matrix shift v3 Decoding

private static string MatrixDecrypt(string cryptogram, int[] key)
        {
            int[] revKey = new int[key.Length];

            for(int i=0; i<key.Length; i++)
            {
                revKey[i] = (Array.IndexOf(key, i+1)+1);
            }

            var decryptionStringList = Split(cryptogram, revKey.Length);
            string plaintext = string.Empty;
            foreach (string portion in decryptionStringList)
            {
                plaintext += EncryptPortion(portion, revKey);
            }
            return plaintext;
        }