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Internet Engineering Task Force (IETF) J. Schaad
Request for Comments: 6210 Soaring Hawk Consulting
Category: Experimental April 2011
ISSN: 2070-1721
Experiment: Hash Functions with Parameters
in the Cryptographic Message Syntax (CMS) and S/MIME
Abstract
New hash algorithms are being developed that may include parameters.
Cryptographic Message Syntax (CMS) has not currently defined any hash
algorithms with parameters, but anecdotal evidence suggests that
defining one could cause major problems. This document defines just
such an algorithm and describes how to use it so that experiments can
be run to find out how bad including hash parameters will be.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6210.
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RFC 6210 CMS Parameterized Hash April 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. XOR-MD5 Digest Algorithm . . . . . . . . . . . . . . . . . . . 5
3. ASN.1 Encoding . . . . . . . . . . . . . . . . . . . . . . . . 6
4. CMS ASN.1 Handling . . . . . . . . . . . . . . . . . . . . . . 6
5. MIME Handling . . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 9
A.1. Encapsulated Signed Data Example . . . . . . . . . . . . . 9
A.2. Multipart Signed Message . . . . . . . . . . . . . . . . . 10
A.3. Authenticated Data Example . . . . . . . . . . . . . . . . 12
Appendix B. 2008 ASN.1 Module . . . . . . . . . . . . . . . . . . 13
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1. Introduction
At the present time, all hash algorithms that are used in
Cryptographic Message Syntax (CMS) implementations are defined as
having no parameters. Anecdotal evidence suggests that if a hash
algorithm is defined that does require the presence of parameters,
there may be extensive problems. This document presents the details
needed to run an experiment so that the community can find out just
how bad the situation really is and, if needed, either make drastic
changes in implementations or make sure that any hash algorithms
chosen do not have parameters.
In CMS data structures, hash algorithms currently exist in the
following locations:
o SignerInfo.digestAlgorithm - holds the digest algorithm used to
compute the hash value over the content.
o DigestedData.digestAlgorithm - holds the digest algorithm used to
compute the hash value over the content.
o AuthenticatedData.digestAlgorithm - holds the digest algorithm
used to compute the hash value over the content.
o SignedData.digestAlgorithms - an optional location to hold the set
of digest algorithms used in computing the hash value over the
content.
o multipart/signed micalg - holds a textual indicator of the hash
algorithm for multipart signed MIME messages.
The first three locations hold the identification of a single hash,
and would hold the parameters for that hash. It's mandatory to fill
these fields.
The ASN.1 structures defined for the DigestedData and
AuthenticatedData types place the digest algorithm field before the
encapsulated data field. This means that the hash algorithm
(including the parameters) is fully defined, and therfore can be
instantiated, before the hash function would start hashing the
encapsulated data.
In the ASN.1 defined for the SignedData type, the value of
SignerInfo.digestAlgorithm is not seen until the content has been
processed. This is the reason for the existence of the
SignedData.digestAlgorithms field, so that the set of all digest
algorithms used can be seen prior to the content being processed. It
is not currently mandatory to fill in this field, and the signature
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validation process is supposed to succeed even if this field is
absent. (RFC 5652 says signature validation MAY fail if the digest
algorithm is absent.)
For the case of detached content, the ASN.1 structures need to be
processed before processing the detached content in order to obtain
the parameters of the hash function. The MIME multipart/signature
content type attempts to avoid this problem by defining a micalg
field that contains the set of hash algorithms (with parameters) so
that the hash functions can be set up prior to processing the
content.
When processing multipart/signed messages, two paths exists:
1. Process the message content before the ASN.1. The steps involved
are:
* Get a set of hash functions by looking at the micalg parameter
and potentially add a set of generic algorithms.
* Create a hasher for each of those algorithms.
* Hash the message content (the first part of the multipart).
* Process the ASN.1 and have a potential failure point if a hash
algorithm is required but was not computed.
2. Process the message content after the ASN.1. The steps involved
are:
* Save the message content for later processing.
* Parse the ASN.1 and build a list of hash functions based on
its content.
* Create a hasher for each of those algorithms.
* Hash the saved message content.
* Perform the signature validation.
The first path allows for single-pass processing, but has the
potential that a fallback path needs to be added in some cases. The
second path does not need a fallback path, but does not allow for
single-pass processing.
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The fallback path above may also be needed for the encapsulated
content case. Since it is optional to place hash algorithms in the
SignedData.digestAlgorithms field, the content will be completely
parsed before the set of hash algorithms used in the various
SignerInfo structures are determined. It may be that an update to
CMS is required to make population of the SignedData.digestAlgorithms
field mandatory, in the event that a parameterized hash algorithm is
adopted.
In this document, a new hash function is created that is based on the
XOR operator and on MD5. MD5 was deliberately used as the basis of
this digest algorithm since it is known to be insecure, and I do not
want to make any statements that the hash algorithm designed here is
in any way secure. This hash function MUST NOT be released as
shipping code, it is designed only for use in experimentation. An
example of a parameterized hash algorithm that might be standardized
is a scheme developed by Shai Halevi and Hugo Krawczyk [RANDOM-HASH].
1.1. Notation
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 [RFC2119].
2. XOR-MD5 Digest Algorithm
The XOR-MD5 digest algorithm has been designed to use two existing
operators, XOR and the MD5 hash algorithm [MD5]. The hash algorithm
works as follows:
1. A random XOR string consisting of exactly 64 bytes is created.
2. The input content is broken up into 64-byte blocks. The last
block may be less that 64 bytes.
3. Each block is XOR-ed with the random string. The last block uses
the same number of bits from the random string as it contains.
4. The resulting string is run through the MD5 hash function.
The length of the XOR string was designed to match the barrel size of
the MD5 hash function.
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3. ASN.1 Encoding
The following ASN.1 is used to define the algorithm:
mda-xor-md5-EXPERIMENT DIGEST-ALGORITHM ::= {
IDENTIFIER id-alg-MD5-XOR-EXPERIMENT
PARAMS TYPE MD5-XOR-EXPERIMENT ARE required
}
id-alg-MD5-XOR-EXPERIMENT OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) id-alg(3) 13
}
MD5-XOR-EXPERIMENT ::= OCTET STRING (SIZE(64))
The octet string holds the value of the random XOR string.
4. CMS ASN.1 Handling
The algorithm is added to the DigestAlgorithmSet in [CMS].
When this algorithm is used in a signed message, it is REQUIRED that
the algorithm be placed in the SignedData.digestAlgorithms sequence.
The algorithm MUST appear in the sequence at least once for each
unique set of parameters. The algorithm SHOULD NOT appear multiple
times with the same set of parameters.
5. MIME Handling
This section defines the string that appears in the micalg parameter.
The algorithm is identified by the string xor-md5. The parameters
for the algorithm are the hex-encoded Distinguished Encoding Rules
(DER) ASN.1 encoding. The parameters and the identifier string are
separated by a colon. One of the issues that needs to be addressed
is the fact that this will generate very long data values for
parameters. These will be too long for many systems to deal with.
The issue of how to deal with this has been addressed in [RFC2231] by
creating a method to fragment values. An example content-type string
that has been fragmented is:
Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg*0="sha1, xor-md5:04400102030405060708090a0b0c0d0e0f0011";
micalg*1="12131415161718191a1b1c1d1e1f102122232425262728292a2b";
micalg*2="2c2d2e2f203132333435363738";
micalg*3="393a3b3c3d3e3f30"; boundary=boundar42
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Arguments could be made that the string should be base64 encoded
rather than hex encoded. The advantage is that the resulting
encoding is shorter. This could be significant if there are a
substantial number of parameters and of a substantial size. Even
with the above example, it was necessary to break the encoding across
multiple lines. The downside would be the requirement that the
micalg parameter always be quoted.
It may be reasonable to require that whitespace be inserted only on
encoding boundaries, but it seems to be overly restrictive.
6. IANA Considerations
All identifiers are assigned out of the S/MIME OID arc.
7. Security Considerations
The algorithm XOR-MD5 is not designed for general-purpose use. The
hash algorithm included here is designed for running this experiment
and nothing more.
This document makes no representation that XOR-MD5 is a secure digest
algorithm. I believe that the algorithm is no more secure than MD5,
and I consider MD5 to be a broken hash algorithm for many purposes.
One known issue with the algorithm at present is the fact that the
XOR pattern is always 64 bytes long, even if the data is shorter.
This means that there is a section of the data than can be
manipulated without changing the hash. In a real algorithm, this
should either be truncated or forced to a known value.
8. References
8.1. Normative References
[ASN.1-2008] ITU-T, "ITU-T Recommendations X.680, X.681, X.682, and
X.683", 2008.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 5652, September 2009.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm",
RFC 1321, April 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and
Encoded Word Extensions: Character Sets, Languages, and
Continuations", RFC 2231, November 1997.
[SMIME-MSG] Ramsdell, B. and S. Turner, "Secure/Multipurpose
Internet Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
8.2. Informative References
[CMS-ASN] Hoffman, P. and J. Schaad, "New ASN.1 Modules for
Cryptographic Message Syntax (CMS) and S/MIME",
RFC 5911, June 2010.
[RANDOM-HASH] Halevi, S. and H. Krawczyk, "Strengthening Digital
Signatures via Random Hashing", January 2007,
<http://webee.technion.ac.il/~hugo/rhash/rhash.pdf>.
[RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)",
RFC 5912, June 2010.
[SMIME-EXAMPLES]
Hoffman, P., "Examples of S/MIME Messages", RFC 4134,
July 2005.
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Appendix A. Examples
Provided here are a set of simple S/MIME messages [SMIME-MSG] that
are for testing. The content used is the same as that found in
Section 2.1 of [SMIME-EXAMPLES]. The certificates and key pairs
found in [SMIME-EXAMPLES] are also used here.
The Perl script in Appendix A of [SMIME-EXAMPLES] can be used to
extract the binary examples from this file. The MIME examples can be
extracted with a standard text editor.
Note: The examples presented here have not been independently
verified. I was unable to use the Microsoft APIs because of the new
cryptographic hash algorithm. However, for the purposes of this
experiment, I believe that the form of the messages, which can be
verified visually as correct, is more important than the question of
the message validating.
A.1. Encapsulated Signed Data Example
This section contains a detached signed data example. The content
was hashed with the MD5-XOR algorithm defined in this document. The
signature is performed using RSA with MD5. The signature is wrapped
as an embedded signed mime message.
MIME-Version: 1.0
To: BobRSA@example.com
From: AliceDss@example.com
Subject: MD5-XOR example message
Message-Id: <34567809323489fd.esc@example.com>
Date: Wed, 16 Dec 2010 23:13:00 -0500
Content-Type: application/pkcs7-mime; smime-type=signed-data;
name=smime.p7m;
micalg*0="xor-md5: 0440010203405060708090a0b0c0d0e0f10";
micalg*1="111213415161718191a1b1c1d1e1f20212223425262728292a2b2c";
micalg*2="2d2e2f30313233435363738393a3b3c3d3e3f40"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m
MIIEqAYJKoZIhvcNAQcCoIIEmTCCBJUCAQExUTBPBgsqhkiG9w0BCRADDQRAAQIDBAUGBw
gJCgsMDQ4PEBESEwQVFhcYGRobHB0eHyAhIiMEJSYnKCkqKywtLi8wMTIzBDU2Nzg5Ojs8
PT4/QDArBgkqhkiG9w0BBwGgHgQcVGhpcyBpcyBzb21lIHNhbXBsZSBjb250ZW50LqCCAi
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voPGXzjPHCyqaqfrAgMBAAGjfzB9MAwGA1UdEwEB/wQCMAAwDgYDVR0PAQH/BAQDAgUgMB
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RFC 6210 CMS Parameterized Hash April 2011
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A.2. Multipart Signed Message
This section contains a detached signed data example. The content
was hashed with the MD5-XOR algorithm defined in this document. The
signature is performed using RSA with MD5. The signature is wrapped
as a detached signed mime message.
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RFC 6210 CMS Parameterized Hash April 2011
MIME-Version: 1.0
To: User2@example.com
From: BobRSA@example.com
Subject: MD5-XOR signing example
Message-Id: <091218002550300.249@example.com>
Date: Fri, 18 Dec 2010 00:25:21 -0300
Content-Type: multipart/signed;
micalg*0="xor-md5: 0440010203405060708090a0b0c0d0e0f10";
micalg*1="111213415161718191a1b1c1d1e1f20212223425262728292a2b2c2d2e";
micalg*2="2f30313233435363738393a3b3c3d3e3f40";
boundary="----=_NextBoundry____Fri,_18_Dec_2009_00:25:21";
protocol="application/pkcs7-signature"
This is a multi-part message in MIME format.
------=_NextBoundry____Fri,_18_Dec_2009_00:25:21
This is some sample content.
------=_NextBoundry____Fri,_18_Dec_2009_00:25:21
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s
MIIEiAYJKoZIhvcNAQcCoIIEeTCCBHUCAQExUTBPBgsqhkiG9w0BCRADDQRAAQIDBAUGBw
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IB3zCCAdsCAQEwJjASMRAwDgYDVQQDEwdDYXJsUlNBAhBGNGvHgABWvBHTbi7NXXHQME8G
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coKSorLC0uLzAxMjMENTY3ODk6Ozw9Pj9AoIHKMBgGCSqGSIb3DQEJAzELBgkqhkiG9w0B
BwEwHAYJKoZIhvcNAQkFMQ8XDTEwMTIxMDIzMjUwMFowHwYJKoZIhvcNAQkEMRIEEJZprm
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BwgJCgsMDQ4PEBESEwQVFhcYGRobHB0eHyAhIiMEJSYnKCkqKywtLi8wMTIzBDU2Nzg5Oj
s8PT4/QKENBgkqhkiG9w0BAQQFADANBgkqhkiG9w0BAQQFAASBgEDMeyAkXMYqg/wW2B3P
i8HWwGnZVA/4muJJ7+dEPacv3bRqE7n4dP0vXIYR7TJ1eRJk9uB/wry2fRPcnG3Y/Rn0Jy
CqXsb+dXXfwOGK/rvLvJOloXUCy4+HxQk6eaYIBrjiVIUgZjpZXGJcZg2xq5yH1e4aw5Ov
fQlfQXPiKp1l
------=_NextBoundry____Fri,_18_Dec_2009_00:25:21--
Schaad Experimental [Page 11]
RFC 6210 CMS Parameterized Hash April 2011
A.3. Authenticated Data Example
This section contains an authenticated data example. The content was
hashed with the MD5-XOR algorithm defined in this document. The
authentication was done with the HMAC-SHA1 algorithm. The key is
transported using RSA encryption to BobRSASignByCarl certificate.
MIME-Version: 1.0
To: BobRSA@example.com
From: AliceDss@example.com
Subject: MD5-XOR example message
Message-Id: <34567809323489fd.esc@example.com>
Date: Wed, 16 Dec 2010 23:13:00 -0500
Content-Type: application/pkcs7-mime; smime-type=authenticated-data;
name=smime.p7m;
micalg*0="xor-md5: 0440010203405060708090a0b0c0d0e0f10";
micalg*1="111213415161718191a1b1c1d1e1f20212223425262728292a2b2c2d2e";
micalg*2="2f30313233435363738393a3b3c3d3e3f40"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m
MIICRQYLKoZIhvcNAQkQAQKgggI0MIICMAIBADGBwDCBvQIBADAmMBIxEDAOBgNVBAMMB0
NhcmxSU0ECEEY0a8eAAFa8EdNuLs1dcdAwDQYJKoZIhvcNAQEBBQAEgYCH70EpEikY7deb
859YJRAWfFondQv1D4NFltw6C1ceheWnlAU0C2WEXr3LUBXZp1/PSte29FnJxu5bXCTn1g
elMm6zNlZNWNd0KadVBcaxi1n8L52tVM5sWFGJPO5cStOyAka2ucuZM6iAnCSkn1Ju7fgU
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|>ad.bin
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|EIbVbg2xql
|<ad.bin
Schaad Experimental [Page 12]
RFC 6210 CMS Parameterized Hash April 2011
Appendix B. 2008 ASN.1 Module
The ASN.1 module defined uses the 2008 ASN.1 definitions found in
[ASN.1-2008]. This module contains the ASN.1 module that contains
the required definitions for the types and values defined in this
document. The module uses the class defined in [CMS-ASN] and
[RFC5912].
MD5-HASH-EXPERIMENT
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0)
id-mod-MD5-XOR-EXPERIMENT(999) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
-- Cryptographic Message Syntax (CMS) [CMS]
DigestAlgorithmIdentifier, MessageAuthenticationCodeAlgorithm,
SignatureAlgorithmIdentifier, DIGEST-ALGORITHM
FROM CryptographicMessageSyntax-2009
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-2004-02(41) }
-- Common PKIX structures [RFC5912]
ATTRIBUTE
FROM PKIX-CommonTypes-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkixCommon-02(57)};
mda-xor-md5-EXPERIMENT DIGEST-ALGORITHM ::= {
IDENTIFIER id-alg-MD5-XOR-EXPERIMENT
PARAMS TYPE MD5-XOR-EXPERIMENT ARE required
}
id-alg-MD5-XOR-EXPERIMENT OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) id-alg(3) 13
}
MD5-XOR-EXPERIMENT ::= OCTET STRING (SIZE(64))
END
Schaad Experimental [Page 13]
RFC 6210 CMS Parameterized Hash April 2011
Author's Address
Jim Schaad
Soaring Hawk Consulting
EMail: ietf@augustcellars.com
Schaad Experimental [Page 14]