Re: Randomness of Message-ID in IMDN

At Thu, 15 May 2008 00:25:38 +0800,
Eric Burger wrote:
> Thanks for your very, very quick review!  On the one open item for  
> discussion, Message-ID, I would offer (1) it is not a do-or-die  
> situation but that (2) using a cryptographically secure random number
> generator. achieves the same result with better properties.  Again, I  
> will defer back to you: I know the work group will push back strong if  
> a cryptographically secure random number generator is a resource hog.
> 
> Are there memory / CPU efficient cryptographically secure random  
> number generators?

Yes. For instance, the NIST SP 800-90 PRNG requires order
256-512 bits of internal state and two hash operations for
every hash-sized (160-512 bits) chunk of output. I would 
just use OpenSSL's cryptographically secure PRNG myself :)

> Should we give guidance to the range of numbers  
> (i.e., 32-bits, 512-bits, 6 digits, etc.)?

As I understand the situation, the sender the only person who has
to rely on the uniqueness of this header, right?

I would suggest instead of recommending a given number you explain
what the issues are and why this has to be unpredictable.  Perhaps
something like this:

  Because the Message-ID is used by the sender to correlate IMDNs with
  their respective IMs, the Message-ID MUST be selected so that:

  (1) There is a minimal chance of any two Message-IDs accidentally
  colliding within the time period within which an IMDN might be
  received.

  (2) It is prohibitive for an attacker who has seen one or more valid
  Message-IDs to generate additional valid Message-IDs.

  The first requirement is a correctness requirement to ensure correct
  matching. The second requirement prevents off-path attackers from
  forging IMDNs. In order to meet both of these requirements, it is
  RECOMMENDED that Message-IDs be generated using a cryptographically
  secure pseudorandom number generator (see [RFC 4086] and contain at
  least 64 bits of randomness, thus reducing the chance of a
  successful guessing attack to n/2^64, where n is the number of
  outstanding valid messages. Another potential approach is to combine
  a sequence number (providing uniqueness) with a cryptographic MAC
  for unforgeability.

Cheers,
-Ekr

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