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RE: comments on draft-ietf-smime-cms-auth-enveloped-02

2007-03-19 09:29:52



Here is the first round of comments on this draft.

Thanks for your review.

1.  I would like to see all information about padding out to a block
moved from this document to the algorithm documents.  This allows for
algorithm to specify if padding is needed and what type of padding is

As an example, the CCM algorithm already as a padding technique built
in and
therefore the padding is not needed, or is it?  I cannot tell from the
current documents if the data is supposed to be padded before it is
in or not.  Padding is properly a part of the algorithm processing and
the superstructure processing.

The point is that some ciphers require the input to be a multiple of
a block size.  AES-CBC is one such cipher.  We require that all such
ciphers use the same padding mechanism in EnvelopedData.  I thin that
we want similar consistency in AuthEnvelopedDtata.

I understand completely that some ciphers require padding.  However I
disagree that we want all ciphers to use the same padding mechanism.  I
strongly feel that we should remove the padding from EnvelopedData and place
it in the algorithm definitions.  I lost that argument solely on the grounds
of backwards compatibility.  My code has all of the padding done in the low
level encryption code, not in the CMS EnvelopedData code.  I think that this
means that the specification of the padding needs to be specified by the
algorithm not CMS.  

In any event you still need to specify if the AD data needs to be padded for
CCM since the algorithm definition has a padding algorithm and thus does not
need to be padded?

2.  If I implement a "stream" version of the decoder for authenticated
am I permitted to pass the decrypted data along prior to doing the
authentication step or not?   I raise the question because the CCM

   The recipient MUST verify the CBC-MAC before releasing any
   information such as the plaintext.  If the CBC-MAC verification
   fails, the receiver MUST destroy all information, except for the
   that the CBC-MAC verification failed.

Which implies to me that until the validation is finished the
code should not pass on any data but buffer it.

You read it correctly.  Are you suggesting a change?  I think that
the current document is consistent with the AES-CCM and AES-GCM
cipher descriptions.

I think that this needs to be explicitly stated in this document, otherwise
I would not implement this buffering required for this.

3.  Just for clarity I think you need to state that you are not doing
"change the '1' to Set" mess before passing the authenticated
into the authentication algorithm.

I assume that this comment is about the tag.  If so, please propose
text for a "Note that ..." paragraph.

I guess the first question is are you going to do this in the same way that
SignedAttributes are done for compatibility or not?

4.  Need to add text to state that each authenticated attribute must
internally DER encoded as well as the set of attributes.

Since attributes doe not use the "octet hole" syntax, I am not sure
why this is needed.  Please explain.

Same reason that it is needed in the SignedData case.  If you place an
attribute in that I do not have a coder/decoder for, it needs to already be
DER encoded so that I can validate it.

5.  There is actually a downside to including the message-digest
It should be stated that this attribute MUST NOT be included.

These may be a performance downside, but I do not see a security
one.  Do you?  If not, then I do not want to include a MUST NOT

I think that if you place the message-digest of the un-encrypted data in
then I can track the un-encrypted data being sent.  I can also do candidate
looking based on the hash value.

6.  I am having a problem understanding the text ins section 3 para 4.
using the symmetric key-encryption keys (and passwords) the actual AEK
wrapped by the original key and thus would be a "one-time-use" key
with just
as much certainty as for the key-transport method.  A better
description of
the problem is needed here in order to fully justify what you are

Yep, the current text is muddled.  I propose:

Fortunately, the CMS authenticated-enveloped-data content type
provides all of the tools needed to avoid misuse of counter
mode.  All of the existing key management techniques permit a fresh
content-encryption key to be generated for each content.  In
addition, existing authenticated encryption algorithms that make use
of counter mode support the use of an unpredictable nonce value in
the counter block.  This unpredictable nonce value (sometimes called
a "salt") should be carried in an algorithm identifier parameter.

Yes looks better.


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