One document matched: draft-schaad-smime-hash-experiment-01.xml

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<!ENTITY rfc4134 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.4134.xml'>
<rfc ipr="trust200902" docName="draft-schaad-smime-hash-experiment-01" category="exp">
<front>
<title abbrev="CMS Paramertized Hash">Experiment: Hash functions with parameters in CMS and S/MIME</title>
<author initials="J." surname="Schaad" fullname="Jim Schaad">
<organization>Soaring Hawk Consulting</organization>
<address>
<postal>
<street>PO Box 675</street>
<city>Gold Bar</city>
<region>WA</region>
<code>98251</code>
</postal>
<email>jimsch@exmsft.com</email>
</address>
</author>
<date/>
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>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.</t>
<t>In CMS data structures, hash algorithms current exist in the following locations:</t>
<t>- SignerInfo.digestAlgorithm -- holds the digest algorithm actually used to compute the hash value over the content.</t>
<t>- DigestedData.digestAlgorithm -- holds the digest algorithm actually used to compute the hash value over the content</t>
<t>- AuthenticatedData.digestAlgorithm -- holds the digest algorithm actually used to compute the hash value over the content</t>
<t>- SignedData.digestAlgorithms -- an optional location to place information about the set of digest algorithms used in computing the hash value over the content.</t>
<t>- multipart/signed micalg - Holds a textual indicator of the hash algorithm for multipart signed MIME messages.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>When processing multipart/signed messages two paths exists:</t>
<t>1.  Process the content before the ASN.1.  The steps involved are:</t>
<t>- Get a set of hash functions by looking at the micalg parameter and potentially add a set of generic algorithms</t>
<t>- Create a hasher for each of those algorithms</t>
<t>- Hash the content (the first part of the multipart)</t>
<t>- Process the ASN.1 and have a potential failure point if a hash algorithm is required which was not computed.</t>
<t>2.  Process the content after the ASN.1.  The steps involved are:</t>
<t>- Save the content for later processing</t>
<t>- Parse the ASN.1 and build a list of hash functions based on it's content</t>
<t>- Create a hasher for each of those algorithms</t>
<t>- Hash the saved content</t>
<t>- Perform the signature validation.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="XOR-MD5 Digest Algorithm">
<t>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:</t>
<t>1.  A random XOR string consisting of exactly 64 bytes is created.</t>
<t>2.  The input content is broken up into 64 byte blocks.  The last block may be less that 64 bytes.</t>
<t>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.</t>
<t>4.  The resulting string is run through the MD5 hash function.</t>
<t>The length of the XOR string was designed to match the barrel size of the MD5 hash function.</t>
</section>
<section title="ASN.1 Encoding">
<t>The following ASN.1 is used to define the algorithm:</t>
<figure>
<artwork>
mda-xor-md5 DIGEST-ALGORITHM ::= {
    IDENTIFIER {tbd}
    PARAMS OCTET STRING (64) ARE required
}
</artwork>
</figure>
<t>The octet string holds the value of the random XOR string.</t>
</section>
<section title="CMS ASN.1 Handling">
<t>The algorithm is added to the DigestAlgorithmSet in [CMS].</t>
<t>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.</t>
</section>
<section title="MIME handling">
<t>This section defines the string that appears in the micalg parameter.</t>
<t>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:</t>
<figure>
<artwork>Content-Type: multipart/signed; protocol="application/pkcs7-signature";
          micalg=sha1, xor-md5:04400102030405060708090a0b0c0d0e0f00111213141
          5161718191a1b1c1d1e1f102122232425262728292a2b2c2d2e2f2031323334353
          63738393a3b3c3d3e3f30;
         boundary=boundar42
</artwork>
</figure>
<t>Arguments could be made that the string should be base64 encoded rather than hex 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.</t>
<t>It may be reasonable to require that whitespace be inserted only on encoding boundaries, but it seems to be overly restrictive.</t>
</section>
<section title="Security Considerations">
<t>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.</t>
<t>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.</t>
<t>One known issue with the algorithm as 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.</t>
</section>
</middle>
<back>
<references title="Informational References">
<reference anchor="CMS">
<front>
<title>Cryptographic Message Syntax (CMS)</title>
<author initials="R." surname="Housley">
</author>
<date month="July" year="2004">
</date>
</front>
<seriesInfo name="RFC" value="3852">
</seriesInfo>
</reference>
&rfc4134;
</references>
<appendix title="Examples">
<t>Provided here are a set of examples that are provided for testing.   The content used is the same as that found in Section 2.1 of <xref target="RFC4134"/>.  The The certificates and key pairs found in <xref target="RFC4134"/> are also used here.</t>
<t>The perl script in <xref target="RFC4134"/> can be used to extract the binary examples from this file.  The mime examples can be extracted with a standard text editor.</t>
<t>NOTE: THESE EXAMPLES ARE NOT CORRECT AS THE FINAL OIDS HAVE NOT BEEN ASSIGNED.  THE KEYS WERE NOT USED AT THIS POINT.</t>
<appendix title="Encapsulated Signed Data Example">
<t>
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.
</t>
<figure>
<artwork>
MIME-Version: 1.0
To: BobRSA@examples.com
From: AliceDss@examples.com
Subject: MD5-XOR example message
Message-Id: >34567809323489fd.esc@examples.com<
Date: Wed, 16 Dec 2009 23:13:00 -0500
Content-Type: application/pkcs7-mime; smime-type=signed-data;
  name=smime.p7m;
  micalg=xor-md5: 0440010203405060708090a0b0c0d0e0f10
  111213415161718191a1b1c1d1e1f20212223425262728292a2b2c2d2e2f30
  313233435363738393a3b3c3d3e3f40
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m

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</artwork>
</figure>
</appendix>
<appendix title="Multipart Signed Message">
<t>
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.
</t>
<figure>
<artwork>
MIME-Version: 1.0
To: User2@examples.com
From: BobRSA@examples.com
Subject: MD5-XOR signing example
Message-Id: >091218002550300.249@examples.com<
Date: Fri, 18 Dec 2009 00:25:21 -0300
Content-Type: multipart/signed;
    micalg=xor-md5: 0440010203405060708090a0b0c0d0e0f10
     111213415161718191a1b1c1d1e1f20212223425262728292a2b2c2d2e2f30
     313233435363738393a3b3c3d3e3f40
    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

MIIDdwYJKoZIhvcNAQcCoIIDaDCCA2QCAQExCTAHBgUrDgMCGjALBgkqhkiG9w0BBwGgggL
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</artwork>
</figure>
</appendix>
<appendix title="Autenticated Data Example">
<t>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.</t>
<figure>
<artwork>
MIME-Version: 1.0
To: BobRSA@examples.com
From: AliceDss@examples.com
Subject: MD5-XOR example message
Message-Id: <34567809323489fd.esc@examples.com>
Date: Wed, 16 Dec 2009 23:13:00 -0500
Content-Type: application/pkcs7-mime; smime-type=authenticated-data;
  name=smime.p7m; 
  micalg=xor-md5: 0440010203405060708090a0b0c0d0e0f10
   111213415161718191a1b1c1d1e1f20212223425262728292a2b2c2d2e2f30
   313233435363738393a3b3c3d3e3f40
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m

MIAGCyqGSIb3DQEJEAECoIAwgAIBADGAMIACAQAwgDCAMYAwgAYDVQQDDAdDYXJsUlNBAA
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AAAAAAAAAAAAAECszMzMzMzMzMzMwAAAAAAAA=


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</figure>
</appendix>
</appendix>
</back>
</rfc>