Update cf-edhoc patch

[actyp]

Observation

The BigInteger#toByteArray() method returns the minimum number of bytes required to represent the BigInteger, which means that

byte[] bytes = new byte[] { 0, 1, 2 }
bytes /* [0, 1, 2] */  != new BigInteger(1, bytes).toByteArray() /* [1, 2] */

Specific Case

Focusing in the SharedSecretCalculation and specifically the methods recomputeEcdsa256YFromX and recomputeEcdsa384YFromX:

The input byte array publicKeyX should have 32 (resp. 48) byte length.

In the case that has a prefix of 0x00 bytes, the following would hold:

publicKeyX /* 32 bytes */ != new BigInteger(1, publicKeyX).toByteArray() /* < 32 bytes */

Now, because the right version of the above byte array was consequently used, the input would have an incorrect length, which is why the process would fail to find an available Y COSE key.

Partial Solution

One simple solution is to pad left the converted array in order to make it have the correct length.
At least, this solves the problematic cases of zero-prefix inputs.

[kostis]

Looks OK (and we should merge it), but wouldn't it be better if cf-edhoc was patched or properly fixed instead?

[actyp]

Yes, of course, we should report it.