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Re: digested-data, surreptitious forwarding, D-H

2002-07-27 23:20:04

Trevor Perrin <Tperrin(_at_)sigaba(_dot_)com> writes:

1) I'm surprised S/MIME doesn't use CMSs' digested-data with enveloped-data.
In the case of encrypted but not signed mails, doesn't this leave the message
vulnerable to things like cut-and-paste attacks (where an attacker reorders
ciphertext blocks, so upon decrypting the recipient sees reordered plaintext)?

Funny you should mention that, I was discussing this recently with someone who
was quite concerned about this (they were FTP'ing large encrypted EDI messages
around).  What they wanted was a modification to EncryptedData to include a
hash inside the encrypted content, rather than DigestedData, which would
require a second pass (they encrypt the content as they stream it in and out,
so anything requiring two passes is a non-starter).  I have the feeling that
more people would be asking for this if they had any idea that encryption
doesn't guarantee integrity protection.

My suggestion was to put it in as an unprotectedAttrs, just encrypting the MDC
after the content.  In other words, hash the data before you encrypt it, then
after you've finished encrypting the data encrypt the hash and store it as an
unprotectedAttrs.  This is a bog-standard way of appending an MDC to the data,
and is compatible with the existing format.  The MDC info is put in as
originatorInfo, which is a bit of a kludge but seems to be the sensible place
for it.  This would need a new originatorInfo type, but that's the only change

If there's any support for this sort of thing, I could crank out an
informational RFC for this fairly quickly.