Hashing a SecureString in .NET

Question

In .NET, we have the SecureString class, which is all very well until you come to try and use it, as to (for example) hash the string, you need the plaintext. I\'ve had a go

Answer 1

As a complement to Hans’ answer here’s a suggestion how to implement the hasher. Hans suggests passing the pointer to the unmanaged string to the hash function but that means that client code (= the hash function) needs to deal with unmanaged memory. That’s not ideal.

On the other hand, you can replace the callback by an instance of the following interface:

interface Hasher {
    void Reinitialize();
    void AddByte(byte b);
    byte[] Result { get; }
}

That way the hasher (although it becomes slightly more complex) can be implemented wholly in managed land without leaking secure information. Your HashSecureString would then look as follows:

private static byte[] HashSecureString(SecureString ss, Hasher hasher) {
    IntPtr bstr = Marshal.SecureStringToBSTR(ss);
    try {
        int length = Marshal.ReadInt32(bstr, -4);

        hasher.Reinitialize();

        for (int i = 0; i < length; i++)
            hasher.AddByte(Marshal.ReadByte(bstr, i));

        return hasher.Result;
    }
    finally {
        Marshal.ZeroFreeBSTR(bstr);
    }
}

Note the finally block to make sure that the unmanaged memory is zeroed, no matter what shenanigans the hasher instance does.

Here’s a simple (and not very useful) Hasher implementation to illustrate the interface:

sealed class SingleByteXor : Hasher {
    private readonly byte[] data = new byte[1];

    public void Reinitialize() {
        data[0] = 0;
    }

    public void AddByte(byte b) {
        data[0] ^= b;
    }

    public byte[] Result {
        get { return data; }
    }
}