AES decryption error “ The input data is not a complete block.” Error vb.net

Question

I keep getting this \"The input data is not a complete block.\" error while decrypting. The function successfully encrypts plain text and puts the IV in a textbox. I am using t

Answer 1

Unless you are going to manually make sure your input is in multiples of BlockSize (in bits), make sure to specify a padding:

Example code:

byte[] Process(byte[] bInput, bool decrypt = false)
{
    byte[] bOutput = null;

    using (var c = System.Security.Cryptography.AesCryptoServiceProvider.Create())
    {
        c.BlockSize = 128;
        c.IV = new byte[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xa, 0xb, 0xc, 0xd, 0xe, 0xf }; // blocksize / 8 = 16 long
        c.KeySize = 256;
        c.Key = new byte[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xa, 0xb, 0xc, 0xd, 0xe, 0xf, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xa, 0xb, 0xc, 0xd, 0xe, 0xf }; // key = keysize / 8 = 32 bytes
        c.Mode = System.Security.Cryptography.CipherMode.CBC;
        c.Padding = System.Security.Cryptography.PaddingMode.PKCS7;
        if (decrypt)
        {
            using (var t = c.CreateDecryptor())
            {
                bOutput = t.TransformFinalBlock(bInput, 0, bInput.Length);
            }
        }
        else
        {
            using (var t = c.CreateEncryptor())
            {
                bOutput = t.TransformFinalBlock(bInput, 0, bInput.Length);
            }
        }
    }

    return bOutput;
}

Answer 2

There are multiple issues with your code.

In encryption function:

1. Wrong constructor is used for MemoryStream, so it gets created with fixed size and cannot be expanded by future padding. You should use New MemoryStream(datain.Length) instead.
2. First memorystream.Close() is issued and then the contents of memorystream.ToArray() is requested, — but without any explicit cstream.Close() invocation: that way the last blocks, including padding, are not actually written to memorystream. You should call cstream.Close() before extracting memorystream data.
3. It is quite illogical, although not forbidden, to release the AesCryptoServiceProvider resources before closing both CryptoStream and MemoryStream.

In decryption function:

1. The amount of data that will get to decrypteddata is overestimated without taking padding into account. You should shrink this array based on the actual value returned by cryptostream.Read(…).
2. Same issue with premature AES.Clear() as above.
3. While the encryption function expects text input, the decryption function returns its result as a Base64-encoded binary data. You probably meant Return Encoding.UTF8.GetString(decrypteddata.ToArray()) instead.

After fixing those issues, I could run your program without any error.

Answer 3

I had the same problem until I implemented with ICryptoTransform:

...
Dim AES As New AesCryptoServiceProvider
AES.Key = Encryptionkey
AES.Mode = CipherMode.CBC
AES.IV = Convert.FromBase64String(tbIV.Text)

Dim transformer As ICryptoTransform = AES.CreateDecryptor()
dim trnsfrmBlock as Byte() = transformer.TransformFinalBlock(EncryptionIn, 0, EncryptionIn.Length)

Return Convert.ToBase64String(trnsfrmBlock)