Decrypt AES256 encrypted string with VB.net

Question

I want to decrypt in VB.net strings encrypted in PHP via openssl_encrypt function:

$encrypted_string = openssl_encrypt(
    $string,
    'AES256',
    'secret_password',
    0,
    'initialization_vector'
);

I tried with this class:

Public Class Aes256Encrypter
    Public Function Encrypt(ByVal plainText As String, ByVal secretKey As String) As Byte()
        Dim encryptedPassword As Byte()
        Using outputStream As MemoryStream = New MemoryStream()
            Dim algorithm As RijndaelManaged = getAlgorithm(secretKey)
            Using cryptoStream As CryptoStream = New CryptoStream(outputStream, algorithm.CreateEncryptor(), CryptoStreamMode.Write)
                Dim inputBuffer() As Byte = Encoding.Unicode.GetBytes(plainText)
                cryptoStream.Write(inputBuffer, 0, inputBuffer.Length)
                cryptoStream.FlushFinalBlock()
                encryptedPassword = outputStream.ToArray()
            End Using
        End Using
        Return encryptedPassword
    End Function

    Public Function Decrypt(ByVal encryptedBytes As Byte(), ByVal secretKey As String) As String
        Dim plainText As String = Nothing
        Using inputStream As MemoryStream = New MemoryStream(encryptedBytes)
            Dim algorithm As RijndaelManaged = getAlgorithm(secretKey)
            Using cryptoStream As CryptoStream = New    CryptoStream(inputStream, algorithm.CreateDecryptor(), CryptoStreamMode.Read)
                Dim outputBuffer(0 To CType(inputStream.Length - 1, Integer)) As Byte
                Dim readBytes As Integer = cryptoStream.Read(outputBuffer, 0, CType(inputStream.Length, Integer))
                plainText = Encoding.Unicode.GetString(outputBuffer, 0, readBytes)
            End Using
        End Using
        Return plainText
    End Function

    Private Function getAlgorithm(ByVal secretKey As String) As RijndaelManaged
        Const salt As String = "put your salt here"
        Const keySize As Integer = 256

        Dim keyBuilder As Rfc2898DeriveBytes = New Rfc2898DeriveBytes(secretKey, Encoding.Unicode.GetBytes(salt))
        Dim algorithm As RijndaelManaged = New RijndaelManaged()
        algorithm.KeySize = keySize
        algorithm.IV = keyBuilder.GetBytes(CType(algorithm.BlockSize / 8, Integer))
        algorithm.Key = keyBuilder.GetBytes(CType(algorithm.KeySize / 8, Integer))
        algorithm.Padding = PaddingMode.PKCS7
        Return algorithm
    End Function
End Class

I use IV "initialization_vector" as salt string, but at runtime an error is thrown: 'System.Security.Cryptography.CryptographicException' in mscorlib.dll. An error occurred: Padding is invalid and cannot be removed.

Where am I doing wrong?

Possible duplicate of Standard library for AES encryption for VB.NET? — SaggingRufus
– SaggingRufus, Commented Jan 9, 2017 at 13:23
Sorry @SaggingRufus but still nothing. It's not clear how should I use the initialization vector. If I put it as a salt, at runtime it throws an exception, 'System.Security.Cryptography.CryptographicException' in mscorlib.dll: Padding is invalid and cannot be removed — Blazeag
– Blazeag, Commented Jan 9, 2017 at 13:41
@SaggingRufus if with "salt" is meant the initialization vector, it is alphanumeric, with no spaces. I don't know what type of padding does openssl_encrypt function uses as default — Blazeag
– Blazeag, Commented Jan 9, 2017 at 14:10

bartonjs · Accepted Answer · 2017-01-12 02:35:13Z

Okay, so this is a C# answer, and I don't like it; but I've produced a thing which gives the same answer as php's openssl_encrypt.

PHP passes the string as if it were bytes, so Encoding.ASCII (not sure what it does with non-ASCII data, actually)
PHP/OpenSSL silently truncates long keys (so your 60 character password is a 32 character password)
Working with the same data in ASCII strings and hex bytes is a pain in the rear.

Code:

private static byte[] php_encode(string php_string, int expectedLength)
{
    byte[] temp = Encoding.ASCII.GetBytes(php_string);

    if (temp.Length == expectedLength)
    {
        return temp;
    }

    byte[] ret = new byte[expectedLength];
    Buffer.BlockCopy(temp, 0, ret, 0, Math.Min(temp.Length, expectedLength));
    return ret;
}

private static string php_encrypt_aes256(string php_data, string php_password, string php_iv)
{
    byte[] key = php_encode(php_password, 32);
    byte[] iv = php_encode(php_iv, 16);
    byte[] data = Encoding.ASCII.GetBytes(php_data);

    using (Aes aes = Aes.Create())
    using (ICryptoTransform encryptor = aes.CreateEncryptor(key, iv))
    {
        return Convert.ToBase64String(encryptor.TransformFinalBlock(data, 0, data.Length));
    }
}

Test inputs:

PHP:

$ /usr/bin/php -r "print(openssl_encrypt('potato', 'AES256', '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ', 0, '0123456789ABCDEF') . \"\n\");"
wiZGvfABfairN4gKcnqD2Q==
$ /usr/bin/php -r "print(openssl_encrypt('potato', 'AES256', '0123456789abcdefghijklmnopqrstuv', 0, '0123456789ABCDEF') . \"\n\");"
wiZGvfABfairN4gKcnqD2Q==
$ /usr/bin/php -r "print(openssl_encrypt('potato', 'AES256', '0123456789abcdefghijklmnopqrstu', 0, '0123456789ABCDEF') . \"\n\");"
XUkYwaomkIXYstlOQlNoBQ==
$ /usr/bin/php -r "print(openssl_encrypt('potato', 'AES256', '0123456789abcdefghijklmnopqrstuv', 0, '0123456789ABCDE') . \"\n\");"
PHP Warning:  openssl_encrypt(): IV passed is only 15 bytes long, cipher expects an IV of precisely 16 bytes, padding with \0 in Command line code on line 1
ah+yXvQEocgLNES2nh2kXA==

openssl enc

$ openssl enc -aes-256-cbc -K 303132333435363738396162636465666768696a6b6c6d6e6f70717273747576 -in enc.data -a -iv 30313233343536373839414243444546
wiZGvfABfairN4gKcnqD2Q==
$ openssl enc -aes-256-cbc -K 303132333435363738396162636465666768696a6b6c6d6e6f70717273747500 -in enc.data -a -iv 30313233343536373839414243444546
XUkYwaomkIXYstlOQlNoBQ==
$ openssl enc -aes-256-cbc -K 303132333435363738396162636465666768696a6b6c6d6e6f70717273747576 -in enc.data -a -iv 30313233343536373839414243444500
ah+yXvQEocgLNES2nh2kXA==

My code

Console.WriteLine(php_encrypt_aes256("potato", "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ", "0123456789ABCDEF"));
Console.WriteLine(php_encrypt_aes256("potato", "0123456789abcdefghijklmnopqrstuv", "0123456789ABCDEF"));
Console.WriteLine(php_encrypt_aes256("potato", "0123456789abcdefghijklmnopqrstu", "0123456789ABCDEF"));
Console.WriteLine(php_encrypt_aes256("potato", "0123456789abcdefghijklmnopqrstuv", "0123456789ABCDE"));

Output:

wiZGvfABfairN4gKcnqD2Q==
wiZGvfABfairN4gKcnqD2Q==
XUkYwaomkIXYstlOQlNoBQ==
ah+yXvQEocgLNES2nh2kXA==

Great, this works fine! I get the same results as openssl_encrypt. But in order to achieve the reverse operation, what should I do?
Reverse the operations: Convert.FromBase64String the string to data, then use CreateDecryptor instead of CreateEncryptor, and Encoding.ASCII.GetString the bytes back to the original string. Same key/IV handling code.

user18954 · Accepted Answer · 2020-04-22 19:09:59Z

I modified it slightly, but I got decryption working in VB.net if anyone needs it.

Private Shared Function php_decrypt_aes256(ByVal php_password As String, ByVal php_iv As String, ByVal php_data As String) As String
    Dim key As Byte() = php_encode(php_password, 32)
    Dim iv As Byte() = php_encode(php_iv, 16)
    Dim buff As Byte() = Encoding.ASCII.GetBytes(php_data)

    Using aes As Aes = Aes.Create()
        Using decryptor As ICryptoTransform = aes.CreateDecryptor(key, iv)
            buff = Convert.FromBase64String(php_data)
            Return ASCIIEncoding.ASCII.GetString(decryptor.TransformFinalBlock(buff, 0, buff.Length))
        End Using
    End Using
End Function

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Decrypt AES256 encrypted string with VB.net

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Test inputs:

PHP:

openssl enc

My code

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2 Answers 2

Test inputs:

PHP:

openssl enc

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