Bouncy Castle Encrypt in Java Decrypt in .Net

Question

I used the following to encrypt a string using a password

 static String algorithm = "PBEWITHSHA256AND128BITAES-CBC-BC"; static byte[] salt = "b9v4n38s".getBytes(StandardCharsets.UTF_8); static int derivedKeyLength = 128; static int iterations = 20000; public static byte[] encrypt(String plainText, String password) throws NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException, InvalidKeySpecException, NoSuchAlgorithmException { Security.addProvider(new BouncyCastleProvider()); KeySpec spec = new PBEKeySpec(password.toCharArray(), salt, iterations, derivedKeyLength); SecretKeyFactory f = SecretKeyFactory.getInstance(algorithm); SecretKey key = f.generateSecret(spec); Cipher cipher = Cipher.getInstance(algorithm); cipher.init(Cipher.ENCRYPT_MODE, key); byte[] text = plainText.getBytes(StandardCharsets.UTF_8); byte[] encrypted = cipher.doFinal(text); return encrypted; }

The result of this is base64 encoded and sent as arg[0] to .Net (arg[1] is the same password). Now I'm trying to decrypt that string in .Net with this code

 private static string Decrypt(string[] args) { int derivedKeyLength = 128; int iterations = 20000; string algorithm = "PBEWITHSHA256AND128BITAES-CBC-BC"; byte[] salt = Encoding.UTF8.GetBytes("b9v4n38s"); PbeParametersGenerator pGen = new Pkcs12ParametersGenerator(new Sha256Digest()); pGen.Init(Encoding.UTF8.GetBytes(args[1]), salt, iterations); ICipherParameters par = pGen.GenerateDerivedParameters("AES256", derivedKeyLength); IBufferedCipher c = CipherUtilities.GetCipher(algorithm); c.Init(false, par); var input = Convert.FromBase64String(args[0]); byte[] enc = c.DoFinal(input); var decoded = Encoding.UTF8.GetString(enc); return decoded; }

Unfortunately it fails on DoFinal with message Org.BouncyCastle.Crypto.InvalidCipherTextException: 'pad block corrupted'

SecretKeyFactory.getInstance(algorithm) uses the same algorithm string as Cipher.getInstance(algorithm) in java but if I try pGen.GenerateDerivedParameters(algorithm, derivedKeyLength); in .Net it throws Org.BouncyCastle.Security.SecurityUtilityException: 'Algorithm PBEWITHSHA256AND128BITAES-CBC-BC not recognised.'

I'm not set on this algorithm, just looking for a way to encrypt a string in Java and decrypt it in .Net.

Topaco · Accepted Answer · 2021-05-12 23:08:07Z

A possible C#/BC code to decrypt a ciphertext generated with the posted Java code is:

using System; using System.Text; using Org.BouncyCastle.Asn1; using Org.BouncyCastle.Crypto; using Org.BouncyCastle.Security; ... private static string algorithm = "PBEWITHSHA256AND128BITAES-CBC-BC"; private static byte[] salt = Encoding.UTF8.GetBytes("b9v4n38s"); private static int iterations = 20000; public static string Decrypt(string ciphertextB64, string password) { IBufferedCipher cipher = CipherUtilities.GetCipher(algorithm); Asn1Encodable algParams = PbeUtilities.GenerateAlgorithmParameters(algorithm, salt, iterations); ICipherParameters cipherParams = PbeUtilities.GenerateCipherParameters(algorithm, password.ToCharArray(), algParams); cipher.Init(false, cipherParams); byte[] cipherBytes = Convert.FromBase64String(ciphertextB64); byte[] decrypted = cipher.DoFinal(cipherBytes); return Encoding.UTF8.GetString(decrypted); }

Test:

string decrypted = Decrypt("mBy4YwAvUpvoSJhzBnpOCJw2kCayvdYfLJ/12x0BgUKh5m5bvArSheMMs2U5rYyE", "MyPassword"); Console.WriteLine(decrypted); // The quick brown fox jumps over the lazy dog

where the ciphertext was generated with the Java code using the password MyPassword.

Please note that a static salt is generally insecure (except for testing purposes of course).

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