Re: secure sign & encrypt

[Derek Atkins <[e-mail]>]

This doesn't help.  Any recipient could re-encrypt the message and
change the list of encrypted recipients.

-derek

[e-mail] writes:

> disastry wrote:
> > fake pubkey encryption packets can be added
> > by man in the middle so that recipient thinks that message was encrypted
> > to him and to other preson.
> > 
> > I wrote about it here:
> > http://lists.gnupg.org/pipermail/gnupg-devel/2001-August/006285.html
> 
> I think this can be solved by modifying
> Sym. Encrypted Integrity Protected Data Packet (Tag 18).
> 
> Now it is:
> 
> version byte == 1
> encrypted data
> 
> encrypted data consists of:
>   encrypted iv
>   encrypted plaintext
>   encrypted Modification Detection Code Packet (Tag 19)
> 
> I suggest:
> 
> version byte == 2
> encrypted data
> 
> encrypted data consists of:
>   encrypted iv
>   encrypted Recipients packet (Tag 20)
>     (put it before plaintext - if it would be after it would
>      be difficult to find where plaintext ends, when decrypting)
>   encrypted plaintext
>   encrypted Modification Detection Code Packet (Tag 19)
> 
> Recipients packet
>   version byte == 1
>   number of recipients, 2 bytes (should be enough..)
>   number_of_recipients*20 byte list of fingerprints recipient keys
>     (16 byte RSA v3 key fingerprints are appended with 4 zeros
>      (or maybe with 4 lowest keyid bytes? I think, it's even better))
> 
> 
> this ensures that recipient list is intact not only for signed & encrypted messages
> but also for encrypted only messages.
> 
> __
> Disastry  http://disastry.dhs.org/

-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       [e-mail]                        PGP key available