draft-ietf-smime-cms-rsaes-oaep-03.txt   draft-ietf-smime-cms-rsaes-oaep-04.txt 
S/MIME Working Group R. Housley S/MIME Working Group R. Housley
Internet Draft RSA Laboratories Internet Draft RSA Laboratories
expires in six months June 2002 expires in six months July 2002
Use of the RSAES-OAEP Key Transport Algorithm in CMS Use of the RSAES-OAEP Key Transport Algorithm in CMS
<draft-ietf-smime-cms-rsaes-oaep-04.txt>
<draft-ietf-smime-cms-rsaes-oaep-03.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working all provisions of Section 10 of RFC2026. Internet-Drafts are working
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract Abstract
This document describes the use of the RSAES-OAEP key transport This document describes the use of the RSAES-OAEP key transport
method of key management within the Cryptographic Message Syntax. method of key management within the Cryptographic Message Syntax
(CMS).
1 Introduction 1 Introduction
This draft is being discussed on the "ietf-smime" mailing list. To This draft is being discussed on the "ietf-smime" mailing list. To
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smime/>. smime/>.
PKCS #1 Version 1.5 [PKCS#1v1.5] specifies a widely deployed variant PKCS #1 Version 1.5 [PKCS#1v1.5] specifies a widely deployed variant
skipping to change at page 2, line 28 skipping to change at page 2, line 28
Version 1.5 key transport is used in conjunction with the Version 1.5 key transport is used in conjunction with the
Cryptographic Message Syntax (CMS) [CMS]. Cryptographic Message Syntax (CMS) [CMS].
When PKCS #1 Version 1.5 key transport is applied as an intermediate When PKCS #1 Version 1.5 key transport is applied as an intermediate
encryption layer within an interactive request-response encryption layer within an interactive request-response
communications environment, exploitation could be more feasible. communications environment, exploitation could be more feasible.
However, Secure Sockets Layer (SSL) [SSL] and Transport Layer However, Secure Sockets Layer (SSL) [SSL] and Transport Layer
Security (TLS) [TLS] protocol implementations could include Security (TLS) [TLS] protocol implementations could include
countermeasures that detect and prevent the Million Message Attack countermeasures that detect and prevent the Million Message Attack
and other chosen-ciphertext attacks. These countermeasures are and other chosen-ciphertext attacks. These countermeasures are
performed within the protocol level. In the interest of long-term performed within the protocol level.
security assurance, it is prudent to adopt an improved cryptographic
technique rather than embedding countermeasures within protocols.
An updated version of PKCS #1 has been published, PKCS #1 Version 2.0 In the interest of long-term security assurance, it is prudent to
[PKCS#1v2.0]. This new document supersedes RFC 2313. PKCS #1 adopt an improved cryptographic technique rather than embedding
Version 2.0 preserves support for the encryption padding format countermeasures within protocols. To this end, an updated version of
defined in PKCS #1 Version 1.5 [PKCS#1v1.5], and it also defines a PKCS #1 has been published. PKCS #1 Version 2.0 [PKCS#1v2.0]
new alternative. To resolve the adaptive chosen ciphertext supersedes RFC 2313. It preserves support for the PKCS #1 Version
vulnerability, the PKCS #1 Version 2.0 specifies and recommends use 1.5 encryption padding format, and it also defines a new one. To
of Optimal Asymmetric Encryption Padding (OAEP) for RSA key resolve the adaptive chosen ciphertext vulnerability, the PKCS #1
transport. Version 2.0 specifies and recommends use of Optimal Asymmetric
Encryption Padding (OAEP) for RSA key transport.
This document specifies the use of RSAES-OAEP key transport algorithm This document specifies the use of RSAES-OAEP key transport algorithm
in the CMS. The CMS can be used in either a store-and-forward or an in the CMS. The CMS can be used in either a store-and-forward or an
interactive request-response environment. interactive request-response environment.
The CMS supports variety of architectures for certificate-based key The CMS supports variety of architectures for certificate-based key
management, particularly the one defined by the PKIX working group management, particularly the one defined by the PKIX working group
[PROFILE]. PKCS #1 Version 1.5 and PKCS #1 Version 2.0 require the [PROFILE]. PKCS #1 Version 1.5 and PKCS #1 Version 2.0 require the
same RSA public key information. Thus, a certified RSA public key same RSA public key information. Thus, a certified RSA public key
may be used with either RSA key transport technique. may be used with either RSA key transport technique.
The CMS uses ASN.1 [X.208-88], the Basic Encoding Rules (BER) The CMS uses ASN.1 [X.208-88], the Basic Encoding Rules (BER)
[X.209-88], and the Distinguished Encoding Rules (DER) [X.509-88]. [X.209-88], and the Distinguished Encoding Rules (DER) [X.509-88].
Throughout this document, when the terms MUST, MUST NOT, SHOULD and Throughout this document, when the terms "MUST," "MUST NOT,"
MAY are used in capital letters, their use conforms to the "SHOULD," and "MAY" are used in capital letters, their use conforms
definitions in [STDWORDS]. These key word definitions help make the to the definitions in RFC 2119 [STDWORDS]. These key word
intent of standards documents as clear as possible. These key words definitions help make the intent of standards documents as clear as
are used in this document to help implementers achieve possible. These key words are used in this document to help
interoperability. implementers achieve interoperability.
2 Enveloped-data Conventions 2 Enveloped-data Conventions
The CMS enveloped-data content type consists of an encrypted content The CMS enveloped-data content type consists of an encrypted content
and wrapped content-encryption keys for one or more recipients. The and wrapped content-encryption keys for one or more recipients. The
RSAES-OAEP key transport algorithm is used to wrap the content- RSAES-OAEP key transport algorithm is used to wrap the content-
encryption key for one recipient. encryption key for one recipient.
Compliant software MUST meet the requirements for constructing an Compliant software MUST meet the requirements for constructing an
enveloped-data content type stated in [CMS] Section 6, "Enveloped- enveloped-data content type stated in [CMS] Section 6, "Enveloped-
data Content Type". data Content Type".
A content-encryption key MUST be randomly generated for each instance A content-encryption key MUST be randomly generated for each instance
of an enveloped-data content type. The content-encryption key is of an enveloped-data content type. The content-encryption key is
used to encipher the content. used to encipher the content.
2.1 EnvelopedData Fields 2.1 EnvelopedData Fields
The enveloped-data content type is ASN.1 encoded using the The enveloped-data content type is ASN.1 encoded using the
EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be
populated as follows: populated as described in this section when RSAES-OAEP key transport
is employed for one or more recipients.
The EnvelopedData version MUST be 0, 2, or 3. The EnvelopedData version MUST be 0, 2, or 3.
The EnvelopedData originatorInfo field is not used for the RSAES-OAEP The EnvelopedData originatorInfo field is not used for the RSAES-OAEP
key transport algorithm. However, this field MAY be present to key transport algorithm. However, this field MAY be present to
support recipients using other key management algorithms. support recipients using other key management algorithms.
The EnvelopedData recipientInfos CHOICE MUST be The EnvelopedData recipientInfos CHOICE MUST be
KeyTransRecipientInfo. See section 2.2 for further discussion of KeyTransRecipientInfo. See section 2.2 for further discussion of
KeyTransRecipientInfo. KeyTransRecipientInfo.
The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
field MUST be a symmetric encryption algorithm identifier. field MUST be a symmetric encryption algorithm identifier.
The EnvelopedData unprotectedAttrs MAY be present. The EnvelopedData unprotectedAttrs MAY be present.
2.2 KeyTransRecipientInfo Fields 2.2 KeyTransRecipientInfo Fields
The enveloped-data content type is ASN.1 encoded using the The fields of the KeyTransRecipientInfo syntax MUST be populated as
EnvelopedData syntax. The fields of the EnvelopedData syntax must be described in this section when RSAES-OAEP key transport is employed
populated as follows: for one or more recipients.
The KeyTransRecipientInfo version MUST be 0 or 2. If the The KeyTransRecipientInfo version MUST be 0 or 2. If the
RecipientIdentifier uses the issuerAndSerialNumber alternative, then RecipientIdentifier uses the issuerAndSerialNumber alternative, then
the version MUST be 0. If the RecipientIdentifier uses the the version MUST be 0. If the RecipientIdentifier uses the
subjectKeyIdentifier alternative, then the version MUST be 2. subjectKeyIdentifier alternative, then the version MUST be 2.
The KeyTransRecipientInfo RecipientIdentifier provides two The KeyTransRecipientInfo RecipientIdentifier provides two
alternatives for specifying the recipient's certificate, and thereby alternatives for specifying the recipient's certificate, and thereby
the recipient's public key. The recipient's certificate MUST contain the recipient's public key. The recipient's certificate MUST contain
a RSA public key. The content-encryption key is encrypted with the a RSA public key. The content-encryption key is encrypted with the
recipient's RSA public key. The issuerAndSerialNumber alternative recipient's RSA public key. The issuerAndSerialNumber alternative
identifies the recipient's certificate by the issuer's distinguished identifies the recipient's certificate by the issuer's distinguished
name and the certificate serial number; the subjectKeyIdentifier name and the certificate serial number; the subjectKeyIdentifier
identifies the recipient's certificate by the X.509 identifies the recipient's certificate by the X.509
subjectKeyIdentifier extension value. subjectKeyIdentifier extension value.
The KeyTransRecipientInfo keyEncryptionAlgorithm specifies that the The KeyTransRecipientInfo keyEncryptionAlgorithm specifies use of the
RSAES-OAEP algorithm, and its associated parameters, was used to RSAES-OAEP algorithm, and its associated parameters, to encrypt the
encrypt the content-encryption key for the recipient. The key- content-encryption key for the recipient. The key-encryption process
encryption process is described in [PKCS#1v2.0]. See section 3 of is described in [PKCS#1v2.0]. See section 3 of this document for the
this document for the algorithm identifier and the parameter syntax. algorithm identifier and the parameter syntax.
The KeyTransRecipientInfo encryptedKey is the result of encrypting The KeyTransRecipientInfo encryptedKey is the result of encrypting
the content-encryption key in the recipient's RSA public key using the content-encryption key in the recipient's RSA public key using
the RSAES-OAEP algorithm. When using a Triple-DES [3DES] content- the RSAES-OAEP algorithm. The RSA public key MUST be at least 1024
encryption key, implementations MUST adjust the parity bits to ensure bits in length. When using a Triple-DES [3DES] content-encryption
odd parity for each octet of each DES key comprising the Triple-DES key, implementations MUST adjust the parity bits to ensure odd parity
key prior to RSAES-OAEP encryption. for each octet of each DES key comprising the Triple-DES key prior to
RSAES-OAEP encryption.
3 RSAES-OAEP Algorithm Identifiers and Parameters 3 RSAES-OAEP Algorithm Identifiers and Parameters
The RSAES-OAEP key transport algorithm is the RSA encryption scheme The RSAES-OAEP key transport algorithm is the RSA encryption scheme
defined in RFC 2437 [PKCS#1v2.0], where the message to be encrypted defined in RFC 2437 [PKCS#1v2.0], where the message to be encrypted
is the content-encryption key. The algorithm identifier for RSAES- is the content-encryption key. The algorithm identifier for RSAES-
OAEP is: OAEP is:
id-RSAES-OAEP OBJECT IDENTIFIER ::= { iso(1) member-body(2) id-RSAES-OAEP OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 7 } us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 7 }
The AlgorithmIdentifier parameters field must be present, and the The AlgorithmIdentifier parameters field MUST be present, and the
parameters field must contain RSAES-OAEP-params. RSAES-OAEP-params parameters field MUST contain RSAES-OAEP-params. RSAES-OAEP-params
have the following syntax: has the following syntax:
RSAES-OAEP-params ::= SEQUENCE { RSAES-OAEP-params ::= SEQUENCE {
hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier, hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier,
maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier, maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier,
pSourceFunc [2] AlgorithmIdentifier DEFAULT pSourceFunc [2] AlgorithmIdentifier DEFAULT
pSpecifiedEmptyIdentifier } pSpecifiedEmptyIdentifier }
sha1Identifier AlgorithmIdentifier ::= { id-sha1, NULL } sha1Identifier AlgorithmIdentifier ::= { id-sha1, NULL }
mgf1SHA1Identifier AlgorithmIdentifier ::= mgf1SHA1Identifier AlgorithmIdentifier ::=
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identified-organization(3) oiw(14) identified-organization(3) oiw(14)
secsig(3) algorithms(2) 26 } secsig(3) algorithms(2) 26 }
pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2) pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) } us(840) rsadsi(113549) pkcs(1) pkcs-1(1) }
id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 } id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 }
id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 } id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 }
The fields of type RSAES-OAEP-params have the following meanings: The fields within RSAES-OAEP-params have the following meanings:
hashFunc identifies the one-way hash function. Implementations hashFunc identifies the one-way hash function. Implementations
MUST support SHA-1 [SHA1]. The SHA-1 algorithm identifier is MUST support SHA-1 [SHA1], and implementations MAY support other
comprised of the id-sha1 object identifier and a parameter of one-way hash functions in addition to SHA-1. The SHA-1 algorithm
NULL. Implementations that perform encryption MUST omit the identifier is comprised of the id-sha1 object identifier and a
hashFunc field when SHA-1 is used, indicating that the default parameter of NULL. Implementations that perform encryption MUST
algorithm was used. Implementations that perform decryption MUST omit the hashFunc field when SHA-1 is used, indicating that the
recognize both the id-sha1 object identifier and an absent default algorithm was used. Implementations that perform
hashFunc field as an indication that SHA-1 was used. decryption MUST recognize both the id-sha1 object identifier and
an absent hashFunc field as an indication that SHA-1 was used.
maskGenFunc identifies the mask generation function. maskGenFunc identifies the mask generation function.
Implementations MUST support MFG1 [PKCS#1v2.0]. MFG1 requires a Implementations MUST support MFG1 [PKCS#1v2.0]. MFG1 requires a
one-way hash function, and it is identified in the parameter field one-way hash function, and it is identified in the parameter field
of the MFG1 algorithm identifier. Implementations MUST support of the MFG1 algorithm identifier. Implementations MUST support
SHA-1 [SHA1]. The MFG1 algorithm identifier is comprised of the SHA-1 [SHA1], and implementations MAY support other one-way hash
id-mgf1 object identifier and a parameter that contains the functions. The MFG1 algorithm identifier is comprised of the id-
algorithm identifier of the one-way hash function employed with mgf1 object identifier and a parameter that contains the algorithm
MFG1. The SHA-1 algorithm identifier is comprised of the id-sha1 identifier of the one-way hash function employed with MFG1. The
object identifier and a parameter of NULL. Implementations that SHA-1 algorithm identifier is comprised of the id-sha1 object
perform encryption MUST omit the maskGenFunc field when MFG1 with identifier and a parameter of NULL. Implementations that perform
SHA-1 is used, indicating that the default algorithm was used. encryption MUST omit the maskGenFunc field when MFG1 with SHA-1 is
used, indicating that the default algorithm was used.
Implementations that perform decryption MUST recognize both the Implementations that perform decryption MUST recognize both the
id-mgf1 and id-sha1 object identifiers as well as an absent id-mgf1 and id-sha1 object identifiers as well as an absent
maskGenFunc field as an indication that MFG1 with SHA-1 was used. maskGenFunc field as an indication that MFG1 with SHA-1 was used.
pSourceFunc identifies the source (and possibly the value) of the pSourceFunc identifies the source (and possibly the value) of the
encoding parameters, commonly called P. Implementations MUST encoding parameters, commonly called P. Implementations MUST
represent P by an algorithm identifier, id-pSpecified, indicating represent P by an algorithm identifier, id-pSpecified, indicating
that P is explicitly provided as an OCTET STRING in the that P is explicitly provided as an OCTET STRING in the
parameters. The default value for P is an empty string. In this parameters. The default value for P is an empty string. In this
case, pHash in EME-OAEP contains the hash of a zero length string. case, pHash in EME-OAEP contains the hash of a zero length string.
skipping to change at page 7, line 29 skipping to change at page 7, line 29
PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0. PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0.
RFC 2437. October 1998. RFC 2437. October 1998.
PROFILE Housley, R., W. Polk, W. Ford, and D. Solo. Internet PROFILE Housley, R., W. Polk, W. Ford, and D. Solo. Internet
X.509 Public Key Infrastructure: Certificate and X.509 Public Key Infrastructure: Certificate and
Certificate Revocation List (CRL) Profile. RFC 3280. Certificate Revocation List (CRL) Profile. RFC 3280.
April 2002. April 2002.
SHA1 National Institute of Standards and Technology. SHA1 National Institute of Standards and Technology.
FIPS Pub 180-1: Secure Hash Standard. 17 April 1995. FIPS Pub 180-1: "Secure Hash Standard." April 1995.
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 Encoding Rules for Abstract Syntax Notation One
(ASN.1). 1988. (ASN.1). 1988.
X.509-88 CCITT. Recommendation X.509: The Directory - X.509-88 CCITT. Recommendation X.509: The Directory -
Authentication Framework. 1988. Authentication Framework. 1988.
5.2 Informative References 5.2 Informative References
CRYPTO98 Bleichenbacher, D. "Chosen Ciphertext Attacks Against CRYPTO98 Bleichenbacher, D. "Chosen Ciphertext Attacks Against
Protocols Based on the RSA Encryption Standard PKCS #1," Protocols Based on the RSA Encryption Standard PKCS #1,"
in H. Krawczyk (editor), Advances in Cryptology - CRYPTO '98 in H. Krawczyk (editor), Advances in Cryptology -
Proceedings, Lecture Notes in Computer Science 1462 (1998), CRYPTO '98 Proceedings, Lecture Notes in Computer
Springer-Verlag, pp. 1-12. Science 1462 (1998), Springer-Verlag, pp. 1-12.
MMA Rescorla, E. Preventing the Million Message Attack on MMA Rescorla, E. Preventing the Million Message Attack on
Cryptographic Message Syntax. RFC 3218. January 2002. Cryptographic Message Syntax. RFC 3218. January 2002.
NISTGUIDE National Institute of Standards and Technology.
Second Draft: "Key Management Guideline, Part 1:
General Guidance." June 2002.
[http://csrc.nist.gov/encryption/kms/guideline-1.pdf]
PKCS#1v1.5 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5. PKCS#1v1.5 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5.
RFC 2313. March 1998. RFC 2313. March 1998.
RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness
Recommendations for Security. RFC 1750. December 1994. Recommendations for Security. RFC 1750. December 1994.
RSALABS Bleichenbacher, D., B. Kaliski, and J. Staddon. RSALABS Bleichenbacher, D., B. Kaliski, and J. Staddon.
Recent Results on PKCS #1: RSA Encryption Standard. Recent Results on PKCS #1: RSA Encryption Standard.
RSA Laboratories' Bulletin No. 7, June 26, 1998. RSA Laboratories' Bulletin No. 7, June 26, 1998.
[http://www.rsasecurity.com/rsalabs/bulletins] [http://www.rsasecurity.com/rsalabs/bulletins]
SHA2 National Institute of Standards and Technology.
Draft FIPS Pub 180-2: "Specifications for the Secure
Hash Standard." May 2001.
[http://csrc.nist.gov/encryption/shs/dfips-180-2.pdf]
SSL Freier, A., P. Karlton, and P. Kocher. The SSL Protocol, SSL Freier, A., P. Karlton, and P. Kocher. The SSL Protocol,
Version 3.0. Netscape Communications. November 1996. Version 3.0. Netscape Communications. November 1996.
[http://wp.netscape.com/eng/ssl3/draft302.txt] [http://wp.netscape.com/eng/ssl3/draft302.txt]
TLS Dierks, T. and C. Allen. The TLS Protocol Version 1.0. TLS Dierks, T. and C. Allen. The TLS Protocol Version 1.0.
RFC 2246. January 1999. RFC 2246. January 1999.
6 Security Considerations 6 Security Considerations
Implementations must protect the RSA private key and the content- Implementations must protect the RSA private key and the content-
encryption key. Compromise of the RSA private key may result in the encryption key. Compromise of the RSA private key may result in the
disclosure of all messages protected with that key. Compromise of disclosure of all messages protected with that key. Compromise of
the content-encryption key may result in disclosure of the associated the content-encryption key may result in disclosure of the associated
encrypted content. encrypted content.
Implementations must protect the key management private key and the
message-authentication key. Compromise of the key management private
key permits masquerade of authenticated data. Compromise of the
message-authentication key may result in undetectable modification of
the authenticated content.
The generation of RSA public/private key pairs relies on a random The generation of RSA public/private key pairs relies on a random
numbers. The use of inadequate pseudo-random number generators numbers. The use of inadequate pseudo-random number generators
(PRNGs) to generate cryptographic keys can result in little or no (PRNGs) to generate cryptographic keys can result in little or no
security. An attacker may find it much easier to reproduce the PRNG security. An attacker may find it much easier to reproduce the PRNG
environment that produced the keys, searching the resulting small set environment that produced the keys, searching the resulting small set
of possibilities, rather than brute force searching the whole key of possibilities, rather than brute force searching the whole key
space. The generation of quality random numbers is difficult. RFC space. The generation of quality random numbers is difficult. RFC
1750 [RANDOM] offers important guidance in this area. 1750 [RANDOM] offers important guidance in this area.
Generally, good cryptographic practice employs a given RSA key pair
in only one scheme. This practice avoids the risk that vulnerability
in one scheme may compromise the security of the other, and may be
essential to maintain provable security. While PKCS #1 Version 1.5
[PKCS#1v1.5] has been employed for both key transport and digital
signature without any known bad interactions, such a combined use of
an RSA key pair is not recommended in the future. Therefore, an RSA
key pair used for RSAES-OAEP key transport should not also be used
for other purposes.
This specification requires implementation to support the SHA-1 one-
way hash function for interoperability, but support for other one-way
hash function is permitted. At the time of this writing, the best
(known) collision attacks against SHA-1 are generic attacks with
complexity 2^80, where 80 is one-half the bit length of the hash
value. When a one-way hash function is used with a digital signature
scheme, a collision attack is easily translated into a signature
forgery. Therefore, the use of SHA-1 in a digital signature scheme
provides a security level of no more than 80 bits. If a greater
level of security is desired, then a secure one-way hash function
with a longer hash value is needed. SHA-256, SHA-384, and SHA-512
are likely candidates [SHA2].
The metrics for choosing a one-way hash function for use in digital
signatures do not directly apply to the RSAES-OAEP key transport
algorithm, since a collision attack on the one-way hash function does
not directly translate into an attack on the key transport algorithm,
unless the encoding parameters P varies (in which case a collision
the hash value for different encoding parameters might be exploited).
Nevertheless, for consistency with the practice for digital signature
schemes, and in case the encoding parameters P is not the empty
string, it is recommended that the same rule of thumb be applied to
selection of a one-way hash function for use with RSAES-OAEP. That
is, the one-way hash function should be selected so that the bit
length of the hash value is at least twice as long as the desired
security level in bits.
A 1024-bit RSA public key and SHA-1 both provide a security level of
about 80 bits. In [NISTGUIDE], the National Institute of Standards
and Technology suggests that a security level of 80 bits is adequate
for most applications until 2015. If a security level greater than
80 bits is needed, then a longer RSA public key and a secure one-way
hash function with a longer hash value are needed. Again, SHA-256,
SHA-384, and SHA-512 are likely candidates for such a one-way hash
function. For this reason, the algorithm identifiers for these one-
way hash functions are included in the ASN.1 module in Appendix A.
7 Acknowledgments 7 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. Further, I extend a special thanks to Burt Kaliski. Group. Further, I extend a special thanks to Burt Kaliski, Jakob
Jonsson, and Francois Rousseau.
8 Author Address 8 Author Address
Russell Housley Russell Housley
RSA Laboratories RSA Laboratories
918 Spring Knoll Drive 918 Spring Knoll Drive
Herndon, VA 20170 Herndon, VA 20170
USA USA
rhousley@rsasecurity.com rhousley@rsasecurity.com
Appendix A ASN.1 Module Appendix A ASN.1 Module
CMS-RSAES-OAEP {iso(1) member-body(2) us(840) rsadsi(113549) CMS-RSAES-OAEP
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-rsaes-oaep(20) } { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) smime(16) modules(0) cms-rsaes-oaep(20) }
DEFINITIONS IMPLICIT TAGS ::= DEFINITIONS IMPLICIT TAGS ::=
BEGIN BEGIN
-- EXPORTS ALL -- -- EXPORTS ALL --
IMPORTS IMPORTS
-- From PKIX Certificate and CRL Profile
AlgorithmIdentifier AlgorithmIdentifier
FROM PKIXExplicit88 { iso(1) identified-organization(3) FROM PKIX1Explicit88 -- RFC 3280
dod(6) internet(1) security(5) mechanisms(5) pkix(7) { iso(1) identified-organization(3) dod(6) internet(1)
id-mod(0) id-pkix1-explicit(18) }; security(5) mechanisms(5) pkix(7) id-mod(0)
id-pkix1-explicit(18) };
pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-1(1) } rsadsi(113549) pkcs(1) pkcs-1(1) }
id-RSAES-OAEP OBJECT IDENTIFIER ::= { pkcs-1 7 } id-RSAES-OAEP OBJECT IDENTIFIER ::= { pkcs-1 7 }
RSAES-OAEP-params ::= SEQUENCE { RSAES-OAEP-params ::= SEQUENCE {
hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier, hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier,
maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier, maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier,
pSourceFunc [2] AlgorithmIdentifier DEFAULT pSourceFunc [2] AlgorithmIdentifier DEFAULT
pSpecifiedEmptyIdentifier } pSpecifiedEmptyIdentifier }
sha1Identifier AlgorithmIdentifier ::= { id-sha1, NULL } sha1Identifier AlgorithmIdentifier ::= { id-sha1, NULL }
sha256Identifier AlgorithmIdentifier ::= { id-sha256, NULL }
sha384Identifier AlgorithmIdentifier ::= { id-sha384, NULL }
sha512Identifier AlgorithmIdentifier ::= { id-sha512, NULL }
mgf1SHA1Identifier AlgorithmIdentifier ::= mgf1SHA1Identifier AlgorithmIdentifier ::=
{ id-mgf1, sha1Identifier } { id-mgf1, sha1Identifier }
pSpecifiedEmptyIdentifier AlgorithmIdentifier ::= pSpecifiedEmptyIdentifier AlgorithmIdentifier ::=
{ id-pSpecified, nullOctetString } { id-pSpecified, nullOctetString }
nullOctetString OCTET STRING (SIZE (0)) ::= { ''H } nullOctetString OCTET STRING (SIZE (0)) ::= { ''H }
id-sha1 OBJECT IDENTIFIER ::= { iso(1) id-sha1 OBJECT IDENTIFIER ::= { iso(1)
identified-organization(3) oiw(14) identified-organization(3) oiw(14)
secsig(3) algorithms(2) 26 } secsig(3) algorithms(2) 26 }
id-sha256 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
country(16) us(840) organization(1) gov(101)
csor(3) nistalgorithm(4) hashalgs(2) 1 }
id-sha384 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
country(16) us(840) organization(1) gov(101)
csor(3) nistalgorithm(4) hashalgs(2) 2 }
id-sha512 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
country(16) us(840) organization(1) gov(101)
csor(3) nistalgorithm(4) hashalgs(2) 3 }
id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 } id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 }
id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 } id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 }
rSAES-OAEP-Default-Identifier AlgorithmIdentifier ::= rSAES-OAEP-Default-Identifier AlgorithmIdentifier ::=
{ id-RSAES-OAEP, { id-RSAES-OAEP,
{ sha1Identifier, { sha1Identifier,
mgf1SHA1Identifier, mgf1SHA1Identifier,
pSpecifiedEmptyIdentifier } } pSpecifiedEmptyIdentifier } }
 End of changes. 

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