code: function(algo) {
      switch (algo) {
        //   Algorithm-Specific Fields for RSA encrypted session keys:
        //       - MPI of RSA encrypted value m**e mod n.
        case enums.publicKey.rsa_encrypt:
        case enums.publicKey.rsa_encrypt_sign:
          return [type_mpi];
  
        //   Algorithm-Specific Fields for Elgamal encrypted session keys:
        //       - MPI of Elgamal value g**k mod p
        //       - MPI of Elgamal value m * y**k mod p
        case enums.publicKey.elgamal:
          return [type_mpi, type_mpi];
        //   Algorithm-Specific Fields for ECDH encrypted session keys:
        //       - MPI containing the ephemeral key used to establish the shared secret
        //       - ECDH Symmetric Key
        case enums.publicKey.ecdh:
          return [type_mpi, type_ecdh_symkey];
        default:
          throw new Error('Invalid public key encryption algorithm.');
      }
    }
docs: Returns the types comprising the encrypted session key of an algorithm
  @param {String} algo The public key algorithm
  @returns {Array<String>} The array of types