On Mon, May 01, 2023 at 11:00:48PM -0700, Laurence Lundblade wrote:
> Here’s a clarification request. Here’s the proposal for the new kty:
>
> 4.2. Key Type for COSE_Key
>
> A new generic kty(1) value HPKE-KEM(T.B.D.) is defined to represent
> the private and public key used for the HPKE KEM. A key with this
> kty has the following parameters:
>
> * The parameter kty(1) MUST be HPKE-KEM(T.B.D).
>
> * The parameter hkc(T.B.D.) MUST be present and contains the HPKE
> Key Configuration defined in Section 3.2.1.
>
> * The parameter pub(-1) MUST be present and contains the public key
> encoded in a byte string (bstr type).
>
> * The parameter priv(-2) MUST be present if the key is private key
> and contains the private key encoded in a byte string (bstr type).
Using hkc there is a bad idea (it says what KEM is supported, not what
KEM it is). I think it should be something like:
* The parameter kty(1) MUST be HPKE-KEM(T.B.D).
* The parameter kem(-1) MUST be present and contains the HPKE
KEM identifier (uint type).
* The parameter pub(-2) MUST be present and contains the public key
encoded in a byte string (bstr type).
* The parameter priv(-3) MUST be present if the key is private key
and contains the private key encoded in a byte string (bstr type).
> How will you know what the actual serializations for pub(-1) and
> priv(-2) will be? They have to vary by KEM, right?
Opaque for COSE-HPKE; It is defined by the KEM.
Of course, one needs to know the HPKE KEM identifier to make any sense
of the key. E.g., both X25519 and CP-256 have 32-byte public keys, and
one would expect >10% of bit patterns be valid for both.
> Also tried to figure out what Ilari’s 1212481536 is. Most KEM and
> curve IDs are small integers.
It is offset between HPKE kem and COSE crv spaces.
Such offset needs to exist, because this mapping needs a contiguous
block of 65536 crv, and values like 4 are already allocated on COSE
side, but available for allocation on HPKE side.
And to not hog 1-, 2- or 3-byte space, the offset needs to be at
least 65536. And for keeping it in 5-byte space (which is plentiful),
the offset can be at most 4294901760.
BTW, 1212481536 = 0x48450000. And in ASCII, 0x48 => H, 0x45 => E.
-Ilari
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