SecureString to Byte[] C#

Question

How would I get a byte[] equivalent of a SecureString (which I get from a PasswordBox)?

My objective is to write these bytes using a CryptoStream to a file, and the Write method of that class takes a byte[] input, so I want to convert the SecureString to the byte[] so I can use in with a CryptoStream.

EDIT: I don't want to use string as it defeats the point of having a SecureString

Answer 1

I modified from the original answer to handle unicode

IntPtr unmanagedBytes = Marshal.SecureStringToGlobalAllocUnicode(password);
byte[] bValue = null;
try
{
    byte* byteArray = (byte*)unmanagedBytes.GetPointer();

    // Find the end of the string
    byte* pEnd = byteArray;
    char c='\0';
    do
    {
        byte b1=*pEnd++;
        byte b2=*pEnd++;
        c = '\0';
        c= (char)(b1 << 8);                 
        c += (char)b2;
    }while (c != '\0');

    // Length is effectively the difference here (note we're 2 past end) 
    int length = (int)((pEnd - byteArray) - 2);
    bValue = new byte[length];
    for (int i=0;i<length;++i)
    {
        // Work with data in byte array as necessary, via pointers, here
        bValue[i] = *(byteArray + i);
    }
}
finally
{
    // This will completely remove the data from memory
    Marshal.ZeroFreeGlobalAllocUnicode(unmanagedBytes);
}