We would like to simplify the APIs by removing the sequence requirement on methods that take CryptoOperationData:

typedef (ArrayBuffer or ArrayBufferView) CryptoOperationData;

interface SubtleCrypto {
  Promise<any> encrypt(AlgorithmIdentifier algorithm,
                       Key key,
                       sequence<CryptoOperationData> data);
  Promise<any> decrypt(AlgorithmIdentifier algorithm,
                       Key key,
                       sequence<CryptoOperationData> data);
  Promise<any> sign(AlgorithmIdentifier algorithm,
                    Key key,
                    sequence<CryptoOperationData> data);
  Promise<any> verify(AlgorithmIdentifier algorithm,
                      Key key,
                      CryptoOperationData signature,
                      sequence<CryptoOperationData> data);
  Promise<any> digest(AlgorithmIdentifier algorithm,
                      sequence<CryptoOperationData> data);

Doing this will simplify the API implementation and testing.  It will also help us bound the amount of data that can be supplied by developers when using only one ArrayBuffer or ArrayBufferView.

The new methods should look something like this:

interface SubtleCrypto {
  Promise<any> encrypt(AlgorithmIdentifier algorithm,
                       Key key,
                       CryptoOperationData data);
  Promise<any> decrypt(AlgorithmIdentifier algorithm,
                       Key key,
                       CryptoOperationData data);
  Promise<any> sign(AlgorithmIdentifier algorithm,
                    Key key,
                    CryptoOperationData data);
  Promise<any> verify(AlgorithmIdentifier algorithm,
                      Key key,
                      CryptoOperationData signature,
                      CryptoOperationData data);
  Promise<any> digest(AlgorithmIdentifier algorithm,
                      CryptoOperationData data);

This was discussed and agreed on during the TPAC F2F.