[OTR-dev] hash commitment in DH key exchange

[Ben Laurie]

On 28 May 2014 22:59, Ian Goldberg <ian at cypherpunks.ca> wrote:
> On Wed, May 28, 2014 at 10:55:10PM +0100, Ben Laurie wrote:
>> On 28 May 2014 19:57, Ian Goldberg <ian at cypherpunks.ca> wrote:
>> > On Wed, May 28, 2014 at 05:56:30PM +0100, Ximin Luo wrote:
>> >> Thanks! I suppose this is the same reasoning as the DH-commit to protect the SAS in ZRTP[1]?
>> >
>> > Probably.
>> >
>> >> To clarify, does this mean the DH-commit is unnecessary if either:
>> >>
>> >> a. the session key is longer, say 128 bits or 256 bits (but this would
>> >> make it "less useable" for verification), or
>> >> b. we use a verification method that doesn't depend on the session id,
>> >> such as direct fingerprint verification
>> >
>> > At first glance, those seem plausible to me.
>>
>> Now I'm curious: why is the session ID short?
>
> Usability of verification in the (long-since-deprecated) "compare
> session IDs" method, which works even if you *know* your private keys
> have been compromised (but only for the current session).

Confused. Why not verify a truncated hash of the (long) session ID?