How do a use a SecureString to create a SHA1 or SHA512 Hash?

Question

I would like to use a SecureString varible within VB.NET and convert that to a SHA1 or SHA512 hash. How would I securely convert the SecureString to the Byte array that HashAlgorithm.ComputeHash will accept?

Answer 1

Typed it up in c# and converted to VB. Hopefully it still works!

Dim input As [Char]() = "Super Secret String".ToCharArray()
Dim secret As New SecureString()

For idx As Integer = 0 To input.Length - 1
    secret.AppendChar(input(idx))
Next
SecurePassword.MakeReadOnly()

Dim pBStr As IntPtr = Marshal.SecureStringToBSTR(secret)

Dim output As String = Marshal.PtrToStringBSTR(pBStr)
Marshal.FreeBSTR(pBStr)

Dim sha As SHA512 = New SHA512Managed()
Dim result As Byte() = sha.ComputeHash(Encoding.UTF8.GetBytes(output))

edit:

As it has been pointed out by myself and a few others in the comments, I want to bring this to attention here. Doing this isn't a good idea. You are moving the bytes to a place that is no longer secure. Sure you CAN do it, but you probably shouldn't

Answer 2

What about that, if we avoid the only used String instance (output) and replace it with a character array. This would enable us to wipe this array after use:

    public static String SecureStringToMD5( SecureString password )
    {
        int passwordLength = password.Length;
        char[] passwordChars = new char[passwordLength];

        // Copy the password from SecureString to our char array
        IntPtr passwortPointer = Marshal.SecureStringToBSTR( password );
        Marshal.Copy( passwortPointer, passwordChars, 0, passwordLength );
        Marshal.ZeroFreeBSTR( passwortPointer );

        // Hash the char array
        MD5 md5Hasher = MD5.Create();
        byte[] hashedPasswordBytes = md5Hasher.ComputeHash( Encoding.Default.GetBytes( passwordChars ) );

        // Wipe the character array from memory
        for (int i = 0; i < passwordChars.Length; i++)
        {
            passwordChars[i] = '\0';
        }

        // Your implementation of representing the hash in a readable manner
        String hashString = ConvertToHexString( hashedPasswordBytes );

        // Return the result
        return hashString;
    }

Is there anything I missed?