Enable legacy signatures for Defender relayers (v == 27 || 28)

[gambino]

We're using Relayer (Defender 2) to sign messages that are sent to an old contract (github > opengsn/gsn/tree/releases/v2.2.x).
This contract internally verifies the signature is valid according to the legacy constraints:
length == 65 bytes
v == 27 || 28

It's normal that the Relayer uses the latest method to sign the message, using the chain id and leading to longer signatures and specific v values per chain.
The problem is we cannot modify the already deployed [old] contract so we would need to, somehow, generate a legacy signature.

Is it possible to get a legacy signature (65 bytes long && v equal to 27 || 28) from the Relayer?
If there's no way to get it now, would it be possible to add a parameter to the /sign & /sign-typed-data endpoints to generate a legacy signature for backwards compatibility?

Related topics, that don't include any solution to that but bringing more info about newer method to sign and validate the signature are:

Relayer.sign() returns 67 bytes instead of 65 bytes

When using a Defender Relayer to sign messages, Relayer.sign() returns 67 bytes instead of the required 65 bytes for the recover method in utils/cryptography/ECDSA.sol. Is there something I can do to bring down the amount of bytes? When I use ethers for signing the same message, it correctly returns 65 bytes so there must be going something wrong in the defender-relay-client library. Thanks in advance! Code: import { Relayer } from 'defender-relay-client'; const relayer = new Relayer({ apiK…

Why does polygon relayer signature is 66 bytes long?

I have created an autotask which signs a hash using a relayer provider and web3js. The autotask returns the signature and when the relayer's network is polygon the signature is 66 bytes (134 chars with 0x included) long, while when using the same relayer in other networks the signature length is 65 bytes 134 chars with 0x included) long as ECDSA is supposed to be. Environment I am using an openzeppelin relayer and an autotask. :memo:Details I've tried same code with same relayer i…

To reproduce, just call the /sign endpoint of the relayer and see the json response has a v different than 27 or 28:

import { Defender } from '@openzeppelin/defender-sdk';
const creds = { relayerApiKey: 'YOUR_RELAYER_API_KEY', relayerApiSecret: 'YOUR_RELAYER_API_SECRET' };
const client = new Defender(creds);
const result = await client.relaySigner.sign({ "message": "0x0" });
console.log(result);

Response for a mainnet relayer:

{
  r: '0x123...',
  s: '0x456...',
  v: 37,
  sig: '0x678...'
}