aspnet core encrypting and decrypting a string

Question

I am trying to decrypt a string using some pretty standard algorithm.

public static string DecryptString(string cipherText)
{
    string keyString = string.Empty;

    // Check whether the environment variable exists.
    keyString = Environment.GetEnvironmentVariable("EncryptKey");

    if (keyString == null)
    {
        keyString = "E546C8DF278CD5931069B522E695D4F2";
    }

    var fullCipher = Convert.FromBase64String(cipherText);
    using (var aesAlg = Aes.Create())
    {
        byte[] iv = new byte[aesAlg.BlockSize / 8];
        var cipher = new byte[16];

        Buffer.BlockCopy(fullCipher, 0, iv, 0, iv.Length);
        Buffer.BlockCopy(fullCipher, iv.Length, cipher, 0, iv.Length);
        var key = Encoding.UTF8.GetBytes(keyString);

        string result;
        using (var decryptor = aesAlg.CreateDecryptor(key, iv))
        using (var msDecrypt = new MemoryStream(cipher))
        using (var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
        using (var srDecrypt = new StreamReader(csDecrypt))
        {
            result = srDecrypt.ReadToEnd();
        }
        return result;
    }
}

I keep getting the error :

System.Security.Cryptography.CryptographicException: Specified padding mode is not valid for this algorithm.

I have tried multiple ways like this

var iv = new byte[16];
var cipher = new byte[16];

Or

var iv = aesAlg.IV;

I still get an error at this point. What am I doing wrong ?

Possible duplicate of [Specified key is not a valid size for this algorithm](http://stackoverflow.com/questions/2919228/specified-key-is-not-a-valid-size-for-this-algorithm) — Eris, May 01 '17 at 22:17
How was the message encrypted, what language and implementation such as PHP mcrypt — zaph, Jul 14 '17 at 03:32
@Eris This is not a duplicate 1) This is AES, that's not. 2) If we define key length 15, then we got the key size exception, but with 16 we got other exception (this padding thing, what the question is about) — g.pickardou, Sep 18 '17 at 09:10
@w2olves can you show the EncryptString method? A possible reason for the exception to be fired could be if you concatenate IV + cipher in a wrong order — Ivan Zaruba, May 03 '18 at 10:28
By changing the value for `cipher` and altering the second `Buffer.BlockCopy` statement as mentioned in this link solves this issue: https://stackoverflow.com/a/46541503/4745542 — Vikas C, May 04 '18 at 03:58

score 1 · Answer 1 · answered Nov 15 '18 at 17:16

1

Two changes are required

var cipher = new byte[fullCipher.Length - iv.Length];

and

Buffer.BlockCopy(fullCipher, iv.Length, cipher, 0, fullCipher.Length - iv.Length);

answered Nov 15 '18 at 17:16

Clayton Harbich

505
1
7
16

score -1 · Answer 2 · answered Aug 30 '19 at 10:55

public static string Decrypt(string cipherText)
  {
     string EncryptionKey  = string.Empty;

    // Check whether the environment variable exists.
    EncryptionKey = Environment.GetEnvironmentVariable("EncryptKey");

    if (EncryptionKey == null)
    {
        EncryptionKey = "E546C8DF278CD5931069B522E695D4F2";
    }
            byte[] cipherBytes;
            try
            {
                cipherBytes = Convert.FromBase64String(cipherText);
            }
            catch
            {
                return 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;
        }

Note :- You need to provide same key which you provided for encryption the string.

This is a block of code that's copy-pasted all over the web and this site. Mention where you found it, and why it's a good choice to do what the OP wants. — CodeCaster, Aug 30 '19 at 10:59
@CodeCaster I am using this code. i did not copy from anywhere. I add this code from multiple places. — Love Pandey, Aug 30 '19 at 16:02

aspnet core encrypting and decrypting a string

2 Answers2