Re: [CFRG] compact representation and HPKE

   Hi Benjamin,

On 2/10/21 10:24 AM, Benjamin Beurdouche wrote:
> Hi all,
>
> Isn't the uncompressed form way more common?
> I guess you can certainly use some compressed form for transport in that case...

   Many other uses require that both sides agree on the sign and therefore
either the point is sent uncompressed or as the x-coordinate plus a sign 
bit.
But in this case we don't care about the sign. We are doing "compact output"
(per RFC 6090) and therefore can take advantage of "compact representation"
(ibid).

   regards,

   Dan.

>
> B.
>
>
>> On 6 Nov 2020, at 19:19, Dan Harkins <dharkins@lounge.org> wrote:
>>
>>
>>    Hello,
>>
>>    When doing a DH-based KEM with the NIST curves, HPKE specifies that
>> SerializePublicKey and DeserializePublicKey use the uncompressed format
>> from SECG. This ends up using 2*Ndh+1 octets to represent the serial
>> form of the public key.
>>
>>    Since compact output is being used in DH-based KEMs-- that is, the
>> secret result of DH() is the x-coordinate of the resulting EC point--
>> it would also be possible to use compact representation (per RFC 6090)
>> and have SerializePublicKey merely do integer-to-octet string
>> conversions of the x-coordinate. DeserializePublicKey would then
>> do octet string-to-integer conversion for the x-coordinate and use the
>> equation of the curve to choose the y-coordinate. The sign isn't
>> important because we're doing compact output.
>>
>>    This would make the interface for the NIST curves and the Bernstein
>> curves be uniform-- Serialize would produce an octet string of Ndh
>> and Deserialize would consume an octet string of Ndh-- at the cost
>> of some CPU inside DeserializePublicKey.
>>
>>    Please consider this suggestion.
>>
>>    regards,
>>
>>    Dan.
>>
>> -- 
>> "The object of life is not to be on the side of the majority, but to
>> escape finding oneself in the ranks of the insane." -- Marcus Aurelius
>>
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-- 
"The object of life is not to be on the side of the majority, but to
escape finding oneself in the ranks of the insane." -- Marcus Aurelius