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RE: Possible ambiguities in encoding of signatures, encrypted keys

1999-04-12 02:22:07
Hi Peter,

Currently both RFC 2459 and CMS refer to RFC 2313/2437 for the encoding of RSA
signatures/encrypted data (RFC 2459, 7.2.1; CMS, and 12.2.2 - what 
about to describe applies to other algorithms as well, but I'll stick with RSA
to keep it simple).  These RFC's make the assumption that the encoded value
will be of the same length as the modulus, zero-padding the value if required
(RFC 2437, 7.2.1 and 8.1.1), however when this padding is used the encoded
value doesn't follow the DER any more.

I'm not sure this is right. The signature is an octet string or a
bit string, not an integer, and it's perfectly legal to have an
OCTET STRING or BIT STRING with leading null bytes. RFC 2313 says:

8.4 Integer-to-octet-string conversion

   The integer encrypted data y shall be converted to an octet string ED
   of length k, the encrypted data. 

and it's the octet string that's encoded.



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