S/MIME Working Group J. Randall
Internet Draft RSA
Document: draft-ietf-smime-cms-rsa-kem-06.txt B.Kaliski
Category: Standards EMC Corp.
Expires: March 2009 September 2008
Use of the RSA-KEM Key Transport Algorithm in CMS
Intellectual Property
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Copyright (C) The IETF Trust (2008).
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at:
http://www.ietf.org/1id-abstracts.html
Randall & Kaliski Standards - Expires: March 2009 [Page 1]
INTERNET DRAFT September 2008
The list of Internet-Draft Shadow Directories can be accessed at:
http://www.ietf.org/shadow.html
Comments or suggestions for improvement may be made on the "ietf-
smime" mailing list, or directly to the author.
Abstract
The RSA-KEM Key Transport Algorithm is a one-pass (store-and-forward)
mechanism for transporting keying data to a recipient using the
recipient's RSA public key. This document specifies the conventions
for using the RSA-KEM Key Transport Algorithm with the Cryptographic
Message Syntax (CMS). The ASN.1 syntax is aligned with ANS X9.44 and
ISO/IEC 18033-2.
Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119
[STDWORDS].
1. Introduction
The RSA-KEM Key Transport Algorithm is a one-pass (store-and-forward)
mechanism for transporting keying data to a recipient using the
recipient's RSA public key.
Most previous key transport algorithms based on the RSA public-key
cryptosystem (e.g., the popular PKCS #1 v1.5 algorithm [PKCS1]) have
the following general form:
1. Format or "pad" the keying data to obtain an integer m.
2. Encrypt the integer m with the recipient's RSA public key:
c = m^e mod n
3. Output c as the encrypted keying data.
The RSA-KEM Key Transport Algorithm takes a different approach that
provides higher security assurance, by encrypting a _random_ integer
with the recipient's public key, and using a symmetric key-wrapping
scheme to encrypt the keying data. It has the following form:
1. Generate a random integer z between 0 and n-1.
2. Encrypt the integer z with the recipient's RSA public key:
c = z^e mod n.
3. Derive a key-encrypting key KEK from the integer z.
Randall & Kaliski Standards - Expires: March 2009 [Page 2]
INTERNET DRAFT September 2008
4. Wrap the keying data using KEK to obtain wrapped keying data
WK.
5. Output c and WK as the encrypted keying data.
This different approach provides higher security assurance because
(a) the input to the underlying RSA operation is effectively a
random integer between 0 and n-1, where n is the RSA modulus, so it
does not have any structure that could be exploited by an adversary,
and (b) the input is independent of the keying data so the result of
the RSA decryption operation is not directly available to an
adversary. As a result, the algorithm enjoys a "tight" security
proof in the random oracle model. (In other padding schemes, such
as PKCS #1 v1.5, the input has structure and/or depends on the
keying data, and the provable security assurances are not as
strong.) The approach is also architecturally convenient because the
public-key operations are separate from the symmetric operations on
the keying data. One benefit is that the length of the keying data
is bounded only by the symmetric key-wrapping scheme, not the size
of the RSA modulus.
The RSA-KEM Key Transport Algorithm in various forms is being adopted
in several draft standards as well as in ANS-X9.44 and ISO/IEC
18033-2. It has also been recommended by the NESSIE project [NESSIE].
For completeness, a specification of the algorithm is given in
Appendix A of this document; ASN.1 syntax is given in Appendix B.
NOTE: The term KEM stands for "key encapsulation mechanism" and
refers to the first three steps of the process above. The
formalization of key transport algorithms (or more generally,
asymmetric encryption schemes) in terms of key encapsulation
mechanisms is described further in research by Victor Shoup leading
to the development of the ISO/IEC 18033-2 standard [SHOUP].
2. Use in CMS
The RSA-KEM Key Transport Algorithm MAY be employed for one or more
recipients in the CMS enveloped-data content type (Section 6 of
[CMS]), where the keying data processed by the algorithm is the CMS
content-encryption key.
The RSA-KEM Key Transport Algorithm SHOULD be considered for new
CMS-based applications as a replacement for the widely implemented
RSA encryption algorithm specified originally in PKCS #1 v1.5 (see
[PKCS1] and Section 4.2.1 of [CMSALGS]), which is vulnerable to
chosen-ciphertext attacks. The RSAES-OAEP Key Transport Algorithm
has also been proposed as a replacement (see [PKCS1] and [CMS-
OAEP]). RSA-KEM has the advantage over RSAES-OAEP of a tighter
security proof, but the disadvantage of slightly longer encrypted
keying data.
Randall & Kaliski Standards - Expires: March 2009 [Page 3]
INTERNET DRAFT September 2008
2.1 Underlying Components
A CMS implementation that supports the RSA-KEM Key Transport
Algorithm MUST support at least the following underlying components:
* For the key derivation function, KDF2 or KDF3 (see [ANS-X9.44]
[IEEE-P1363a]) based on SHA-1 (see [FIPS-180-2]) (this function
is also specified as the key derivation function in
[ANS-X9.63]).
* For the key-wrapping scheme, AES-Wrap-128, i.e., the AES Key
Wrap with a 128-bit key encrypting key (see [AES-WRAP])
An implementation SHOULD also support KDF2 and KDF3 based on SHA-256
(see [FIPS-180-2]). The Camillia key wrap algorithm (see [CAMILLIA])
should be supported, and, if 3DES is supported as a content-
encryption cipher, then the Triple-DES Key Wrap (see [3DES-WRAP])
SHOULD also be supported.
It MAY support other underlying components. When AES or Camilla are
used the data block size is 128 bits while the key size can be 128,
192, or 256 bits while Triple DES requires a data block size of 64
bits and a key size of 112 or 168 bits.
2.2 RecipientInfo Conventions
When the RSA-KEM Key Transport Algorithm is employed for a recipient,
the RecipientInfo alternative for that recipient MUST be
KeyTransRecipientInfo. The algorithm-specific fields of the
KeyTransRecipientInfo value MUST have the following values:
* keyEncryptionAlgorithm.algorithm MUST be id-ac-generic-hybrid
(see Appendix B)
* keyEncryptionAlgorithm.parameters MUST be a value of type
GenericHybridParameters, identifying the RSA-KEM key
encapsulation mechanism (see Appendix B)
* encryptedKey MUST be the encrypted keying data output by the
algorithm, where the keying data is the content-encryption key.
(see Appendix A)
2.3 Certificate Conventions
The conventions specified in this section augment RFC 3280 [PROFILE].
A recipient who employs the RSA-KEM Key Transport Algorithm MAY
identify the public key in a certificate by the same
AlgorithmIdentifier as for the PKCS #1 v1.5 algorithm, i.e., using
the rsaEncryption object identifier [PKCS1].
Randall & Kaliski Standards - Expires: March 2009 [Page 4]
INTERNET DRAFT September 2008
If the recipient wishes only to employ the RSA-KEM Key Transport
Algorithm with a given public key, the recipient MUST identify the
public key in the certificate using the id-ac-generic-hybrid object
identifier (see Appendix B) where the associated
GenericHybridParameters value indicates the underlying components
with which the algorithm is to be employed. The certificate user MUST
perform the RSA-KEM Key Transport algorithm using only those
components.
Regardless of the AlgorithmIdentifier used, the RSA public key is
encoded in the same manner in the subject public key information.
The RSA public key MUST be encoded using the type RSAPublicKey type:
RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER -- e
}
Here, the modulus is the modulus n, and publicExponent is the public
exponent e. The DER encoded RSAPublicKey is carried in the
subjectPublicKey BIT STRING within the subject public key
information.
The intended application for the key MAY be indicated in the key
usage certificate extension (see [PROFILE], Section 4.2.1.3). If the
keyUsage extension is present in a certificate that conveys an RSA
public key with the id-ac-generic-hybrid object identifier as
discussed above, then the key usage extension MUST contain the
following value:
keyEncipherment.
dataEncipherment SHOULD NOT be present. That is, a key intended to be
employed only with the RSA-KEM Key Transport Algorithm SHOULD NOT
also be employed for data encryption or for authentication such as in
signatures. 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.
2.4 SMIMECapabilities Attribute Conventions
RFC 2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed
attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be
used to specify a partial list of algorithms that the software
announcing the SMIMECapabilities can support. When constructing a
signedData object, compliant software MAY include the
SMIMECapabilities signed attribute announcing that it supports the
RSA-KEM Key Transport algorithm.
The SMIMECapability SEQUENCE representing the RSA-KEM Key Transport
Algorithm MUST include the id-ac-generic-hybrid object identifier
(see Appendix B) in the capabilityID field and MUST include a
Randall & Kaliski Standards - Expires: March 2009 [Page 5]
INTERNET DRAFT September 2008
GenericHybridParameters value in the parameters field identifying the
components with which the algorithm is to be employed.
The DER encoding of a SMIMECapability SEQUENCE is the same as the DER
encoding of an AlgorithmIdentifier. Example DER encodings for typical
sets of components are given in Appendix B.4.
3. Security Considerations
The security of the RSA-KEM Key Transport Algorithm described in
this document can be shown to be tightly related to the difficulty
of either solving the RSA problem or breaking the underlying
symmetric key-wrapping scheme, if the underlying key derivation
function is modeled as a random oracle, and assuming that the
symmetric key-wrapping scheme satisfies the properties of a data
encapsulation mechanism [SHOUP]. While in practice a random-oracle
result does not provide an actual security proof for any particular
key derivation function, the result does provide assurance that the
general construction is reasonable; a key derivation function would
need to be particularly weak to lead to an attack that is not
possible in the random oracle model.
The RSA key size and the underlying components should be selected
consistent with the desired symmetric security level for an
application. Several security levels have been identified in [NIST-
FIPS PUB 800-57]. For brevity, the first three levels are mentioned
here:
* 80-bit security. The RSA key size SHOULD be at least 1024 bits,
the hash function underlying the KDF SHOULD be SHA-1 or above,
and the symmetric key-wrapping scheme SHOULD be AES Key Wrap,
Triple-DES Key Wrap, or Camillia Key Wrap.
* 112-bit security. The RSA key size SHOULD be at least 2048
bits, the hash function underlying the KDF SHOULD be SHA-224 or
above, and the symmetric key-wrapping scheme SHOULD be AES Key
Wrap, Triple-DES Key Wrap, or Camillia Key Wrap.
* 128-bit security. The RSA key size SHOULD be at least 3072
bits, the hash function underlying the KDF SHOULD be SHA-256 or
above, and the symmetric key-wrapping scheme SHOULD be AES Key
Wrap or Camillia Key Wrap.
Note that the AES Key Wrap or Camillia Key Wrap MAY be used at all
three of these levels; the use of AES or Camillia does not require a
128-bit security level for other components.
Implementations MUST protect the RSA private key and the content-
encryption key. Compromise of the RSA private key may result in the
disclosure of all messages protected with that key. Compromise of the
content-encryption key may result in disclosure of the associated
encrypted content.
Randall & Kaliski Standards - Expires: March 2009 [Page 6]
INTERNET DRAFT September 2008
Additional considerations related to key management may be found in
[NIST-GUIDELINE].
The security of the algorithm also depends on the strength of the
random number generator, which SHOULD have a comparable security
level. For further discussion on random number generation, please
see [RANDOM].
Implementations SHOULD NOT reveal information about intermediate
values or calculations, whether by timing or other "side channels",
or otherwise an opponent may be able to determine information about
the keying data and/or the recipient's private key. Although not all
intermediate information may be useful to an opponent, it is
preferable to conceal as much information as is practical, unless
analysis specifically indicates that the information would not be
useful.
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 RSA public keys have
often been employed for multiple purposes such as key transport and
digital signature without any known bad interactions, for increased
security assurance, such combined use of an RSA key pair is NOT
RECOMMENDED in the future (unless the different schemes are
specifically designed to be used together).
Accordingly, an RSA key pair used for the RSA-KEM Key Transport
Algorithm SHOULD NOT also be used for digital signatures. (Indeed,
ASC X9 requires such a separation between key establishment key pairs
and digital signature key pairs.) Continuing this principle of key
separation, a key pair used for the RSA-KEM Key Transport Algorithm
SHOULD NOT be used with other key establishment schemes, or for data
encryption, or with more than one set of underlying algorithm
components.
Parties MAY formalize the assurance that one another's
implementations are correct through implementation validation, e.g.
NIST's Cryptographic Module Validation Program (CMVP).
4. References
4.1 Normative References
[3DES-WRAP] Housley, R. Triple-DES and RC2 Key Wrapping. RFC
3217. December 2001.
[AES-WRAP] Schaad, J. and R. Housley. Advanced Encryption
Standard (AES) Key Wrap Algorithm. RFC 3394.
September 2002.
[ANS-X9.63] American National Standard X9.63-2002: Public Key
Cryptography for the Financial Services Industry:
Randall & Kaliski Standards - Expires: March 2009 [Page 7]
INTERNET DRAFT September 2008
Key Agreement and Key Transport Using Elliptic
Curve Cryptography.
[CAMILLIA] Kato, A., Moriai, S., and Kanda, M.: The Camellia
Cipher Algorithm and Its Use With IPsec. RFC 3657.
December 2005.
[CMS] Housley, R. Cryptographic Message Syntax. RFC
3852. July 2004.
[CMSALGS] Housley, R. Cryptographic Message Syntax (CMS)
Algorithms. RFC 3370. August 2002.
[FIPS-180-2] National Institute of Standards and Technology
(NIST). FIPS 180-2: Secure Hash Standard. August
2002.
[MSG] Ramsdell, B. S/MIME Version 3 Message
Specification. RFC 3851. July 2004.
[PROFILE] Housley, R., Polk, W., Ford, W. and D. Solo.
Internet X.509 Public Key Infrastructure:
Certificate and Certificate Revocation List (CRL)
Profile. RFC 3280. April 2002.
[STDWORDS] Bradner, S. Key Words for Use in RFCs to Indicate
Requirement Levels. RFC 2119. March 1997.
4.2 Informative References
[ANS-X9.44] ASC X9F1 Working Group. American National
Standard X9.44: Public Key Cryptography for the
Financial Services Industry -- Key Establishment
Using Integer Factorization Cryptography. 2007
[CMS-OAEP] Housley, R. Use of the RSAES-OAEP Key Transport
Algorithm in the Cryptographic Message Syntax
(CMS). RFC 3560. July 2003.
[IEEE-P1363a] IEEE Std 1363a-2004: Standard Specifications for
Public Key Cryptography: Additional Techniques.
IEEE, 2004.
[ISO-IEC-18033-2] ISO/IEC 18033-2:2005 Information technology --
Security techniques -- Encryption algorithms --
Part 2: Asymmetric Ciphers. ISO/IEC, 2005.
[NESSIE] NESSIE Consortium. Portfolio of Recommended
Cryptographic Primitives. February 27, 2003.
Available via http://www.cryptonessie.org/.
[NIST-GUIDELINE] National Institute of Standards and Technology.
Special Publication 800-57: Recommendation for Key
Management. Part 1: General Guideline. August 2005.
Randall & Kaliski Standards - Expires: March 2009 [Page 8]
INTERNET DRAFT September 2008
Available via:
http://csrc.nist.gov/publications/index.html.
[PKCS1] Jonsson, J. and B. Kaliski. PKCS #1: RSA
Cryptography Specifications Version 2.1. RFC 3447.
February 2003.
[RANDOM] Eastlake, D., S. Crocker, and J. Schiller.
Randomness Recommendations for Security. RFC 4086.
June 2005.
[SHOUP] Shoup, V. A Proposal for an ISO Standard for
Public Key Encryption. Version 2.1, December 20,
2001. Available via http://www.shoup.net/papers/.
5. IANA Considerations
Within the CMS, algorithms are identified by object identifiers
(OIDs). With one exception, all of the OIDs used in this document
were assigned in other IETF documents, in ISO/IEC standards
documents, by the National Institute of Standards and Technology
(NIST), and in Public-Key Cryptography Standards (PKCS) documents.
The one exception is that the ASN.1 module's identifier (see Appendix
B.3) is assigned in this document. No further action by the IANA is
necessary for this document or any anticipated updates.
6. Acknowledgments
This document is one part of a strategy to align algorithm standards
produced by ASC X9, ISO/IEC JTC1 SC27, NIST, and the IETF. We would
like to thank the members of the ASC X9F1 working group for their
contributions to drafts of ANS X9.44 which led to this specification.
Our thanks to Russ Housley as well for his guidance and
encouragement. We also appreciate the helpful direction we've
received from Blake Ramsdell and Jim Schaad in bringing this document
to fruition. A special thanks to Magnus Nystrom for his assistance on
Appendix B.
7. Authors' Addresses
James Randall
RSA, The Security Division of EMC
174 Middlesex Turnpike
Bedford, MA 01730
USA
e-mail: jrandall@rsa.com
Burt Kaliski
EMC
176 South Street
Hopkinton, MA 01748
USA
e-mail: kaliski_burt@emc.com
Randall & Kaliski Standards - Expires: March 2009 [Page 9]
INTERNET DRAFT September 2008
Appendix A. RSA-KEM Key Transport Algorithm
The RSA-KEM Key Transport Algorithm is a one-pass (store-and-forward)
mechanism for transporting keying data to a recipient using the
recipient's RSA public key.
With this type of algorithm, a sender encrypts the keying data using
the recipient's public key to obtain encrypted keying data. The
recipient decrypts the encrypted keying data using the recipient's
private key to recover the keying data.
A.1 Underlying Components
The algorithm has the following underlying components:
* KDF, a key derivation function, which derives keying data of a
specified length from a shared secret value
* Wrap, a symmetric key-wrapping scheme, which encrypts keying
data using a key-encrypting key
In the following, kekLen denotes the length in bytes of the key-
encrypting key for the underlying symmetric key-wrapping scheme.
In this scheme, the length of the keying data to be transported MUST
be among the lengths supported by the underlying symmetric key-
wrapping scheme. (Both the AES and Camillia Key Wraps, for instance,
require the length of the keying data to be a multiple of 8 bytes,
and at least 16 bytes.) Usage and formatting of the keying data
(e.g., parity adjustment for Triple-DES keys) is outside the scope of
this algorithm. With some key derivation functions, it is possible to
include other information besides the shared secret value in the
input to the function. Also, with some symmetric key-wrapping
schemes, it is possible to associate a label with the keying data.
Such uses are outside the scope of this document, as they are not
directly supported by CMS.
A.2 Sender's Operations
Let (n,e) be the recipient's RSA public key (see [PKCS1] for details)
and let K be the keying data to be transported.
Let nLen denote the length in bytes of the modulus n, i.e., the least
integer such that 2^{8*nLen} > n.
The sender performs the following operations:
1. Generate a random integer z between 0 and n-1 (see Note), and
convert z to a byte string Z of length nLen, most significant
byte first:
z = RandomInteger (0, n-1)
Z = IntegerToString (z, nLen)
Randall & Kaliski Standards - Expires: March 2009 [Page 10]
INTERNET DRAFT September 2008
2. Encrypt the random integer z using the recipient's public key
(n,e) and convert the resulting integer c to a ciphertext C, a
byte string of length nLen:
c = z^e mod n
C = IntegerToString (c, nLen)
3. Derive a key-encrypting key KEK of length kekLen bytes from the
byte string Z using the underlying key derivation function:
KEK = KDF (Z, kekLen)
4. Wrap the keying data K with the key-encrypting key KEK using
the underlying key-wrapping scheme to obtain wrapped keying
data WK:
WK = Wrap (KEK, K)
5. Concatenate the ciphertext C and the wrapped keying data WK to
obtain the encrypted keying data EK:
EK = C || WK
6. Output the encrypted keying data EK.
NOTE: The random integer z MUST be generated independently at random
for different encryption operations, whether for the same or
different recipients.
A.3 Recipient's Operations
Let (n,d) be the recipient's RSA private key (see [PKCS1]; other
private key formats are allowed) and let EK be the encrypted keying
data.
Let nLen denote the length in bytes of the modulus n.
The recipient performs the following operations:
1. Separate the encrypted keying data EK into a ciphertext C of
length nLen bytes and wrapped keying data WK:
C || WK = EK
If the length of the encrypted keying data is less than nLen
bytes, output "decryption error" and stop.
2. Convert the ciphertext C to an integer c, most significant
byte first. Decrypt the integer c using the recipient's
private key (n,d) to recover an integer z (see Note):
c = StringToInteger (C)
z = c^d mod n
Randall & Kaliski Standards - Expires: March 2009 [Page 11]
INTERNET DRAFT September 2008
If the integer c is not between 0 and n-1, output "decryption
error" and stop.
3. Convert the integer z to a byte string Z of length nLen, most
significant byte first (see Note):
Z = IntegerToString (z, nLen)
4. Derive a key-encrypting key KEK of length kekLen bytes from
the byte string Z using the underlying key derivation function
(see Note):
KEK = KDF (Z, kekLen)
5. Unwrap the wrapped keying data WK with the key-encrypting key
KEK using the underlying key-wrapping scheme to recover the
keying data K:
K = Unwrap (KEK, WK)
If the unwrapping operation outputs an error, output
"decryption error" and stop.
6. Output the keying data K.
NOTE: Implementations SHOULD NOT reveal information about the integer
z and the string Z, nor about the calculation of the exponentiation
in Step 2, the conversion in Step 3, or the key derivation in Step 4,
whether by timing or other "side channels". The observable behavior
of the implementation SHOULD be the same at these steps for all
ciphertexts C that are in range. (For example, IntegerToString
conversion should take the same amount of time regardless of the
actual value of the integer z.) The integer z, the string Z and other
intermediate results MUST be securely deleted when they are no longer
needed.
Appendix B. ASN.1 Syntax
The ASN.1 syntax for identifying the RSA-KEM Key Transport Algorithm
is an extension of the syntax for the "generic hybrid cipher" in
ISO/IEC 18033-2 [ISO-IEC-18033-2], and is the same as employed in
ANS X9.44 [ANS-X9.44]. The syntax for the scheme is given in Section
B.1. The syntax for selected underlying components including those
mentioned above is given in B.2.
The following object identifier prefixes are used in the definitions
below:
is18033-2 OID ::= { iso(1) standard(0) is18033(18033) part2(2) }
nistAlgorithm OID ::= {
joint-iso-itu-t(2) country(16) us(840) organization(1)
gov(101) csor(3) nistAlgorithm(4)
}
Randall & Kaliski Standards - Expires: March 2009 [Page 12]
INTERNET DRAFT September 2008
pkcs-1 OID ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
}
NullParms is a more descriptive synonym for NULL when an algorithm
identifier has null parameters:
NullParms ::= NULL
The material in this Appendix is based on ANS X9.44.
B.1 RSA-KEM Key Transport Algorithm
The object identifier for the RSA-KEM Key Transport Algorithm is the
same as for the "generic hybrid cipher" in ISO/IEC 18033-2,
id-ac-generic-hybrid, which is defined in the draft as:
id-ac-generic-hybrid OID ::= {
is18033-2 asymmetric-cipher(1) generic-hybrid(2)
}
The associated parameters for id-ac-generic-hybrid have type
GenericHybridParameters:
GenericHybridParameters ::= {
kem KeyEncapsulationMechanism,
dem DataEncapsulationMechanism
}
The fields of type GenericHybridParameters have the following
meanings:
* kem identifies the underlying key encapsulation mechanism. For
the RSA-KEM Key Transport Algorithm, the scheme is RSA-KEM from
ISO/IEC 18033-2.
The object identifier for RSA-KEM (as a key encapsulation
mechanism) is id-kem-rsa, which is defined in ISO/IEC 18033-2
as
id-kem-rsa OID ::= {
is18033-2 key-encapsulation-mechanism(2) rsa(4)
}
The associated parameters for id-kem-rsa have type
RsaKemParameters:
RsaKemParameters ::= {
keyDerivationFunction KeyDerivationFunction,
keyLength KeyLength
}
The fields of type RsaKemParameters have the following
meanings:
Randall & Kaliski Standards - Expires: March 2009 [Page 13]
INTERNET DRAFT September 2008
* keyDerivationFunction identifies the underlying key
derivation function. For alignment with ANS X9.44, it
MUST be KDF2 or KDF3. However, other key derivation
functions MAY be used with CMS. Please see B.2.1 for the
syntax for KDF2 and KDF3.
KeyDerivationFunction ::=
AlgorithmIdentifier {{KDFAlgorithms}}
KDFAlgorithms ALGORITHM ::= {
kdf2 | kdf3,
... -- implementations may define other methods
}
* keyLength is the length in bytes of the key-encrypting
key, which depends on the underlying symmetric key-
wrapping scheme.
KeyLength ::= INTEGER (1..MAX)
* dem identifies the underlying data encapsulation mechanism.
For alignment with ANS X9.44, it MUST be an X9-approved
symmetric key-wrapping scheme. (See Note.) However, other
symmetric key-wrapping schemes MAY be used with CMS. Please see
B.2.2 for the syntax for the AES, Triple-DES, and Camillia Key
Wraps.
DataEncapsulationMechanism ::=
AlgorithmIdentifier {{DEMAlgorithms}}
DEMAlgorithms ALGORITHM ::= {
X9-SymmetricKeyWrappingSchemes,
Camillia-KeyWrappingSchemes,
... -- implementations may define other methods
}
X9-SymmetricKeyWrappingSchemes ALGORITHM ::= {
aes128-Wrap | aes192-Wrap | aes256-Wrap | tdes-Wrap,
... -- allows for future expansion
}
Camillia-KeyWrappingSchemes ALGORITHM ::= {
camillia128-Wrap | camillia192-Wrap | camillia256-Wrap
}
NOTE: The generic hybrid cipher in ISO/IEC 18033-2 can encrypt
arbitrary data, hence the term "data encapsulation mechanism". The
symmetric key-wrapping schemes take the role of data encapsulation
mechanisms in the RSA-KEM Key Transport Algorithm. ISO/IEC 18033-2
allows only three specific data encapsulation mechanisms, not
including any of these symmetric key-wrapping schemes. However, the
ASN.1 syntax in that document expects that additional algorithms will
be allowed.
Randall & Kaliski Standards - Expires: March 2009 [Page 14]
INTERNET DRAFT September 2008
B.2 Selected Underlying Components
B.2.1 Key Derivation Functions
The object identifier for KDF2 (see [ANS X9.44]) is:
id-kdf-kdf2 OID ::= { x9-44-components kdf2(1) }
The associated parameters identify the underlying hash function. For
alignment with ANS X9.44, the hash function MUST be an ASC
X9-approved hash function. However, other hash functions MAY be used
with CMS.
kdf2 ALGORITHM ::= { OID id-kdf-kdf2 PARMS KDF2-HashFunction }
KDF2-HashFunction ::= AlgorithmIdentifier {{KDF2-HashFunctions}}
KDF2-HashFunctions ALGORITHM ::= {
X9-HashFunctions,
... -- implementations may define other methods
}
X9-HashFunctions ALGORITHM ::= {
sha1 | sha224 | sha256 | sha384 | sha512,
... -- allows for future expansion
}
The object identifier for SHA-1 is
id-sha1 OID ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithms(2) sha1(26)
}
The object identifiers for SHA-224, SHA-256, SHA-384 and SHA-512 are
id-sha224 OID ::= { nistAlgorithm hashAlgs(2) sha224(4) }
id-sha256 OID ::= { nistAlgorithm hashAlgs(2) sha256(1) }
id-sha384 OID ::= { nistAlgorithm hashAlgs(2) sha384(2) }
id-sha512 OID ::= { nistAlgorithm hashAlgs(2) sha512(3) }
There has been some confusion over whether the various SHA object
identifiers have a NULL parameter, or no associated parameters. As
also discussed in [PKCS1], implementations SHOULD generate algorithm
identifiers without parameters, and MUST accept algorithm identifiers
either without parameters, or with NULL parameters.
sha1 ALGORITHM ::= { OID id-sha1 } -- NULLParms MUST be
sha224 ALGORITHM ::= { OID id-sha224 } -- accepted for these
sha256 ALGORITHM ::= { OID id-sha256 } -- OIDs
sha384 ALGORITHM ::= { OID id-sha384 } -- ""
sha512 ALGORITHM ::= { OID id-sha512 } -- ""
Randall & Kaliski Standards - Expires: March 2009 [Page 15]
INTERNET DRAFT September 2008
The object identifier for KDF3 (see [ANS X9.44]) is:
id-kdf-kdf3 OID ::= { x9-44-components kdf3(2) }
The associated parameters identify the underlying hash function. For
alignment with the draft ANS X9.44, the hash function MUST be an ASC
X9-approved hash function. (See Note.) However, other hash functions
MAY be used with CMS.
kdf3 ALGORITHM ::= { OID id-kdf-kdf3 PARMS KDF3-HashFunction }
KDF3-HashFunction ::= AlgorithmIdentifier { KDF3-HashFunctions }
KDF3-HashFunctions ALGORITHM ::= {
X9-HashFunctions,
... -- implementations may define other methods
}
B.2.2 Symmetric Key-Wrapping Schemes
The object identifiers for the AES Key Wrap depends on the size of
the key encrypting key. There are three object identifiers (see
[AES-WRAP]):
id-aes128-Wrap OID ::= { nistAlgorithm aes(1) aes128-Wrap(5) }
id-aes192-Wrap OID ::= { nistAlgorithm aes(1) aes192-Wrap(25) }
id-aes256-Wrap OID ::= { nistAlgorithm aes(1) aes256-Wrap(45) }
These object identifiers have no associated parameters.
aes128-Wrap ALGORITHM ::= { OID id-aes128-Wrap }
aes192-Wrap ALGORITHM ::= { OID id-aes192-Wrap }
aes256-Wrap ALGORITHM ::= { OID id-aes256-Wrap }
The object identifier for the Triple-DES Key Wrap (see [3DES-WRAP])
is
id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) alg(3) 6
}
This object identifier has a NULL parameter.
tdes-Wrap ALGORITHM ::=
{ OID id-alg-CMS3DESwrap PARMS NullParms }
NOTE: As of this writing, the AES Key Wrap and the Triple-DES Key
Wrap are in the process of being approved by ASC X9.
The object identifiers for the Camillia Key Wrap depends on the size
of the key encrypting key. There are three object identifiers:
Randall & Kaliski Standards - Expires: March 2009 [Page 16]
INTERNET DRAFT September 2008
id-camellia128-Wrap OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) 392 200011 61 security(1)
algorithm(1) key-wrap-algorithm(3)
camellia128-wrap(2) }
id-camellia192-Wrap OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) 392 200011 61 security(1)
algorithm(1) key-wrap-algorithm(3)
camellia192-wrap(3) }
id-camellia256-Wrap OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) 392 200011 61 security(1)
algorithm(1) key-wrap-algorithm(3)
camellia256-wrap(4) }
These object identifiers have no associated parameters.
camellia128-Wrap ALGORITHM ::= { OID id-camellia128-Wrap }
camellia192-Wrap ALGORITHM ::= { OID id-camellia192-Wrap }
camellia256-Wrap ALGORITHM ::= { OID id-camellia256-Wrap }
B.3 ASN.1 module
CMS-RSA-KEM
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) smime(16) modules(0) cms-rsa-kem(21) }
DEFINITIONS ::=
BEGIN
-- EXPORTS ALL
-- IMPORTS None
-- Useful types and definitions
OID ::= OBJECT IDENTIFIER -- alias
-- Unless otherwise stated, if an object identifier has associated
-- parameters (i.e., the PARMS element is specified), the parameters
-- field shall be included in algorithm identifier values. The
-- parameters field shall be omitted if and only if the object
-- identifier does not have associated parameters (i.e., the PARMS
-- element is omitted), unless otherwise stated.
ALGORITHM ::= CLASS {
&id OBJECT IDENTIFIER UNIQUE,
&Type OPTIONAL
}
WITH SYNTAX { OID &id [PARMS &Type] }
Randall & Kaliski Standards - Expires: March 2009 [Page 17]
INTERNET DRAFT September 2008
AlgorithmIdentifier { ALGORITHM:IOSet } ::= SEQUENCE {
algorithm ALGORITHM.&id( {IOSet} ),
parameters ALGORITHM.&Type( {IOSet}{@algorithm} ) OPTIONAL
}
NullParms ::= NULL
-- ISO/IEC 18033-2 arc
is18033-2 OID ::= { iso(1) standard(0) is18033(18033) part2(2) }
-- NIST algorithm arc
nistAlgorithm OID ::= {
joint-iso-itu-t(2) country(16) us(840) organization(1)
gov(101) csor(3) nistAlgorithm(4)
}
-- PKCS #1 arc
pkcs-1 OID ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
}
-- RSA-KEM Key Transport Algorithm, based on Generic Hybrid Cipher
id-ac-generic-hybrid OID ::= {
is18033-2 asymmetric-cipher(1) generic-hybrid(2)
}
GenericHybridParameters ::= SEQUENCE {
kem KeyEncapsulationMechanism,
dem DataEncapsulationMechanism
}
KeyEncapsulationMechanism ::= AlgorithmIdentifier {{KEMAlgorithms}}
KEMAlgorithms ALGORITHM ::= {
... -- Don't know what you want in here
}
id-kem-rsa OID ::= {
is18033-2 key-encapsulation-mechanism(2) rsa(4)
}
RsaKemParameters ::= SEQUENCE {
keyDerivationFunction KeyDerivationFunction,
keyLength KeyLength
}
KeyDerivationFunction ::= AlgorithmIdentifier {{KDFAlgorithms}}
KDFAlgorithms ALGORITHM ::= {
kdf2 | kdf3,
Randall & Kaliski Standards - Expires: March 2009 [Page 18]
INTERNET DRAFT September 2008
... -- implementations may define other methods
}
KeyLength ::= INTEGER (1..MAX)
DataEncapsulationMechanism ::= AlgorithmIdentifier {{DEMAlgorithms}}
DEMAlgorithms ALGORITHM ::= {
X9-SymmetricKeyWrappingSchemes |
Camillia-KeyWrappingSchemes,
... -- implementations may define other methods
}
X9-SymmetricKeyWrappingSchemes ALGORITHM ::= {
aes128-Wrap | aes192-Wrap | aes256-Wrap | tdes-Wrap,
... -- allows for future expansion
}
X9-SymmetricKeyWrappingScheme ::=
AlgorithmIdentifier {{ X9-SymmetricKeyWrappingSchemes }}
Camillia-KeyWrappingSchemes ALGORITHM ::= {
camellia128-Wrap | camellia192-Wrap | camellia256-Wrap,
... -- allows for future expansion
}
Camillia-KeyWrappingScheme ::=
AlgorithmIdentifier {{ Camillia-KeyWrappingSchemes }}
-- Key Derivation Functions
id-kdf-kdf2 OID ::= { x9-44-components kdf2(1) }
-- Base arc
x9-44 OID ::= {
iso(1) identified-organization(3) tc68(133) country(16) x9(840)
x9Standards(9) x9-44(44)
}
x9-44-components OID ::= { x9-44 components(1) }
kdf2 ALGORITHM ::= { OID id-kdf-kdf2 PARMS KDF2-HashFunction }
KDF2-HashFunction ::= AlgorithmIdentifier {{ KDF2-HashFunctions }}
KDF2-HashFunctions ALGORITHM ::= {
X9-HashFunctions,
... -- implementations may define other methods
}
-- id-kdf-kdf3 OID ::= { x9-44-components kdf3(2) }
Randall & Kaliski Standards - Expires: March 2009 [Page 19]
INTERNET DRAFT September 2008
kdf3 ALGORITHM ::= { OID id-kdf-kdf2 PARMS KDF3-HashFunction }
KDF3-HashFunction ::= AlgorithmIdentifier {{ KDF3-HashFunctions }}
KDF3-HashFunctions ALGORITHM ::= {
X9-HashFunctions,
... -- implementations may define other methods
}
-- Hash Functions
X9-HashFunctions ALGORITHM ::= {
sha1 | sha224 | sha256 | sha384 | sha512,
... -- allows for future expansion
}
id-sha1 OID ::= {
iso(1) identified-organization(3) oiw(14) secsig(3)
algorithms(2) sha1(26)
}
id-sha224 OID ::= { nistAlgorithm hashAlgs(2) sha256(4) }
id-sha256 OID ::= { nistAlgorithm hashAlgs(2) sha256(1) }
id-sha384 OID ::= { nistAlgorithm hashAlgs(2) sha384(2) }
id-sha512 OID ::= { nistAlgorithm hashAlgs(2) sha512(3) }
sha1 ALGORITHM ::= { OID id-sha1 } -- NullParms MUST be
sha224 ALGORITHM ::= { OID id-sha224 } -- accepted for these
sha256 ALGORITHM ::= { OID id-sha256 } -- OIDs
sha384 ALGORITHM ::= { OID id-sha384 } -- ""
sha512 ALGORITHM ::= { OID id-sha512 } -- ""
-- Symmetric Key-Wrapping Schemes
id-aes128-Wrap OID ::= { nistAlgorithm aes(1) aes128-Wrap(5) }
id-aes192-Wrap OID ::= { nistAlgorithm aes(1) aes192-Wrap(25) }
id-aes256-Wrap OID ::= { nistAlgorithm aes(1) aes256-Wrap(45) }
aes128-Wrap ALGORITHM ::= { OID id-aes128-Wrap }
aes192-Wrap ALGORITHM ::= { OID id-aes192-Wrap }
aes256-Wrap ALGORITHM ::= { OID id-aes256-Wrap }
id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) alg(3) 6
}
tdes-Wrap ALGORITHM ::= { OID id-alg-CMS3DESwrap PARMS NullParms }
id-camellia128-Wrap OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) 392 200011 61 security(1)
algorithm(1) key-wrap-algorithm(3)
camellia128-wrap(2) }
Randall & Kaliski Standards - Expires: March 2009 [Page 20]
INTERNET DRAFT September 2008
id-camellia192-Wrap OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) 392 200011 61 security(1)
algorithm(1) key-wrap-algorithm(3)
camellia192-wrap(3) }
id-camellia256-Wrap OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) 392 200011 61 security(1)
algorithm(1) key-wrap-algorithm(3)
camellia256-wrap(4) }
camellia128-Wrap ALGORITHM ::= { OID id-camellia128-Wrap }
camellia192-Wrap ALGORITHM ::= { OID id-camellia192-Wrap }
camellia256-Wrap ALGORITHM ::= { OID id-camellia256-Wrap }
END
B.4 Examples
As an example, if the key derivation function is KDF2 based on
SHA-256 and the symmetric key-wrapping scheme is the AES Key Wrap
with a 128-bit KEK, the AlgorithmIdentifier for the RSA-KEM Key
Transport Algorithm will have the following value:
SEQUENCE {
id-ac-generic-hybrid, -- generic cipher
SEQUENCE { -- GenericHybridParameters
SEQUENCE { -- key encapsulation mechanism
id-kem-rsa, -- RSA-KEM
SEQUENCE { -- RsaKemParameters
SEQUENCE { -- key derivation function
id-kdf-kdf2, -- KDF2
SEQUENCE { -- KDF2-HashFunction
id-sha256 -- SHA-256; no parameters (preferred)
},
16 -- KEK length in bytes
},
SEQUENCE { -- data encapsulation mechanism
id-aes128-Wrap -- AES-128 Wrap; no parameters
}
}
}
Randall & Kaliski Standards - Expires: March 2009 [Page 21]
INTERNET DRAFT September 2008
This AlgorithmIdentifier value has the following DER encoding:
30 4f
06 07 28 81 8c 71 02 01 02 -- id-ac-generic-hybrid
30 44
30 25
06 07 28 81 8c 71 02 02 04 -- id-kem-rsa
30 1a
30 16
06 07 28 81 8c 71 02 05 02 -- id-kdf-kdf2
30 0b
06 09 60 86 48 01 65 03 04 02 01 -- id-sha256
02 10 -- 16 bytes
30 0b
06 09 60 86 48 01 65 03 04 01 05 -- id-aes128-Wrap
The DER encodings for other typical sets of underlying components are
as follows:
* KDF2 based on SHA-384, AES Key Wrap with a 192-bit KEK
30 4f 06 07 28 81 8c 71 02 01 02 30 44 30 25 06
07 28 81 8c 71 02 02 04 30 1a 30 16 06 07 28 81
8c 71 02 05 02 30 0b 06 09 60 86 48 01 65 03 04
02 02 02 18 30 0b 06 09 60 86 48 01 65 03 04 01
19
* KDF2 based on SHA-512, AES Key Wrap with a 256-bit KEK
30 4f 06 07 28 81 8c 71 02 01 02 30 44 30 25 06
07 28 81 8c 71 02 02 04 30 1a 30 16 06 07 28 81
8c 71 02 05 02 30 0b 06 09 60 86 48 01 65 03 04
02 03 02 20 30 0b 06 09 60 86 48 01 65 03 04 01
2d
* KDF2 based on SHA-1, Triple-DES Key Wrap with a 128-bit KEK
(two-key triple-DES)
30 4f 06 07 28 81 8c 71 02 01 02 30 44 30 21 06
07 28 81 8c 71 02 02 04 30 16 30 12 06 07 28 81
8c 71 02 05 02 30 07 06 05 2b 0e 03 02 1a 02 10
30 0f 06 0b 2a 86 48 86 f7 0d 01 09 10 03 06 05
00
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
Randall & Kaliski Standards - Expires: March 2009 [Page 22]
INTERNET DRAFT September 2008
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
Disclaimer Statement
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Randall & Kaliski Standards - Expires: March 2009 [Page 23]