of authenticated data. Similarly, compromise key permits masquerade of authenticated data. Similarly, compromise
of the message-authentication key may result in undetectable of the message-authentication key may result in undetectable
modification of the authenticated content. modification of the authenticated content.
The key management technique employed to distribute message- The key management technique employed to distribute message-
authentication keys must itself provide data origin authentication, authentication keys must itself provide data origin authentication,
otherwise the contents are delivered with integrity from an unknown otherwise the contents are delivered with integrity from an unknown
source. Neither RSA [PKCS#1, NEWPKCS#1] nor Ephemeral-Static Diffie- source. Neither RSA [PKCS#1, NEWPKCS#1] nor Ephemeral-Static
Hellman [DH-X9.42] provide the necessary data origin authentication. Diffie-Hellman [DH-X9.42] provide the necessary data origin
Static-Static Diffie-Hellman [DH-X9.42] does provide the necessary authentication. Static-Static Diffie-Hellman [DH-X9.42] does provide
data origin authentication when both the originator and recipient the necessary data origin authentication when both the originator and
public keys are bound to appropriate identities in X.509 recipient public keys are bound to appropriate identities in X.509
certificates. certificates.
When more than two parties share the same message-authentication key, When more than two parties share the same message-authentication key,
data origin authentication is not provided. Any party that knows the data origin authentication is not provided. Any party that knows the
message-authentication key can compute a valid MAC, therefore the message-authentication key can compute a valid MAC, therefore the
contents could originate from any one of the parties. contents could originate from any one of the parties.
Implementations must randomly generate content-encryption keys, Implementations must randomly generate content-encryption keys,
message-authentication keys, initialization vectors (IVs), and message-authentication keys, initialization vectors (IVs), and
padding. Also, the generation of public/private key pairs relies on padding. Also, the generation of public/private key pairs relies on
skipping to change at page 55, line 7 skipping to change at page 55, line 5
countersigning process need not know the original signed content. countersigning process need not know the original signed content.
This structure permits implementation efficiency advantages; however, This structure permits implementation efficiency advantages; however,
this structure may also permit the countersigning of an inappropriate this structure may also permit the countersigning of an inappropriate
signature value. Therefore, implementations that perform signature value. Therefore, implementations that perform
countersignatures should either verify the original signature value countersignatures should either verify the original signature value
prior to countersigning it (this verification requires processing of prior to countersigning it (this verification requires processing of
the original content), or implementations should perform the original content), or implementations should perform
countersigning in a context that ensures that only appropriate countersigning in a context that ensures that only appropriate
signature values are countersigned. signature values are countersigned.
16. Acknowledgments 15. Acknowledgments
This document is the result of contributions from many professionals. This document is the result of contributions from many professionals.
I appreciate the hard work of all members of the IETF S/MIME Working I appreciate the hard work of all members of the IETF S/MIME Working
Group. I extend a special thanks to Rich Ankney, Simon Blake-Wilson, Group. I extend a special thanks to Rich Ankney, Simon Blake-Wilson,
Tim Dean, Steve Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman, Tim Dean, Steve Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman,
Stephen Henson, Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt Stephen Henson, Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt
Kaliski, John Linn, John Pawling, Blake Ramsdell, Francois Rousseau, Kaliski, John Linn, John Pawling, Blake Ramsdell, Francois Rousseau,
Jim Schaad, Dave Solo, Paul Timmel, and Sean Turner for their efforts Jim Schaad, Dave Solo, Paul Timmel, and Sean Turner for their efforts
and support. and support.
17. Authors' Address 16. Author's Address
Russell Housley Russell Housley
Vigil Security, LLC Vigil Security, LLC
918 Spring Knoll Drive 918 Spring Knoll Drive
Herndon, VA 20170 Herndon, VA 20170
USA USA
EMail: housley@vigilsec.com EMail: housley@vigilsec.com
18. Full Copyright Statement 17. Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved. Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and translations of it may be copied and furnished to This document and the information contained herein are provided on an
others, and derivative works that comment on or otherwise explain it "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
or assist in its implementation may be prepared, copied, published and OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
distributed, in whole or in part, without restriction of any kind, ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
provided that the above copyright notice and this paragraph are INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
included on all such copies and derivative works. However, this INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
document itself may not be modified in any way, such as by removing WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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The limited permissions granted above are perpetual and will not be Intellectual Property
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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