Which ciphers can TPPs use to sign their on-boarding & authorisation requests?

Open Banking (OB) will sign the SSA request with the PS256 algorithim. TPPs should sign requests with the algorithim advertised by the ASPSPs on their well known configuration which should be advertised as PS256.

However, if any ASPSP can not process PS256 signed JWTs, OB will fall back to signing the JWT's as RS256.