ECDSA with Curve P-521

[verocri]

This issue refers to the security review requested in issue w3c/security-request#55 and it is related to the issue w3c/vc-data-integrity#337.

About parameters, this spec does not consider P-521 (256 bits of security) but just P-256 and P-384.

SING group discussed this topic during the meeting SING_2025-04-01 and the emerged reasons are for reducing the amount of optionality and for interoperability (since P-521 is not widely used).

[brentzundel]

We are grateful for this response from SING and look forward to considering it as part of a future version of the specification