Converting .Net decryption to Java

问题

Currently I'm working on a project where they use AES encryption with RFC2898 derived bytes. This the decryption method that I've provided. Now I need to implement it in java.

private string Decrypt(string cipherText)
    {
        string EncryptionKey = "MAKV2SPBNI657328B";
        cipherText = cipherText.Replace(" ", "+");
        byte[] cipherBytes = Convert.FromBase64String(cipherText);
        using (Aes encryptor = Aes.Create())
        {
            Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] {
                0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 
                });
            encryptor.Key = pdb.GetBytes(32);
            encryptor.IV = pdb.GetBytes(16);
            using (MemoryStream ms = new MemoryStream())
            {
                using (CryptoStream cs = new CryptoStream(ms, encryptor.CreateDecryptor(), CryptoStreamMode.Write))
                {
                    cs.Write(cipherBytes, 0, cipherBytes.Length);
                    cs.Close();
                }
                cipherText = Encoding.Unicode.GetString(ms.ToArray());
            }
        }
        return cipherText;
    }

This is what I go so far:

   String EncryptionKey = "MAKV2SPBNI657328B";
   String userName="5L9p7pXPxc1N7ey6tpJOla8n10dfCNaSJFs%2bp5U0srs0GdH3OcMWs%2fDxMW69BQb7"; 
   byte[] salt =  new byte[] {0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76};
   try {
        userName = java.net.URLDecoder.decode(userName, StandardCharsets.UTF_8.name());
        SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
        PBEKeySpec pbeKeySpec = new PBEKeySpec(EncryptionKey.toCharArray(), salt, 1000);
        Key secretKey = factory.generateSecret(pbeKeySpec);
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        cipher.init(Cipher.DECRYPT_MODE, secretKey);
        byte[] result = cipher.doFinal(userName.getBytes("UTF-8"));
        System.out.println(result.toString());
   } catch (Exception e) {
        System.out.println(e.getMessage());
   }

But I'm getting errors as below:

Key length not found java.security.spec.InvalidKeySpecException: Key length not found

回答1:

There are some issues in the Java code: The number of bits to be generated must be specified, besides the key the IV must be derived, the IV must be applied for decryption, the ciphertext must be Base64 decoded and Utf-16LE must be used when decoding the plaintext. In detail:

When instantiating PBEKeySpec, the number of bits to be generated must be specified in the 4th parameter. Since both, key (256 bits) and IV (128 bits) are derived in the C# code, 384 (= 256 + 128) must be applied:
```
SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
PBEKeySpec pbeKeySpec = new PBEKeySpec(encryptionKey.toCharArray(), salt, 1000, 384);   
```

The first 256 bits are the key, the following 128 bits the IV:

byte[] derivedData = factory.generateSecret(pbeKeySpec).getEncoded();
byte[] key = new byte[32];
byte[] iv = new byte[16];
System.arraycopy(derivedData, 0, key, 0, key.length);
System.arraycopy(derivedData, key.length, iv, 0, iv.length);

The IV must be passed in the 3rd parameter of the Cipher#init-call using an IvParameterSpec instance:

SecretKeySpec secretKeySpec = new SecretKeySpec(key, "AES");
IvParameterSpec ivSpec = new IvParameterSpec(iv);
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
cipher.init(Cipher.DECRYPT_MODE, secretKeySpec, ivSpec);

The ciphertext must be Base64-decoded before it can be decrypted:

byte[] result = cipher.doFinal(Base64.getDecoder().decode(userName));

From the decrypted byte array a string has to be created using Utf-16LE encoding (which corresponds to Encoding.Unicode in C#):
```
System.out.println(new String(result, StandardCharsets.UTF_16LE)); // Output: Mohammad Al Mamun
```

Note that for CBC mode, it's important that a key/IV combination is only used once for security reasons. For the C# (or Java) code here, this means that for the same password, different salts must be used for each encryption, see here.

来源：https://stackoverflow.com/questions/60387628/converting-net-decryption-to-java

标签

java

.net

encryption

aes

rfc2898