code: def keys_by_alg_and_usage(self, issuer, alg, usage):
          
          if usage in ["sig", "ver"]:
              ktype = jws_alg2keytype(alg)
          else:
              ktype = jwe_alg2keytype(alg)
  
          return self.get(usage, ktype, issuer)
docs: Find all keys that can be used for a specific crypto algorithm and
          usage by key owner.
  
          :param issuer: Key owner
          :param alg: a crypto algorithm
          :param usage: What the key should be used for
          :return: A possibly empty list of keys