pwCallbackWrapper = function(passphrase) {
      return goog.async.Deferred.succeed(passphrase).addCallback(
          passphraseCallback);
    }
Internal implementation of the verification/decryption operation.
@param {!e2e.ByteArray} encryptedMessage The encrypted data.
@param {function(string):!goog.Thenable<string>} passphraseCallback This
    callback is used for requesting an action-specific passphrase from the
    user.
@param {string=} opt_charset The (optional) charset to decrypt with.
@param {!Array<!e2e.openpgp.Key>=} opt_decryptionKeys If present,
    only those keys will be used for decryption. Otherwise, Context2 uses
    Key ID hints in the message to resolve keys on its own.
@param {!Array<!e2e.openpgp.Key>=} opt_verificationKeys If present,
    only those keys will be used for signature verification. Otherwise,
    Key ID hints in the message will be used to resolve the keys.
@protected
@return {!e2e.openpgp.VerifyDecryptPromise}