Update Secp256r1 Instruction with Message Length as Immediate

[SilentCicero]

Abstract

The current specified implementation of the Secp256r1 opcode assumes a 32 byte message being signed over, however, in the current widespread use of the curve in WebAuthn and passkeys, the signed message is larger than 32 bytes (e.g. 69 bytes).

Solution

Allow the immediate value to specify a more flexible message length beyond or below 32 bytes.

Considerations

• For consistency, we should perhaps also enable a more flexible message length for the secp256k1 opcode.

[Dentosal]

The thing we're signing with the opcode is not a message, but the hash of a message. In WebAuthn, the original message is condensed into the required 32 bytes using SHA-256, which is already available using S256 instruction.