[Android] CryptographicException: Bad PKCS7 padding. Invalid length 0.

Unity Engine
1

Sometimes (but not always) I get this exception on Android device (but not in Editor), built with Unity 5.3.6f1.

CryptographicException: Bad PKCS7 padding. Invalid length 0.
  at Mono.Security.Cryptography.SymmetricTransform.ThrowBadPaddingException (PaddingMode padding, Int32 length, Int32 position) 
  at Mono.Security.Cryptography.SymmetricTransform.FinalDecrypt (System.Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount) 
  at Mono.Security.Cryptography.SymmetricTransform.TransformFinalBlock (System.Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount)
  at System.Security.Cryptography.CryptoStream.FlushFinalBlock ()
  ...

I’m using AesManaged { Mode = CipherMode.CFB, Padding = PaddingMode.PKCS7 } with properly specified key and iv for symmetric cryptography.

The code looks like:

using (var decryptor = _aes.CreateDecryptor(_aesKeyBuffer, _aesIVBuffer))
using (var memoryStream = new MemoryStream())
using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Write))
{
    cryptoStream.Write(paddedCiphertext.Array, paddedCiphertext.Offset, paddedCiphertext.Count);
    cryptoStream.Flush();
    // http://stackoverflow.com/questions/18783182/aes-encrypt-memorystream-toarray-empty
    cryptoStream.FlushFinalBlock();
    byte[] plaintextArray = memoryStream.ToArray();
    ...
}

Any ideas?

2

When I use another approach, still get the same exception (with another trace):

CryptographicException: Bad PKCS7 padding. Invalid length 0.
  at Mono.Security.Cryptography.SymmetricTransform.ThrowBadPaddingException (PaddingMode padding, Int32 length, Int32 position)
  at Mono.Security.Cryptography.SymmetricTransform.FinalDecrypt (System.Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount)
  at Mono.Security.Cryptography.SymmetricTransform.TransformFinalBlock (System.Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount)
  at System.Security.Cryptography.CryptoStream.Read (System.Byte[] buffer, Int32 offset, Int32 count)
  at System.IO.BinaryReader.ReadBytes (Int32 count)
  ...

using (var decryptor = _aes.CreateDecryptor(_aesKeyBuffer, _aesIVBuffer))
using (var memoryStream = new MemoryStream(paddedCiphertext.Array, paddedCiphertext.Offset, paddedCiphertext.Count))
using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
{
    using (var reader = new BinaryReader(cryptoStream))
    {
        byte[] plaintextArray = reader.ReadBytes(paddedCiphertext.Count);
        ...
    }
}

Works fine in Windows Editor. Looks like a bug in Mono crypto. Can anyone confirm this?

3

Fixed by replacing AesManaged with equivalent Rijndael (BlockSize = 16 bytes, KeySize = 32 bytes). The error has gone. Mono cryptography is just broken.

Remind me please, why we are still stuck to out-of-date, suboptimal, and buggy version of Mono?

2 Likes
4

I have the same issue in 2017.3.1, works fine in 5.6.5

5

It appears that not every encryption mode is supported by .net 2.0 subset, switching to full .net 2.0 api compatibility or choosing a different encryption mode might work.

1 Like
6

Same issue, but in editor too. 2018.2.10f1. But…But… My unit tests are passes: one test encrypts, another decrypts.

7

AesCryptoServiceProvider not supported by .net 2.0 subset.
AesManaged supported.
I tried DES then. For DES “Bad PKCS7 padding. Invalid lengh x” exception was fixed by specifying padding in encryptor and decryptor methods for DesCryptoServiseProvider.

8

Nope. It is not fixed. Encrypt → Decrypt → (change some data) → Encrypt → Decrypt(Bad PKCS7 Exception).

9

I confirm the problem still exist in 2021.2.x.

10

I get the other error,when do Decrypt.
Error is : CryptographicException: Bad PKCS7 padding. Invalid length 154.
at System.Security.Cryptography.CryptoStream.Read

my code is:

7999329--1028442--upload_2022-3-28_9-30-5.png
7999329--1028442--upload_2022-3-28_9-30-5.png1086Ă—446 59.2 KB

some one say:
Removed the call to CryptoStream.FlushFinalBlock() in the Encrypt method.
I will try

11

Can anyone please share a solution? I am running into the same issue. What makes me curious is that it is a sometimes issue and doesn’t happen on the editor (or maybe not enough lucky to see it appear on the editor?). My code is pasted below:

private const string KeyString = "my-32-byte-secret-key-go-here...";

public static byte[] EncryptToByte(string plainText)
{
    byte[] cipherData;
    Aes aes = Aes.Create();
    aes.Key = Encoding.UTF8.GetBytes(KeyString);
    aes.GenerateIV();
    aes.Mode = CipherMode.CBC;
    aes.Padding = PaddingMode.PKCS7;
    ICryptoTransform cipher = aes.CreateEncryptor(aes.Key, aes.IV);

    using (MemoryStream ms = new MemoryStream())
    {
        using (CryptoStream cs = new CryptoStream(ms, cipher, CryptoStreamMode.Write))
        {
            using (StreamWriter sw = new StreamWriter(cs))
            {
                sw.Write(plainText);
            }
        }
        cipherData = ms.ToArray();
    }

    byte[] combinedData = new byte[aes.IV.Length + cipherData.Length];
    Array.Copy(aes.IV, 0, combinedData, 0, aes.IV.Length);
    Array.Copy(cipherData, 0, combinedData, aes.IV.Length, cipherData.Length);
    return combinedData;
}

public static string DecryptFromByte(byte[] combinedData)
{
    string plainText;
    Aes aes = Aes.Create();
    aes.Key = Encoding.UTF8.GetBytes(KeyString);
    byte[] iv = new byte[aes.BlockSize / 8];
    byte[] cipherText = new byte[combinedData.Length - iv.Length];
    Array.Copy(combinedData, iv, iv.Length);
    Array.Copy(combinedData, iv.Length, cipherText, 0, cipherText.Length);
    aes.IV = iv;
    aes.Mode = CipherMode.CBC;
    aes.Padding = PaddingMode.PKCS7;
    ICryptoTransform decipher = aes.CreateDecryptor(aes.Key, aes.IV);

    using (MemoryStream ms = new MemoryStream(cipherText))
    {
        using (CryptoStream cs = new CryptoStream(ms, decipher, CryptoStreamMode.Read))
        {
            using (StreamReader sr = new StreamReader(cs))
            {
                plainText = sr.ReadToEnd();
            }
        }
        return plainText;
    }
}
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