S/MIME Working Group R. Housley
Internet Draft RSA Laboratories
expires in six months June 2002
Use of the RSAES-OAEP Key Transport Algorithm in CMS
<draft-ietf-smime-cms-rsaes-oaep-03.txt>
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes the use of the RSAES-OAEP key transport
method of key management within the Cryptographic Message Syntax.
1 Introduction
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PKCS #1 Version 1.5 [PKCS#1v1.5] specifies a widely deployed variant
of the RSA key transport algorithm. PKCS #1 Version 1.5 key
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transport is vulnerable to adaptive chosen ciphertext attacks,
especially when it is used to for key management in interactive
applications. This attack is often referred to as the "Million
Message Attack," and it explained in [RSALABS] and [CRYPTO98].
Exploitation of this vulnerability, which reveals 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
that provide information on the successes or failures of attempted
decryption operations.
The attack is significantly less feasible in store-and-forward
environments, such as S/MIME. RFC 3218 [MMA] discussed the
countermeasures to this attack that are available when PKCS #1
Version 1.5 key transport is used in conjunction with the
Cryptographic Message Syntax (CMS) [CMS].
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.
However, Secure Sockets Layer (SSL) [SSL] and Transport Layer
Security (TLS) [TLS] protocol implementations could include
countermeasures that detect and prevent the Million Message Attack
and other chosen-ciphertext attacks. These countermeasures are
performed within the protocol level. In the interest of long-term
security assurance, it is prudent to adopt an improved cryptographic
technique rather than embedding countermeasures within protocols.
An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
[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) for RSA key
transport.
This document specifies the use of RSAES-OAEP key transport algorithm
in the CMS. The CMS can be used in either a store-and-forward or an
interactive request-response environment.
The CMS supports variety of architectures for certificate-based key
management, particularly the one defined by the PKIX working group
[PROFILE]. PKCS #1 Version 1.5 and PKCS #1 Version 2.0 require the
same RSA public key information. Thus, a certified RSA public key
may be used with either RSA key transport technique.
The CMS uses ASN.1 [X.208-88], the Basic Encoding Rules (BER)
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[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 [STDWORDS]. These key word definitions help make the
intent of standards documents as clear as possible. These key words
are used in this document to help implementers achieve
interoperability.
2 Enveloped-data Conventions
The CMS enveloped-data content type consists of an 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".
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 0, 2, or 3.
The EnvelopedData originatorInfo field is not used for the RSAES-OAEP
key transport algorithm. However, this field MAY be present to
support recipients using other key management algorithms.
The EnvelopedData recipientInfos CHOICE MUST be
KeyTransRecipientInfo. See section 2.2 for further discussion of
KeyTransRecipientInfo.
The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
field MUST be a symmetric encryption algorithm identifier.
The EnvelopedData unprotectedAttrs MAY be present.
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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 0 or 2. If the
RecipientIdentifier uses the issuerAndSerialNumber alternative, then
the version MUST be 0. If the RecipientIdentifier uses the
subjectKeyIdentifier alternative, 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 [3DES] content-
encryption key, implementations MUST adjust the parity bits to ensure
odd parity for each octet of 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 2437 [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:
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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 }
pSpecifiedEmptyIdentifier AlgorithmIdentifier ::=
{ id-pSpecified, nullOctetString }
nullOctetString OCTET STRING (SIZE (0)) ::= { ''H }
id-sha1 OBJECT IDENTIFIER ::= { iso(1)
identified-organization(3) oiw(14)
secsig(3) algorithms(2) 26 }
pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) }
id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 }
id-pSpecified OBJECT IDENTIFIER ::= { pkcs-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. Implementations that perform encryption MUST omit the
hashFunc field when SHA-1 is used, indicating that the default
algorithm was used. Implementations that perform decryption MUST
recognize both the id-sha1 object identifier and an absent
hashFunc field as an indication that SHA-1 was used.
maskGenFunc identifies the mask generation function.
Implementations MUST support MFG1 [PKCS#1v2.0]. MFG1 requires a
one-way hash function, and it is identified in the parameter field
of the MFG1 algorithm identifier. Implementations MUST support
SHA-1 [SHA1]. The MFG1 algorithm identifier is comprised of the
id-mgf1 object identifier and a parameter that contains the
algorithm identifier of the one-way hash function employed with
MFG1. The SHA-1 algorithm identifier is comprised of the id-sha1
object identifier and a parameter of NULL. Implementations that
perform encryption MUST omit the maskGenFunc field when MFG1 with
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SHA-1 is used, indicating that the default algorithm was used.
Implementations that perform decryption MUST recognize both the
id-mgf1 and id-sha1 object identifiers as well as an absent
maskGenFunc field as an indication that MFG1 with SHA-1 was used.
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.
Implementations MUST support a zero length P value.
Implementations that perform encryption MUST omit the pSourceFunc
field when a zero length P value is used, indicating that the
default value was used. Implementations that perform decryption
MUST recognize both the id-pSpecified object identifier and an
absent pSourceFunc field as an indication that a zero length P
value was used.
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
RSAES-OAEP algorithm.
The SMIMECapability SEQUENCE representing RSAES-OAEP MUST include the
id-RSAES-OAEP object identifier in the capabilityID field and MUST
include the RSAES-OAEP-Default-Identifier SEQUENCE in the parameters
field.
rSAES-OAEP-Default-Identifier AlgorithmIdentifier ::=
{ id-RSAES-OAEP,
{ sha1Identifier,
mgf1SHA1Identifier,
pSpecifiedEmptyIdentifier } }
When all of the default settings are selected, the SMIMECapability
SEQUENCE representing RSAES-OAEP MUST be DER-encoded as the following
hexadecimal string:
30 0D 06 09 2A 86 48 86 F7 0D 01 01 07 30 00
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5 References
This section provides normative and informative references.
5.1 Normative References
3DES American National Standards Institute. ANSI X9.52-1998,
Triple Data Encryption Algorithm Modes of Operation. 1998.
CMS Housley, R. Cryptographic Message Syntax. RFC <TBD>.
<TBD DATE>.
MSG Ramsdell, B. S/MIME Version 3 Message Specification.
RFC 2633. June 1999.
PKCS#1v2.0 Kaliski, B. PKCS #1: RSA Encryption, Version 2.0.
RFC 2437. October 1998.
PROFILE Housley, R., W. Polk, W. Ford, and D. Solo. Internet
X.509 Public Key Infrastructure: Certificate and
Certificate Revocation List (CRL) Profile. RFC 3280.
April 2002.
SHA1 National Institute of Standards and Technology.
FIPS Pub 180-1: Secure Hash Standard. 17 April 1995.
STDWORDS Bradner, S. Key Words for Use in RFCs to Indicate
Requirement Levels. BCP 14, RFC 2119. March 1997.
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.
5.2 Informative References
CRYPTO98 Bleichenbacher, D. "Chosen Ciphertext Attacks Against
Protocols Based on the RSA Encryption Standard PKCS #1,"
in H. Krawczyk (editor), Advances in Cryptology - CRYPTO '98
Proceedings, Lecture Notes in Computer Science 1462 (1998),
Springer-Verlag, pp. 1-12.
MMA Rescorla, E. Preventing the Million Message Attack on
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Cryptographic Message Syntax. RFC 3218. January 2002.
PKCS#1v1.5 Kaliski, B. PKCS #1: RSA Encryption, Version 1.5.
RFC 2313. March 1998.
RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness
Recommendations for Security. RFC 1750. December 1994.
RSALABS Bleichenbacher, D., B. Kaliski, and J. Staddon.
Recent Results on PKCS #1: RSA Encryption Standard.
RSA Laboratories' Bulletin No. 7, June 26, 1998.
[http://www.rsasecurity.com/rsalabs/bulletins]
SSL Freier, A., P. Karlton, and P. Kocher. The SSL Protocol,
Version 3.0. Netscape Communications. November 1996.
[http://wp.netscape.com/eng/ssl3/draft302.txt]
TLS Dierks, T. and C. Allen. The TLS Protocol Version 1.0.
RFC 2246. January 1999.
6 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
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.
7 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. Further, I extend a special thanks to Burt Kaliski.
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8 Author Address
Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Herndon, VA 20170
USA
rhousley@rsasecurity.com
Appendix A ASN.1 Module
CMS-RSAES-OAEP {iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-rsaes-oaep(20) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL --
IMPORTS
-- From PKIX Certificate and CRL Profile
AlgorithmIdentifier
FROM PKIXExplicit88 { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7)
id-mod(0) id-pkix1-explicit(18) };
pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-1(1) }
id-RSAES-OAEP OBJECT IDENTIFIER ::= { pkcs-1 7 }
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 }
pSpecifiedEmptyIdentifier AlgorithmIdentifier ::=
{ id-pSpecified, nullOctetString }
nullOctetString OCTET STRING (SIZE (0)) ::= { ''H }
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id-sha1 OBJECT IDENTIFIER ::= { iso(1)
identified-organization(3) oiw(14)
secsig(3) algorithms(2) 26 }
id-mgf1 OBJECT IDENTIFIER ::= { pkcs-1 8 }
id-pSpecified OBJECT IDENTIFIER ::= { pkcs-1 9 }
rSAES-OAEP-Default-Identifier AlgorithmIdentifier ::=
{ id-RSAES-OAEP,
{ sha1Identifier,
mgf1SHA1Identifier,
pSpecifiedEmptyIdentifier } }
END
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Full Copyright Statement
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included on all such copies and derivative works. In addition, the
ASN.1 module presented in Appendix A may be used in whole or in part
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