Internet DRAFT - draft-housley-lamps-rfc5990bis
draft-housley-lamps-rfc5990bis
LAMPS Working Group R. Housley
Internet-Draft Vigil Security
Obsoletes: 5990 (if approved) S. Turner
Intended status: Standards Track sn3rd
Expires: 18 August 2023 14 February 2023
Use of the RSA-KEM Algorithm in the Cryptographic Message Syntax (CMS)
draft-housley-lamps-rfc5990bis-00
Abstract
The RSA Key Encapsulation Mechanism (RSA-KEM) Algorithm is a one-pass
(store-and-forward) cryptographic mechanism for an originator to
securely send keying material to a recipient using the recipient's
RSA public key. The RSA-KEM Algorithm is specified in Clause 11.5 of
ISO/IEC: 18033-2:2006. This document specifies the conventions for
using the RSA-KEM Algorithm with the Cryptographic Message Syntax
(CMS) using KEMRecipientInfo as specified in draft-housley-lamps-cms-
kemri.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-housley-lamps-rfc5990bis/.
Discussion of this document takes place on the WG LAMPS mailing list
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This Internet-Draft will expire on 18 August 2023.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4
1.2. ASN.1 . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Changes Since RFC 5990 . . . . . . . . . . . . . . . . . 4
2. Use of the RSA-KEM Algorithm in CMS . . . . . . . . . . . . . 5
2.1. Underlying Components . . . . . . . . . . . . . . . . . . 5
2.2. RecipientInfo Conventions . . . . . . . . . . . . . . . . 5
2.3. Certificate Conventions . . . . . . . . . . . . . . . . . 6
2.4. SMIMECapabilities Attribute Conventions . . . . . . . . . 7
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Normative References . . . . . . . . . . . . . . . . . . 10
5.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. RSA-KEM Algorithm . . . . . . . . . . . . . . . . . 12
A.1. Underlying Components . . . . . . . . . . . . . . . . . . 13
A.2. Originator's Operations . . . . . . . . . . . . . . . . . 13
A.3. Recipient's Operations . . . . . . . . . . . . . . . . . 14
Appendix B. ASN.1 Syntax . . . . . . . . . . . . . . . . . . . . 16
B.1. Underlying Components . . . . . . . . . . . . . . . . . . 16
B.2. ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . 17
Appendix C. SMIMECapabilities Examples . . . . . . . . . . . . . 21
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
The RSA Key Encapsulation Mechanism (RSA-KEM) Algorithm is a one-pass
(store-and-forward) cryptographic mechanism for an originator to
securely send keying material to a recipient using the recipient's
RSA public key. The RSA-KEM Algorithm is specified in Clause 11.5 of
[ISO18033-2].
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The RSA-KEM Algorithm takes a different approach than other RSA key
transport mechanisms [RFC8017], with the goal of providing higher
security assurance. The RSA-KEM Algorithm encrypts a random integer
with the recipient's RSA public key, derives a key-encryption key
from the random integer, and wraps a symmetric content-encryption key
with the key-encryption key. In this way, the originator and the
recipient end up with the same content-encryption key. Given a
content-encryption key CEK, RSA-KEM can be summarized as:
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-encryption key KEK from the integer z:
KEK = KDF(z)
4. Wrap the CEK with the KEK to obtain wrapped keying material WK:
WK = WRAP(KEK, CEK)
5. The originator sends c and WK to the recipient.
This different approach provides higher security assurance for two
reasons. First, 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. Second, the input is independent of the keying
material so the result of the RSA decryption operation is not
directly available to an adversary. As a result, the RSA-KEM
Algorithm enjoys a "tight" security proof in the random oracle model.
(In other padding schemes, such as PKCS #1 v1.5 [RFC8017], the input
has structure and/or depends on the keying material, 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 material.
Another benefit is that the length of the keying material is bounded
only by the symmetric key-wrapping algorithm, not the size of the RSA
modulus.
For completeness, a specification of the RSA-KEM Algorithm is given
in Appendix A of this document; ASN.1 syntax is given in Appendix B.
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1.1. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. ASN.1
CMS values are generated using ASN.1 [X.680], which uses the Basic
Encoding Rules (BER) and the Distinguished Encoding Rules (DER)
[X.690].
1.3. Changes Since RFC 5990
RFC 5990 [RFC5990] specified the conventions for using the RSA-KEM
Algorithm in CMS as a key transport algorithm. That is, it used
KeyTransRecipientInfo [RFC5652] for each recipient. This approach
resulted in a very complex parameter definition with the id-rsa-kem
algorithm identifier. Implementation experience with many different
algorithms has shown that complex parameter structures cause
interoperability issues. Since the publication of RFC 5990, a new
KEMRecipientInfo structure [I-D.housley-lamps-cms-kemri] has been
defined to support KEM algorithms, and this new structure avoids the
complex parameters structure that was used in RFC 5990. Likewise,
when the id-rsa-kem algorithm identifier appears in the
SubjectPublicKeyInfo field of a certificate, this document encourages
the omission of any parameters.
RFC 5990 uses EK and the EncryptedKey, which the concatenation of C
and WK (C || WK). The use of EK is necessary to align with the
KeyTransRecipientInfo structure. In this document, C and WK are sent
in separate fields of new KEMRecipientInfo structure. In particular,
C is carried in the kemct field, and WK is carried in the
encryptedKey field.
RFC 5990 supports the future definition of additional KEM algorithms
that use RSA; this document supports only one, and it is identified
by the id-kem-rsa object identifier.
RFC 5990 includes support for Camellia and Triple-DES block ciphers;
discussion of these block ciphers is removed from this document, but
the algorithm identifiers remain in the ASN.1 Module Appendix B.2.
RFC 5990 includes support for SHA-1 hash function; discussion of this
hash function is removed from this document, but the algorithm
identifier remains in the ASN.1 module Appendix B.2.
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RFC 5990 required support for the KDF3 [ANS-X9.44] key-derivation
function; this document continues to require support for the KDF3
key-derivation function, but it requires support for SHA-256 [SHS] as
the hash function.
RFC 5990 recommends support for alternatives to KDF3 and AES-Wrap-
128; this document simply states that other key-derivation functions
and key-encryption algorithms MAY be supported.
RFC 5990 includes an ASN.1 module; this document provides an
alternative ASN.1 module that follows the conventions established in
[RFC5911], [RFC5912], and [RFC6268]. The new ASN.1 module
Appendix B.2 produces the same bits-on-the-wire as the one in RFC
5990.
2. Use of the RSA-KEM Algorithm in CMS
The RSA-KEM Algorithm MAY be employed for one or more recipients in
the CMS enveloped-data content type [RFC5652], the CMS authenticated-
data content type [RFC5652], or the CMS authenticated-enveloped-data
content type [RFC5083]. In each case, the KEMRecipientInfo
[I-D.housley-lamps-cms-kemri] is used with with the RSA-KEM Algorithm
to securely transfer the content-encryption key from the originator
to the recipient.
2.1. Underlying Components
A CMS implementation that supports the RSA-KEM Algorithm MUST support
at least the following underlying components:
* For the key-derivation function, an implementation MUST support
KDF3 [ANS-X9.44] with SHA-256 [SHS].
* For key-wrapping, an implementation MUST support the AES-Wrap-128
[RFC3394] key-encryption algorithm.
An implementation MAY also support other key-derivation functions and
key-encryption algorithms as well.
2.2. RecipientInfo Conventions
When the RSA-KEM Algorithm is employed for a recipient, the
RecipientInfo alternative for that recipient MUST be
OtherRecipientInfo using the KEMRecipientInfo structure
[I-D.housley-lamps-cms-kemri]. The fields of the KEMRecipientInfo
MUST have the following values:
version is the syntax version number; it MUST be 0.
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rid identifies the recipient's certificate or public key.
kem identifies the KEM algorithm; it MUST contain id-kem-rsa.
kemct is the ciphertext produced for this recipient; it contains C
from steps 1 and 2 of Originator's Operations in Appendix A.
kdf identifies the key-derivation algorithm.
kekLength is the size of the key-encryption key in octets.
ukm is an optional random input to the key-derivation function.
wrap identifies a key-encryption algorithm used to encrypt the
content-encryption key.
encryptedKey is the result of encrypting the keying material with
the key-encryption key. When used with the CMS enveloped-data
content type [RFC5652], the keying material is a content-
encryption key. When used with the CMS authenticated-data content
type [RFC5652], the keying material is a message-authentication
key. When used with the CMS authenticated-enveloped-data content
type [RFC5083], the keying material is a content-authenticated-
encryption key.
2.3. Certificate Conventions
The conventions specified in this section augment RFC 5280 [RFC5280].
A recipient who employs the RSA-KEM Algorithm MAY identify the public
key in a certificate by the same AlgorithmIdentifier as for the PKCS
#1 v1.5 algorithm, that is, using the rsaEncryption object identifier
[RFC8017]. The fact that the recipient will accept RSA-KEM with this
public key is not indicated by the use of this object identifier.
The willingness to accept the RSA-KEM Algorithm MAY be signaled by
the use of the appropriate SMIME Capabilities either in a message or
in the certificate.
If the recipient wishes only to employ the RSA-KEM Algorithm with a
given public key, the recipient MUST identify the public key in the
certificate using the id-rsa-kem object identifier; see Appendix B.
When the id-rsa-kem object identifier appears in the
SubjectPublicKeyInfo algorithm field of the certificate, the
parameters field from AlgorithmIdentifier SHOULD be absent. That is,
the AlgorithmIdentifier SHOULD be a SEQUENCE of one component, the
id-rsa-kem object identifier.
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When the AlgorithmIdentifier parameters are present, the
GenericHybridParameters MUST be used. As described in the next
section, the GenericHybridParameters constrain the values that can be
used with the RSA public key for the kdf, kekLength, and wrap fields
of the KEMRecipientInfo structure.
Regardless of the AlgorithmIdentifier used, the RSA public key MUST
be carried in the subjectPublicKey BIT STRING within the
SubjectPublicKeyInfo filed of the certificate using the RSAPublicKey
type defined in [RFC8017].
The intended application for the public key MAY be indicated in the
key usage certificate extension as specified in Section 4.2.1.3 of
[RFC5280]. If the keyUsage extension is present in a certificate
that conveys an RSA public key with the id-rsa-kem object identifier
as discussed above, then the key usage extension MUST contain the
following value:
keyEncipherment
The digitalSignatrure and dataEncipherment values SHOULD NOT be
present. That is, a public key intended to be employed only with the
RSA-KEM Algorithm SHOULD NOT also be employed for data encryption or
for digital 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
Section 2.5.2 of [RFC8551] defines the SMIMECapabilities signed
attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to
announce a partial list of algorithms that an S/MIME implementation
can support. When constructing a CMS signed-data content type
[RFC5652], a compliant implementation MAY include the
SMIMECapabilities signed attribute announcing that it supports the
RSA-KEM Algorithm.
The SMIMECapability SEQUENCE representing the RSA-KEM Algorithm MUST
include the id-rsa-kem object identifier in the capabilityID field;
see Appendix B for the object identifier value, and see Appendix C
for examples. When the id-rsa-kem object identifier appears in the
capabilityID field and the parameters are present, then the
parameters field MUST use the GenericHybridParameters type.
GenericHybridParameters ::= SEQUENCE {
kem KeyEncapsulationMechanism,
dem DataEncapsulationMechanism }
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The fields of the GenericHybridParameters type have the following
meanings:
kem is an AlgorithmIdentifer; the algorithm field MUST be set to
id-kem-rsa; the parameters field MUST be RsaKemParameters, which
is a SEQUENCE of an AlgorithmIdentifier that identifies the
supported key-derivation function and a positive INTEGER that
identifies the length of the key-encryption key in octets. If the
GenericHybridParameters are present, then the provided kem value
MUST be used as the key-derivation function in the kdf field of
KEMRecipientInfo, and the provided key length MUST be used in the
kekLength of KEMRecipientInfo.
dem is an AlgorithmIdentifier; the algorithm field MUST be
present, and it identifies the key-encryption algorithm;
parameters are optional. If the GenericHybridParameters are
present, then the provided dem value MUST be used in the wrap
field of KEMRecipientInfo.
3. Security Considerations
The RSA-KEM Algorithm should be considered as a replacement for the
widely implemented PKCS #1 v1.5 [RFC8017] for new applications that
use CMS to avoid potential vulnerabilities to chosen-ciphertext
attacks and gain a tighter security proof; however, the RSA-KEM
Algorithm has the disadvantage of slightly longer encrypted keying
material.
The security of the RSA-KEM Algorithm can be shown to be tightly
related to the difficulty of either solving the RSA problem or
breaking the underlying symmetric key-encryption algorithm, if the
underlying key-derivation function is modeled as a random oracle, and
assuming that the symmetric key-encryption algorithm 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 need to be selected
consistent with the desired security level. Several security levels
have been identified in the NIST SP 800-57 Part 1
[NISTSP800-57pt1r5]. To achieve 128-bit security, the RSA key size
SHOULD be at least 3072 bits, the key-derivation function SHOULD make
use of SHA-256, and the symmetric key-encryption algorithm SHOULD be
AES Key Wrap with a 128-bit key.
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Implementations MUST protect the RSA private key, the key-encryption
key, the content-encryption key, the content-authenticated-encryption
key. Compromise of the RSA private key could result in the
disclosure of all messages protected with that key. Compromise of
the key-encryption key, the content-encryption key, or content-
authenticated-encryption key could result in disclosure of the
associated encrypted content.
Additional considerations related to key management may be found in
[NISTSP800-57pt1r5].
The security of the RSA-KEM 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, see
[RFC4086].
Implementations SHOULD NOT reveal information about intermediate
values or calculations, whether by timing or other "side channels",
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 to an opponent.
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 Algorithm SHOULD
NOT also be used for digital signatures. Indeed, the Accredited
Standards Committee X9 (ASC X9) requires such a separation between
key pairs used for key establishment and key pairs used for digital
signature [ANS-X9.44]. Continuing this principle of key separation,
a key pair used for the RSA-KEM 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 gain assurance that implementations are correct through
formal implementation validation, such as the NIST Cryptographic
Module Validation Program (CMVP) [CMVP].
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4. IANA Considerations
For the ASN.1 Module in Appendix B.2, IANA is requested to assign an
object identifier (OID) for the module identifier. The OID for the
module should be allocated in the "SMI Security for S/MIME Module
Identifier" registry (1.2.840.113549.1.9.16.0).
5. References
5.1. Normative References
[I-D.housley-lamps-cms-kemri]
Housley, R., Gray, J., and T. Okubo, "Using Key
Encapsulation Mechanism (KEM) Algorithms in the
Cryptographic Message Syntax (CMS)", Work in Progress,
Internet-Draft, draft-housley-lamps-cms-kemri-00, 1
February 2023, <https://datatracker.ietf.org/doc/html/
draft-housley-lamps-cms-kemri-00>.
[ISO18033-2]
ISO/IEC JTC 1/SC 27, "Information technology -- Security
techniques -- Encryption algorithms -- Part 2: Asymmetric
ciphers", ISO/IEC 18033-2:2006, 2006,
<https://www.iso.org/standard/37971.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5083] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
DOI 10.17487/RFC5083, November 2007,
<https://www.rfc-editor.org/info/rfc5083>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>.
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[RFC5911] Hoffman, P. and J. Schaad, "New ASN.1 Modules for
Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911,
DOI 10.17487/RFC5911, June 2010,
<https://www.rfc-editor.org/info/rfc5911>.
[RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
DOI 10.17487/RFC5912, June 2010,
<https://www.rfc-editor.org/info/rfc5912>.
[RFC6268] Schaad, J. and S. Turner, "Additional New ASN.1 Modules
for the Cryptographic Message Syntax (CMS) and the Public
Key Infrastructure Using X.509 (PKIX)", RFC 6268,
DOI 10.17487/RFC6268, July 2011,
<https://www.rfc-editor.org/info/rfc6268>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/info/rfc8551>.
[SHS] National Institute of Standards and Technology, "Secure
Hash Standard", DOI 10.6028/nist.fips.180-4, July 2015,
<https://doi.org/10.6028/nist.fips.180-4>.
[X.680] ITU-T, "Information technology -- Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ITU-T
Recommendation X.680, ISO/IEC 8824-1:2021, February 2021,
<https://www.itu.int/rec/T-REC-X.680>.
[X.690] ITU-T, "Information technology -- ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1:2021,
February 2021, <https://www.itu.int/rec/T-REC-X.680>.
5.2. Informative References
[ANS-X9.44]
American National Standards Institute, "Public Key
Cryptography for the Financial Services Industry -- Key
Establishment Using Integer Factorization Cryptography",
American National Standard X9.44, 2007.
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[CMVP] National Institute of Standards and Technology,
"Cryptographic Module Validation Program", 2016,
<https://csrc.nist.gov/projects/cryptographic-module-
validation-program>.
[NISTSP800-57pt1r5]
Barker, E. and National Institute of Standards and
Technology, "Recommendation for key management:",
DOI 10.6028/nist.sp.800-57pt1r5, May 2020,
<http://dx.doi.org/10.6028/nist.sp.800-57pt1r5>.
[RFC3394] Schaad, J. and R. Housley, "Advanced Encryption Standard
(AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
September 2002, <https://www.rfc-editor.org/info/rfc3394>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC5990] Randall, J., Kaliski, B., Brainard, J., and S. Turner,
"Use of the RSA-KEM Key Transport Algorithm in the
Cryptographic Message Syntax (CMS)", RFC 5990,
DOI 10.17487/RFC5990, September 2010,
<https://www.rfc-editor.org/info/rfc5990>.
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<https://www.rfc-editor.org/info/rfc6194>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<https://www.rfc-editor.org/info/rfc8017>.
[SHOUP] Shoup, V., "A Proposal for an ISO Standard for Public Key
Encryption", Cryptology ePrint Archive Paper 2001/112,
2001, <https://eprint.iacr.org/2001/112>.
Appendix A. RSA-KEM Algorithm
The RSA-KEM Algorithm is a one-pass (store-and-forward) cryptographic
mechanism for an originator to securely send keying material to a
recipient using the recipient's RSA public key.
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With this type of algorithm, an originator encrypts the keying
material using the recipient's public key, and then sends the
resulting encrypted keying material to the recipient. The recipient
decrypts the encrypted keying material using the recipient's private
key to recover the keying material.
A.1. Underlying Components
The RSA-KEM Algorithm has the following underlying components:
* KDF, a key-derivation function, which derives key-encryption key
of a specified length from a shared secret value;
* Wrap, a symmetric key-encryption algorithm, which encrypts keying
material using key-encryption key that was produced by the KDF.
The kekLen value denotes the length in bytes of the key-encryption
key for the underlying symmetric key-encryption algorithm.
The length of the keying material MUST be among the lengths supported
by the underlying symmetric key-encryption algorithm. For example,
the AES-Wrap key-encryption algorithm requires the kekLen to be 16,
24, or 32 octets. Usage and formatting of the keying material is
outside the scope of the RSA-KEM Algorithm.
Many key-derivation functions support the inclusion of other
information in addition to the shared secret value in the input to
the function. Also, with some symmetric key-encryption algorithms,
it is possible to associate a label with the keying material. Such
uses are outside the scope of this document, as they are not directly
supported by CMS.
A.2. Originator's Operations
Let (n,e) be the recipient's RSA public key; see [RFC8017] for
details.
Let K be the keying material to be securely transferred from the
originator to the recipient.
Let nLen denote the length in bytes of the modulus n, i.e., the least
integer such that 2^(8*nLen) > n.
The originator 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:
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z = RandomInteger (0, n-1)
Z = IntegerToString (z, nLen)
2. Encrypt the random integer z using the recipient's RSA 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 symmetric key-encryption 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 material K with the symmetric key-encryption key
KEK using the key-encryption algorithm to obtain wrapped keying
material WK:
WK = Wrap (KEK, K)
5. Send the ciphertext C and the wrapped keying material WK to the
recipient.
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 [RFC8017] for
details, but other private key formats are allowed.
Let WK be the encrypted keying material.
Let C be the ciphertext.
Let nLen denote the length in bytes of the modulus n.
The recipient performs the following operations:
1. If the length of the encrypted keying material is less than nLen
bytes, output "decryption error", and stop.
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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 below):
c = StringToInteger (C)
z = c^d mod n
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 below):
Z = IntegerToString (z, nLen)
4. Derive a symmetric key-encryption key KEK of length kekLen bytes
from the byte string Z using the key-derivation function (see
NOTE below):
KEK = KDF (Z, kekLen)
5. Unwrap the wrapped keying material WK with the symmetric key-
encryption key KEK using the underlying key-encryption algorithm
to recover the keying material K:
K = Unwrap (KEK, WK)
If the unwrapping operation outputs an error, output "decryption
error", and stop.
6. Output the keying material K.
NOTE: Implementations SHOULD NOT reveal information about the integer
z, the string Z, or 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.
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Appendix B. ASN.1 Syntax
The ASN.1 syntax for identifying the RSA-KEM Algorithm is an
extension of the syntax for the "generic hybrid cipher" in ANS X9.44
[ANS-X9.44].
The ASN.1 Module is unchanged from RFC 5990. The id-rsa-kem object
identifier is used in a backward compatible manner in certificates
[RFC5280] and SMIMECapabilities [RFC8551]. Of course, the use of the
id-kem-rsa object identifier in the new KEMRecipientInfo structure
[I-D.housley-lamps-cms-kemri] was not yet defined at the time that
RFC 5990 was written.
B.1. Underlying Components
Implementations that conform to this specification MUST support the
KDF3 [ANS-X9.44] key-derivation function using SHA-256 [SHS].
The object identifier for KDF3 is:
id-kdf-kdf3 OBJECT IDENTIFIER ::= { x9-44-components kdf3(2) }
The KDF3 parameters identify the underlying hash function. For
alignment with the ANS X9.44, the hash function MUST be an ASC
X9-approved hash function. While the SHA-1 hash algorithm is
included in the ASN.1 definitions, SHA-1 MUST NOT be used. SHA-1 is
considered to be obsolete; see [RFC6194]. SHA-1 remains in the ASN.1
module for compatibility with RFC 5990. In addition, other hash
functions MAY be used with CMS.
kda-kdf3 KEY-DERIVATION ::= {
IDENTIFIER id-kdf-kdf3
PARAMS TYPE KDF3-HashFunction ARE required
-- No S/MIME caps defined -- }
KDF3-HashFunction ::=
AlgorithmIdentifier { DIGEST-ALGORITHM, {KDF3-HashFunctions} }
KDF3-HashFunctions DIGEST-ALGORITHM ::= { X9-HashFunctions, ... }
X9-HashFunctions DIGEST-ALGORITHM ::= {
mda-sha1 | mda-sha224 ! mda-sha256 ! mda-sha384 |
mda-sha512, ... }
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Implementations that conform to this specification MUST support the
AES Key Wrap [RFC3394] key-encryption algorithm with a 128-bit key.
There are three object identifiers for the AES Key Wrap, one for each
permitted size of the key-encryption key. There are three object
identifiers imported from [RFC5912], and none of these algorithm
identifiers have associated parameters:
kwa-aes128-wrap KEY-WRAP ::= {
IDENTIFIER id-aes128-wrap
PARAMS ARE absent
SMIME-CAPS { IDENTIFIED BY id-aes128-wrap } }
kwa-aes192-wrap KEY-WRAP ::= {
IDENTIFIER id-aes192-wrap
PARAMS ARE absent
SMIME-CAPS { IDENTIFIED BY id-aes192-wrap } }
kwa-aes256-wrap KEY-WRAP ::= {
IDENTIFIER id-aes256-wrap
PARAMS ARE absent
SMIME-CAPS { IDENTIFIED BY id-aes256-wrap } }
B.2. ASN.1 Module
RFC EDITOR: Please replace TBD2 with the value assigned by IANA
during the publication of [I-D.housley-lamps-cms-kemri].
<CODE BEGINS>
CMS-RSA-KEM-2023
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) smime(16) modules(0) cms-rsa-kem-2023(TBD1) }
DEFINITIONS ::= BEGIN
-- EXPORTS ALL
IMPORTS
KEM-ALGORITHM
FROM KEMAlgorithmInformation-2023 -- [I-D.housley-lamps-cms-kemri]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-kemAlgorithmInformation-2023(TBD3) }
AlgorithmIdentifier{}, PUBLIC-KEY, DIGEST-ALGORITHM,
KEY-DERIVATION, KEY-WRAP
FROM AlgorithmInformation-2009 -- [RFC5912]
{ iso(1) identified-organization(3) dod(6) internet(1)
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security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-algorithmInformation-02(58) }
kwa-aes128-wrap, kwa-aes192-wrap, kwa-aes256-wrap
FROM CMSAesRsaesOaep-2009 -- [RFC5911]
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0)
id-mod-cms-aes-02(38) }
kwa-3DESWrap
FROM CryptographicMessageSyntaxAlgorithms-2009 -- [RFC5911]
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0)
id-mod-cmsalg-2001-02(37) }
id-camellia128-wrap, id-camellia192-wrap, id-camellia256-wrap
FROM CamelliaEncryptionAlgorithmInCMS -- [RFC3657]
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs9(9) smime(16) modules(0)
id-mod-cms-camellia(23) }
mda-sha1, pk-rsa, RSAPublicKey
FROM PKIXAlgs-2009 -- [RFC5912]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-algorithms2008-02(56) }
mda-sha224, mda-sha256, mda-sha384, mda-sha512
FROM PKIX1-PSS-OAEP-Algorithms-2009 -- [RFC5912]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-rsa-pkalgs-02(54) } ;
-- Useful types and definitions
OID ::= OBJECT IDENTIFIER -- alias
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) }
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-- PKCS #1 arc
pkcs-1 OID ::= { iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-1(1) }
-- X9.44 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) }
-- RSA-KEM Algorithm
id-rsa-kem OID ::= { iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) alg(3) 14 }
GenericHybridParameters ::= SEQUENCE {
kem KeyEncapsulationMechanism,
dem DataEncapsulationMechanism }
KeyEncapsulationMechanism ::=
AlgorithmIdentifier { KEM-ALGORITHM, {KEMAlgorithms} }
KEMAlgorithms KEM-ALGORITHM ::= { kema-kem-rsa, ... }
kema-kem-rsa KEM-ALGORITHM ::= {
IDENTIFIER id-kem-rsa
PARAMS TYPE RsaKemParameters ARE optional
PUBLIC-KEYS { pk-rsa | pk-rsa-kem }
UKM ARE optional
SMIME-CAPS { TYPE GenericHybridParameters
IDENTIFIED BY id-rsa-kem } }
id-kem-rsa OID ::= { is18033-2 key-encapsulation-mechanism(2)
rsa(4) }
RsaKemParameters ::= SEQUENCE {
keyDerivationFunction KeyDerivationFunction,
keyLength KeyLength }
pk-rsa-kem PUBLIC-KEY ::= {
IDENTIFIER id-rsa-kem
KEY RSAPublicKey
PARAMS TYPE GenericHybridParameters ARE preferredAbsent
-- Private key format is not specified here --
CERT-KEY-USAGE {keyEncipherment} }
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KeyDerivationFunction ::=
AlgorithmIdentifier { KEY-DERIVATION, {KDFAlgorithms} }
KDFAlgorithms KEY-DERIVATION ::= { kda-kdf2 | kda-kdf3, ... }
KeyLength ::= INTEGER (1..MAX)
DataEncapsulationMechanism ::=
AlgorithmIdentifier { KEY-WRAP, {DEMAlgorithms} }
DEMAlgorithms KEY-WRAP ::= {
X9-SymmetricKeyWrappingSchemes |
Camellia-KeyWrappingSchemes, ... }
X9-SymmetricKeyWrappingSchemes KEY-WRAP ::= {
kwa-aes128-wrap | kwa-aes192-wrap | kwa-aes256-wrap |
kwa-3DESWrap, ... }
X9-SymmetricKeyWrappingScheme ::=
AlgorithmIdentifier { KEY-WRAP, {X9-SymmetricKeyWrappingSchemes} }
Camellia-KeyWrappingSchemes KEY-WRAP ::= {
kwa-camellia128-wrap | kwa-camellia192-wrap |
kwa-camellia256-wrap, ... }
Camellia-KeyWrappingScheme ::=
AlgorithmIdentifier { KEY-WRAP, {Camellia-KeyWrappingSchemes} }
kwa-camellia128-wrap KEY-WRAP ::= {
IDENTIFIER id-camellia128-wrap
PARAMS ARE absent
SMIME-CAPS { IDENTIFIED BY id-camellia128-wrap } }
kwa-camellia192-wrap KEY-WRAP ::= {
IDENTIFIER id-camellia192-wrap
PARAMS ARE absent
SMIME-CAPS { IDENTIFIED BY id-camellia192-wrap } }
kwa-camellia256-wrap KEY-WRAP ::= {
IDENTIFIER id-camellia256-wrap
PARAMS ARE absent
SMIME-CAPS { IDENTIFIED BY id-camellia256-wrap } }
-- Key Derivation Functions
id-kdf-kdf2 OID ::= { x9-44-components kdf2(1) }
kda-kdf2 KEY-DERIVATION ::= {
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IDENTIFIER id-kdf-kdf2
PARAMS TYPE KDF2-HashFunction ARE required
-- No S/MIME caps defined -- }
KDF2-HashFunction ::=
AlgorithmIdentifier { DIGEST-ALGORITHM, {KDF2-HashFunctions} }
KDF2-HashFunctions DIGEST-ALGORITHM ::= { X9-HashFunctions, ... }
id-kdf-kdf3 OID ::= { x9-44-components kdf3(2) }
kda-kdf3 KEY-DERIVATION ::= {
IDENTIFIER id-kdf-kdf3
PARAMS TYPE KDF3-HashFunction ARE required
-- No S/MIME caps defined -- }
KDF3-HashFunction ::=
AlgorithmIdentifier { DIGEST-ALGORITHM, {KDF3-HashFunctions} }
KDF3-HashFunctions DIGEST-ALGORITHM ::= { X9-HashFunctions, ... }
-- Hash Functions
X9-HashFunctions DIGEST-ALGORITHM ::= {
mda-sha1 | mda-sha224 | mda-sha256 | mda-sha384 |
mda-sha512, ... }
END
<CODE ENDS>
Appendix C. SMIMECapabilities Examples
To indicate support for the RSA-KEM algorithm coupled with the KDF3
key-derivation function with SHA-256 and the AES Key Wrap symmetric
key-encryption algorithm 128-bit key-encryption key, the
SMIMECapabilities will include the following entry:
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SEQUENCE {
id-rsa-kem, -- RSA-KEM Algorithm
SEQUENCE { -- GenericHybridParameters
SEQUENCE { -- key encapsulation mechanism
id-kem-rsa, -- RSA-KEM
SEQUENCE { -- RsaKemParameters
SEQUENCE { -- key derivation function
id-kdf-kdf3, -- KDF3
SEQUENCE { -- KDF3-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
}
}
}
This SMIMECapability value has the following DER encoding (in
hexadecimal):
30 47
06 0b 2a 86 48 86 f7 0d 01 09 10 03 0e -- id-rsa-kem
30 38
30 29
06 07 28 81 8c 71 02 02 04 -- id-kem-rsa
30 1e
30 19
06 0a 2b 81 05 10 86 48 09 2c 01 02 -- id-kdf-kdf3
30 0b
06 09 60 86 48 01 65 03 04 02 01 -- id-sha256
02 01 10 -- 16 bytes
30 0b
06 09 60 86 48 01 65 03 04 01 05 -- id-aes128-Wrap
To indicate support for the RSA-KEM algorithm coupled with the KDF3
key-derivation function with SHA-384 and the AES Key Wrap symmetric
key-encryption algorithm 192-bit key-encryption key, the
SMIMECapabilities will include the following SMIMECapability value
(in hexadecimal):
30 47 06 0b 2a 86 48 86 f7 0d 01 09 10 03 0e 30
38 30 29 06 07 28 81 8c 71 02 02 04 30 1e 30 19
06 0a 2b 81 05 10 86 48 09 2c 01 02 30 0b 06 09
60 86 48 01 65 03 04 02 02 02 01 18 30 0b 06 09
60 86 48 01 65 03 04 01 19
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To indicate support for the RSA-KEM algorithm coupled with the KDF3
key-derivation function with SHA-512 and the AES Key Wrap symmetric
key-encryption algorithm 256-bit key-encryption key, the
SMIMECapabilities will include the following SMIMECapability value
(in hexadecimal):
30 47 06 0b 2a 86 48 86 f7 0d 01 09 10 03 0e 30
38 30 29 06 07 28 81 8c 71 02 02 04 30 1e 30 19
06 0a 2b 81 05 10 86 48 09 2c 01 02 30 0b 06 09
60 86 48 01 65 03 04 02 03 02 01 20 30 0b 06 09
60 86 48 01 65 03 04 01 2d
Acknowledgements
We thank James Randall, Burt Kaliski, and John Brainard as the
original authors of RFC 5990; this document is based on their work.
We thank the members of the ASC X9F1 working group for their
contributions to drafts of ANS X9.44, which led to RFC 5990.
We thank Blake Ramsdell, Jim Schaad, Magnus Nystrom, Bob Griffin, and
John Linn for helping bring RFC 5990 to fruition.
Authors' Addresses
Russ Housley
Vigil Security, LLC
516 Dranesville Road
Herndon, VA, 20170
United States of America
Email: housley@vigilsec.com
Sean Turner
sn3rd
Washington, DC, 20002
United States of America
Email: sean@sn3rd.com
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