code: function getBlockHmac(key, blockIndex, blockLength, blockData) {
      return getHmacKey(key, new Int64(blockIndex)).then(function(blockKey) {
          var blockDataForHash = new Uint8Array(blockData.byteLength + 4 + 8);
          var blockDataForHashView = new DataView(blockDataForHash.buffer);
          blockDataForHash.set(new Uint8Array(blockData), 4 + 8);
          blockDataForHashView.setInt32(0, blockIndex, true);
          blockDataForHashView.setInt32(8, blockLength, true);
          return CryptoEngine.hmacSha256(blockKey, blockDataForHash.buffer);
      });
  }
docs: Gets block HMAC
  @param {ArrayBuffer} key
  @param {number} blockIndex
  @param {number} blockLength
  @param {ArrayBuffer} blockData
  @returns {Promise.<ArrayBuffer>}