crypto: fix getDecoder() encoding check

Normalize the encoding in getDecoder() before using it. Fixes an
AssertionError: "Cannot change encoding" when encoding is "ucs2",
"ucs-2" or "utf-16le"

Fixes: nodejs#8236

lib/crypto.js

@@ -100,7 +100,7 @@ Hmac.prototype._transform = Hash.prototype._transform;
 
 
 function getDecoder(decoder, encoding) {
-  if (encoding === 'utf-8') encoding = 'utf8';  // Normalize encoding.
+  encoding = internalUtil.normalizeEncoding(encoding);
   decoder = decoder || new StringDecoder(encoding);
   assert(decoder.encoding === encoding, 'Cannot change encoding');
   return decoder;

test/parallel/test-crypto-cipher-decipher.js

@@ -113,3 +113,29 @@ testCipher2(Buffer.from('0123456789abcdef'));
   c.update('update', 'utf-8');
   c.final('utf8');  // Should not throw.
 }
+
+// Regression tests for https://github.com/nodejs/node/issues/8236.
+{
+  const key = '0123456789abcdef';
+  const plaintext = 'Top secret!!!';
+  const c = crypto.createCipher('aes192', key);
+  var ciph = c.update(plaintext, 'utf16le', 'base64');
+  ciph += c.final('base64');
+
+  var decipher = crypto.createDecipher('aes192', key);
+
+  var txt;
+  assert.doesNotThrow(() => txt = decipher.update(ciph, 'base64', 'ucs2'));
+  assert.doesNotThrow(() => txt += decipher.final('ucs2'));
+  assert.strictEqual(txt, plaintext, 'decrypted result in ucs2');
+
+  decipher = crypto.createDecipher('aes192', key);
+  assert.doesNotThrow(() => txt = decipher.update(ciph, 'base64', 'ucs-2'));
+  assert.doesNotThrow(() => txt += decipher.final('ucs-2'));
+  assert.strictEqual(txt, plaintext, 'decrypted result in ucs-2');
+
+  decipher = crypto.createDecipher('aes192', key);
+  assert.doesNotThrow(() => txt = decipher.update(ciph, 'base64', 'utf-16le'));
+  assert.doesNotThrow(() => txt += decipher.final('utf-16le'));
+  assert.strictEqual(txt, plaintext, 'decrypted result in utf-16le');
+}