Hi Stephen, Sean,
Possibly another item worth including in the Security Considerations section. Suppose MSG1 is sent to a set S1 of users. In the case where MSG2 is sent to only a subset of users in S1, all users from S1 will still be able to decrypt MSG2 (since MSG2.KEK is computed only from MSG1.CEK). I don't think you intended for your solution to be used for such dynamic recipient sets, but it might be worth explicitly mentioning this unfortunate side-effect of key re-use in any case. (Might be enough to mention that the recipient lists must be the same for each message.)
Mike J.
> -----Original Message-----
> From: Internet-Drafts@xxxxxxxx [mailto:Internet-Drafts@xxxxxxxx]
> Sent: Friday, February 09, 2001 7:27 AM
> Cc: ietf-smime@xxxxxxx
> Subject: I-D ACTION:draft-ietf-smime-rcek-01.txt
>
>
> A New Internet-Draft is available from the on-line
> Internet-Drafts directories.
> This draft is a work item of the S/MIME Mail Security Working
> Group of the IETF.
>
> Title : Reuse of CMS Content Encryption Keys
> Author(s) : S. Farrell, S. Turner
> Filename : draft-ietf-smime-rcek-01.txt
> Pages : 7
> Date : 08-Feb-01
>
> This note describes a way to include a key identifier in a CMS
> enveloped data structure, so that the content encryption key can be
> re-used for further enveloped data packets.
>
> A URL for this Internet-Draft is:
> http://www.ietf.org/internet-drafts/draft-ietf-smime-rcek-01.txt
>
> Internet-Drafts are also available by anonymous FTP. Login
> with the username
> "anonymous" and a password of your e-mail address. After logging in,
> type "cd internet-drafts" and then
> "get draft-ietf-smime-rcek-01.txt".
>
> A list of Internet-Drafts directories can be found in
> http://www.ietf.org/shadow.html
> or ftp://ftp.ietf.org/ietf/1shadow-sites.txt
>
>
> Internet-Drafts can also be obtained by e-mail.
>
> Send a message to:
> mailserv@xxxxxxxxx
> In the body type:
> "FILE /internet-drafts/draft-ietf-smime-rcek-01.txt".
>
> NOTE: The mail server at ietf.org can return the document in
> MIME-encoded form by using the "mpack" utility. To use this
> feature, insert the command "ENCODING mime" before the "FILE"
> command. To decode the response(s), you will need "munpack" or
> a MIME-compliant mail reader. Different MIME-compliant
> mail readers
> exhibit different behavior, especially when dealing with
> "multipart" MIME messages (i.e. documents which have been split
> up into multiple messages), so check your local documentation on
> how to manipulate these messages.
>
>
> Below is the data which will enable a MIME compliant mail reader
> implementation to automatically retrieve the ASCII version of the
> Internet-Draft.
>