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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.
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 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 in any way secure. This hash function MUST NOT be released as shipping code, it is designed only for use in experimentation.
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.
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.
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.
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.
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.
| [CMS] | Housley, R., “Cryptographic Message Syntax (CMS),” RFC 3852, July 2004. |
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.
| Jim Schaad | |
| Soaring Hawk Consulting | |
| PO Box 675 | |
| Gold Bar, WA 98251 | |
| Email: | jimsch@exmsft.com |