Network Working Group J. Schaad
Internet-Draft Soaring Hawk Consulting
Intended status: Experimental May 5, 2009
Expires: November 6, 2009
Experiment: Hash functions with parameters in CMS and S/MIME
draft-schaad-smime-hash-experiment-00.txt
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Abstract
New hash algorithms are being developed and these algorithms may
include parameters. CMS has not currently defined any hash
algorithms with parameters, but anecdotic evidence suggests that
defining one could cause major problems. In this document we define
just such an algorithm and describe how to use it so that we can run
experiments to find out how bad including hash parameters will be.
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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 describes presents
the details needed to run an experiment so that we can find out just
how bad the situation really is and if we need to either make drastic
changes in implementations or make sure that any hash algorithms
chosen do not have parameters.
In CMS data structures, hash algorithms current exist in the
following locations:
- SignerInfo.digestAlgorithm -- holds the digest algorithm actually
used to compute the hash value over the content.
- DigestedData.digestAlgorithm -- holds the digest algorithm actually
used to compute the hash value over the content
- AuthenticatedData.digestAlgorithm -- holds the digest algorithm
actually used to compute the hash value over the content
- SignedData.digestAlgorithms -- an optional location to place
information about the set of digest algorithms used in computing the
hash value over the content.
- 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. These fields are
mandatory to be filled in.
DigestedData and AthenticatedData are defined such that in the case
of encapsulated data, these fields are parsed before the content is
parsed and thus the data is not needed until the parameters have been
seen.
SignedData is defined such that even for the encapsulated data case,
the value of SignerInfo.digestedAlgorithm 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.
For the case of detached content, the ASN.1 structures need to be
procssed prior to processing the detached content in order to obtain
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the parameters of the hash function. In the case of MIME, this is
the purpose of the micalg field on the multipart/signature content-
type.
When processing multipart/signed messages two paths exists:
1. Process the 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 content (the first part of the multipart)
- Process the ASN.1 and have a potential failure point if a hash
algorithm is required which was not computed.
2. Process the content after the ASN.1. The steps involved are:
- Save the content for later processing
- Parse the ASN.1 and build a list of hash functions based on it's
content
- Create a hasher for each of those algorithms
- Hash the saved 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.
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 in the SignerInfos are
determined. It may be that we need to require population of the
SignedData.digestAlgorithms field if we adopt a parameterized hash
field.
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
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in any way secure. This hash function MUST NOT be released as
shipping code, it is designed only for use in experimentation.
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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 DIGEST-ALGORITHM ::= { IDENTIFIER {tbd} PARAMS OCTET STRING (64) ARE required }
The octet string holds the value of the random XOR string.
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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.
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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 DER ASN.1 encoding. The
parameters and the identifier string are separated by a colon.
Arbitrary amounts of white space may be inserted between any two
characters in the hex encoded string. An example content-type string
would be:
Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1, xor-md5:04400102030405060708090a0b0c0d0e0f00111213141 5161718191a1b1c1d1e1f102122232425262728292a2b2c2d2e2f2031323334353 63738393a3b3c3d3e3f30; boundary=boundar42
I debated if the string should be hex encoded rather than base64
encoding the string. 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
we needed 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.
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6. 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.
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7. Informational References
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852,
July 2004.
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Appendix A. Examples
The examples encoded in this document can be extracted using the perl
script found in [CMS-EXAMPLES]. The certificates and key pairs found
in that document are also used here.
Appendix A.1. Encapsulated Signed Data Example
Appendix A.2. Detached Signed Data Example
Appendix A.3. Multipart Signed Message
Appendix A.4. Digested Data Example
Appendix A.5. Autenticated Data Example
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Author's Address
Jim Schaad
Soaring Hawk Consulting
PO Box 675
Gold Bar, WA 98251
Email: jimsch@exmsft.com
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