S/MIME Working Group R. Housley
Internet Draft SPYRUS
expires in six months February 2000
CMS RSAES-OAEP Conventions
<draft-ietf-smime-cms-rsaes-oaep-00.txt>
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes the use of the RSAES-OAEP key transport
method of key management within the Cryptographic Message Syntax
[CMS].
This draft is being discussed on the "ietf-smime" mailing list. To
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1 Introduction
When the variant of the RSA algorithm specified in PKCS #1 Version
1.5 [PKCS#1v1.5] is used for key management, it is vulnerable to
adaptive chosen ciphertext attacks. The use of PKCS #1 Version 1.5
key transport in interactive applications is especially vulnerable.
Exploitation of this identified vulnerability, revealing the result
of a particular RSA decryption, requires access to an oracle which
will respond to hundreds of thousands of ciphertexts, which are
constructed adaptively in response to previously-received replies
providing information on the successes or failures of attempted
decryption operations. As a result, the attack appears significantly
less feasible in store-and-forward environments, such as S/MIME.
When PKCS #1 Version 1.5 key transport is applied as an intermediate
encryption layer within an interactive request-response
communications environment, exploitation could be more feasible.
An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
[PKCS#1v2.0]. This new document supersedes RFC 2313. PKCS #1
Version 2.0 preserves support for the encryption padding format
defined in PKCS #1 Version 1.5 [PKCS#1v1.5], and it also defines a
new alternative. To resolve the adaptive chosen ciphertext
vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
is used to provide confidentiality.
This document specifies the use of RSAES-OAEP key transport algorithm
in the Cryptographic Message Syntax (CMS) [CMS].
CMS supports variety of architectures for certificate-based key
management, particularly the one defined by the PKIX working group
[PROFILE].
CMS values are generated using ASN.1 [X.208-88], using the Basic
Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules
(DER) [X.509-88].
Throughout this document, when the terms MUST, MUST NOT, SHOULD and
MAY are used in capital letters, their use conforms to the
definitions in [MUSTSHOULD]. [MUSTSHOULD] defines these key words to
help make the intent of standards track documents as clear as
possible. The same key words are used in this document to help
implementers achieve interoperability. Implementations that claims
compliance with this document MUST provide the capabilities as
indicated by the MUST, MUST NOT, SHOULD and MAY terms.
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2 Enveloped-data Conventions
The CMS enveloped-data content type consists of encrypted content and
wrapped content-encryption keys for one or more recipients. The RSAES-
OAEP key transport algorithm is used to wrap the content-encryption key
for one recipient.
Compliant software MUST meet the requirements for constructing an
enveloped-data content type stated in [CMS] Section 6, "Enveloped-data
Content Type". [CMS] Section 6 should be studied before reading this
section, because this section does not repeat the [CMS] text.
A content-encryption key MUST be randomly generated for each instance of
an enveloped-data content type. The content-encryption key is used to
encipher the content.
2.1 EnvelopedData Fields
The enveloped-data content type is ASN.1 encoded using the
EnvelopedData syntax. The fields of the EnvelopedData syntax must be
populated as follows:
The EnvelopedData version MUST be either 0 or 2.
The EnvelopedData originatorInfo field MUST be absent.
The EnvelopedData recipientInfos CHOICE MUST be
KeyTransRecipientInfo. See section 2.2 for further discussion of
KeyTransRecipientInfo.
The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
field MUST be specify a symmetric encryption algorithm.
Implementations MUST support the encryption of Triple-DES content-
encryption keys, but implementations MAY support other algorithms as
well.
The EnvelopedData unprotectedAttrs MAY be present.
2.2 KeyTransRecipientInfo Fields
The enveloped-data content type is ASN.1 encoded using the
EnvelopedData syntax. The fields of the EnvelopedData syntax must be
populated as follows:
The KeyTransRecipientInfo version MUST be either 0 or 2. If the
RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the
version MUST be 0. If the RecipientIdentifier is
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subjectKeyIdentifier, then the version MUST be 2.
The KeyTransRecipientInfo RecipientIdentifier provides two
alternatives for specifying the recipient's certificate, and thereby
the recipient's public key. The recipient's certificate must contain
a RSA public key. The content-encryption key is encrypted with the
recipient's RSA public key. The issuerAndSerialNumber alternative
identifies the recipient's certificate by the issuer's distinguished
name and the certificate serial number; the subjectKeyIdentifier
identifies the recipient's certificate by the X.509
subjectKeyIdentifier extension value.
The KeyTransRecipientInfo keyEncryptionAlgorithm specifies that the
RSAES-OAEP algorithm, and its associated parameters, was used to
encrypt the content-encryption key for the recipient. The key-
encryption process is described in [PKCS#1v2.0]. See section 3 of
this document for the algorithm identifier and the parameter syntax.
The KeyTransRecipientInfo encryptedKey is the result of encrypting
the content-encryption key in the recipient's RSA public key using
the RSAES-OAEP algorithm. When using a Triple-DES content-encryption
key, implementations MUST adjust the parity bits for each DES key
comprising the Triple-DES key prior to RSAES-OAEP encryption.
3 RSAES-OAEP Algorithm Identifiers and Parameters
The RSAES-OAEP key transport algorithm is the RSA encryption scheme
defined in RFC 2347 [PKCS#1v2.0], where the message to be encrypted
is the content-encryption key. The algorithm identifier for RSAES-
OAEP is:
id-RSAES-OAEP OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 7 }
The AlgorithmIdentifier parameters field must be present, and the
parameters field must contain RSAES-OAEP-params. RSAES-OAEP-params
have the following syntax:
RSAES-OAEP-params ::= SEQUENCE {
hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier,
maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier,
pSourceFunc [2] AlgorithmIdentifier DEFAULT pSpecifiedEmptyIdentifier }
sha1Identifier ::= AlgorithmIdentifier {
id-sha1, NULL }
mgf1SHA1Identifier ::= AlgorithmIdentifier {
id-mgf1, sha1Identifier }
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pSpecifiedEmptyIdentifier ::= AlgorithmIdentifier {
id-pSpecified, OCTET STRING SIZE (0) }
id-sha1 OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) oiw(14) secsig(3) algorithms(2) 26 }
id-mgf1 OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 8 }
id-pSpecified OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 9 }
The fields of type RSAES-OAEP-params have the following meanings:
hashFunc identifies the one-way hash function. Implementations
MUST support SHA-1 [SHA1]. The SHA-1 algorithm identifier is
comprised of the id-sha1 object identifier and a parameter of
NULL.
maskGenFunc identifies the mask generation function.
Implementations MUST support MFG1 [PKCS#1v2.0]. MFG1 requires a
one-way hash function, and it is identified in the parameter field
of the algorithm identifier. Implementations MUST support SHA-1
[SHA1]. The MFG1 algorithm identifier is comprised of the id-mgf1
object identifier and a parameter of the SHA-1 algorithm
identifier. Again, the SHA-1 algorithm identifier is comprised of
the id-sha1 object identifier and a parameter of NULL.
pSourceFunc identifies the source (and possibly the value) of the
encoding parameters, commonly called P. Implementations MUST
represent P by an algorithm identifier, id-pSpecified, indicating
that P is explicitly provided as an OCTET STRING in the
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.
4 SMIMECapabilities Attribute Conventions
RFC 2633, Section 2.5.2 defines the SMIMECapabilities signed
attribute (defined as a SEQUENCE of SMIMECapability SEQUNCEs) 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
RSAES-OAEP algorithm.
The SMIMECapability SEQUENCE representing RSAES-OAEP MUST include the
id-RSAES-OAEP object identifier in the capabilityID field and MUST
include a RSAES-OAEP-Default-Identifier SEQUENCE in the parameters
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field.
RSAES-OAEP-Default-Identifier ::= AlgorithmIdentifier {
id-RSAES-OAEP, {
sha1Identifier, mgf1SHA1Identifier, pSpecifiedEmptyIdentifier } }
The SMIMECapability SEQUENCE representing RSAES-OAEP MUST be DER-
encoded as follows: {{{TBD}}}.
References
CMS Housley, R. Cryptographic Message Syntax. RFC 2630.
June 1999.
MUSTSHOULD Bradner, S. Key Words for Use in RFCs to Indicate
Requirement Levels. BCP 14, RFC 2119. March 1997.
PKCS#1v1.5 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5.
RFC 2313. March 1998.
PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0.
RFC 2347. October 1998.
PROFILE Housley, R., W. Ford, W. Polk, and D. Solo. Internet
X.509 Public Key Infrastructure: Certificate and CRL
Profile. RFC 2459. January 1999.
RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness
Recommendations for Security. RFC 1750. December 1994.
SHA1 National Institute of Standards and Technology.
FIPS Pub 180-1: Secure Hash Standard. 17 April 1995.
X.208-88 CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
X.209-88 CCITT. Recommendation X.209: Specification of Basic Encoding
Rules for Abstract Syntax Notation One (ASN.1). 1988.
X.509-88 CCITT. Recommendation X.509: The Directory - Authentication
Framework. 1988.
Security Considerations
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
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the content-encryption key may result in disclosure of the associated
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
numbers. The use of inadequate pseudo-random number generators
(PRNGs) to generate cryptographic keys can result in little or no
security. An attacker may find it much easier to reproduce the PRNG
environment that produced the keys, searching the resulting small set
of possibilities, rather than brute force searching the whole key
space. The generation of quality random numbers is difficult. RFC
1750 [RANDOM] offers important guidance in this area.
Acknowledgments
This document is the result of contributions from many professionals.
I appreciate the hard work of all members of the IETF S/MIME Working
Group. I wish to extend a special thanks to Burt Kaliski.
Author Address
Russell Housley
SPYRUS
381 Elden Street
Suite 1120
Herndon, VA 20170
USA
housley@spyrus.com
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ASN.1 module presented in Appendix A may be used in whole or in part
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