code: async function(algo) {
      const prefixrandom = await random.getRandomBytes(cipher[algo].blockSize);
      const repeat = new Uint8Array([prefixrandom[prefixrandom.length - 2], prefixrandom[prefixrandom.length - 1]]);
      return util.concat([prefixrandom, repeat]);
    }
docs: Generates a random byte prefix for the specified algorithm
  See {@link https://tools.ietf.org/html/rfc4880#section-9.2|RFC 4880 9.2} for algorithms.
  @param {module:enums.symmetric} algo Symmetric encryption algorithm
  @returns {Uint8Array}                Random bytes with length equal to the block size of the cipher, plus the last two bytes repeated.
  @async