[OTR-dev] Extra symmetric key

[Dev Random]

On 01/14/2013 03:34 AM, Kjell Braden wrote:
> Am 14.01.2013 06:05, schrieb Dev Random:
>> Hi all,
>>
>> I plan to use the Type 8 TLV / extra symmetric key in the Gibberbot
>> project, but I am puzzled by the description in the OTRv3 spec.
>>
>> TLVs are already encrypted inside the message envelope.  What is then
>> the function of the additional key h2(0xFF)?  Since the message is
>> already encrypted with a related key - h1(), there seems to be no added
>> security from another layer of encryption.
>>
>> Could the Type 8 TLV data be used as is (without using h2(0xFF) to
>> encrypt)?
>>
>> I also note that the libotr4 implementation of otrl_message_symkey
>> doesn't do any encryption on sending, and only provides h2(0xFF) as a
>> callback, but doesn't do decryption.
>>
>
> [sent this off-list earlier, sorry]
>
> The TLV8 is used to indicate to your partner's client that you intend
> to use the key, not to exchange key material (both sides know the key
> already). The contents are entirely application-defined.

Yes, I realize that.  But what is the point of the key, if the content
of the TLV is *already encrypted* with the message key?

Is the extra key meant to be used for out of band data?

>
> -Kjell
>
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