Decrypt AES buffer received from PHP service

Question

I'm having trouble decrypting AES encrypted data. I receive base64 encoded, encrypted buffer from remote service. The data is encrypted using AES 128-cbc using OpenSSL library in PHP. I'm trying to decrypt it this way:

var encryptedKey = Encoding.ASCII.GetString(key);
var rsaKey = Convert.FromBase64String(encryptedKey);
var ms = new MemoryStream();
var aes = RijndaelManaged.Create();
aes.Padding = PaddingMode.None;
aes.Mode = CipherMode.CBC;
aes.KeySize = 128;
aes.BlockSize = 128;
aes.Key = pwd;
aes.IV = pwd;
var stream = new CryptoStream(ms, aes.CreateDecryptor(), CryptoStreamMode.Write);
stream.Write(rsaKey, 0, rsaKey.Length);
var ss6 = Encoding.ASCII.GetString(ms.ToArray());

But I get absolutely invalid result.

Here's the code in PHP used to encrypt the data:

($this->source, 'aes-128-cbc', $this->key, true, $this->key)

Anyone has any idea what could be wrong? The key and IV are set correctly.

Thanks

P.S. I've tried all available CipherModes :) also key size is 16 bytes.

Answer 1

This is what I'm using to do that:

Here's what the parameters mean:

/// <summary>
        /// Decrypts specified ciphertext using Rijndael symmetric key algorithm.
        /// </summary>
        /// <param name="cipherText">
        /// Base64-formatted ciphertext value.
        /// </param>
        /// <param name="passPhrase">
        /// Passphrase from which a pseudo-random password will be derived. The
        /// derived password will be used to generate the encryption key.
        /// Passphrase can be any string. In this example we assume that this
        /// passphrase is an ASCII string.
        /// </param>
        /// <param name="saltValue">
        /// Salt value used along with passphrase to generate password. Salt can
        /// be any string. In this example we assume that salt is an ASCII string.
        /// </param>
        /// <param name="hashAlgorithm">
        /// Hash algorithm used to generate password. Allowed values are: "MD5" and
        /// "SHA1". SHA1 hashes are a bit slower, but more secure than MD5 hashes.
        /// </param>
        /// <param name="passwordIterations">
        /// Number of iterations used to generate password. One or two iterations
        /// should be enough.
        /// </param>
        /// <param name="initVector">
        /// Initialization vector (or IV). This value is required to encrypt the
        /// first block of plaintext data. For RijndaelManaged class IV must be
        /// exactly 16 ASCII characters long.
        /// </param>
        /// <param name="keySize">
        /// Size of encryption key in bits. Allowed values are: 128, 192, and 256.
        /// Longer keys are more secure than shorter keys.
        /// </param>
        /// <returns>
        /// Decrypted string value.
        /// </returns>
        /// <remarks>
        /// Most of the logic in this function is similar to the Encrypt
        /// logic. In order for decryption to work, all parameters of this function
        /// - except cipherText value - must match the corresponding parameters of
        /// the Encrypt function which was called to generate the
        /// ciphertext.
        /// </remarks>

This is the actual function:

 public static string Decrypt(string cipherText,
                                     string passPhrase,
                                     string saltValue,
                                     string hashAlgorithm,
                                     int passwordIterations,
                                     string initVector,
                                     int keySize)
        {
            // Convert strings defining encryption key characteristics into byte
            // arrays. Let us assume that strings only contain ASCII codes.
            // If strings include Unicode characters, use Unicode, UTF7, or UTF8
            // encoding.
            byte[] initVectorBytes = Encoding.ASCII.GetBytes(initVector);
            byte[] saltValueBytes = Encoding.ASCII.GetBytes(saltValue);

            // Convert our ciphertext into a byte array.
            byte[] cipherTextBytes = Convert.FromBase64String(cipherText);

            // First, we must create a password, from which the key will be 
            // derived. This password will be generated from the specified 
            // passphrase and salt value. The password will be created using
            // the specified hash algorithm. Password creation can be done in
            // several iterations.
            PasswordDeriveBytes password = new PasswordDeriveBytes(
                                                            passPhrase,
                                                            saltValueBytes,
                                                            hashAlgorithm,
                                                            passwordIterations);

            // Use the password to generate pseudo-random bytes for the encryption
            // key. Specify the size of the key in bytes (instead of bits).
            byte[] keyBytes = password.GetBytes(keySize / 8);

            // Create uninitialized Rijndael encryption object.
            RijndaelManaged symmetricKey = new RijndaelManaged();

            // It is reasonable to set encryption mode to Cipher Block Chaining
            // (CBC). Use default options for other symmetric key parameters.
            symmetricKey.Mode = CipherMode.CBC;

            // Generate decryptor from the existing key bytes and initialization 
            // vector. Key size will be defined based on the number of the key 
            // bytes.
            ICryptoTransform decryptor = symmetricKey.CreateDecryptor(
                                                             keyBytes,
                                                             initVectorBytes);

            // Define memory stream which will be used to hold encrypted data.
            MemoryStream memoryStream = new MemoryStream(cipherTextBytes);

            // Define cryptographic stream (always use Read mode for encryption).
            CryptoStream cryptoStream = new CryptoStream(memoryStream,
                                                          decryptor,
                                                          CryptoStreamMode.Read);

            // Since at this point we don't know what the size of decrypted data
            // will be, allocate the buffer long enough to hold ciphertext;
            // plaintext is never longer than ciphertext.
            byte[] plainTextBytes = new byte[cipherTextBytes.Length];

            // Start decrypting.
            int decryptedByteCount = cryptoStream.Read(plainTextBytes,
                                                       0,
                                                       plainTextBytes.Length);

            // Close both streams.
            memoryStream.Close();
            cryptoStream.Close();

            // Convert decrypted data into a string. 
            // Let us assume that the original plaintext string was UTF8-encoded.
            string plainText = Encoding.UTF8.GetString(plainTextBytes,
                                                       0,
                                                       decryptedByteCount);

            // Return decrypted string.   
            return plainText;
        }