I want to create an allocator which provides memory with the following attributes:
The idea is that this will contain sensitive information (like licence information) which should be inaccessible to the user. I have done the usual research online and asked a few other people about this, but I cannot find a good place start on this problem.
Your best bet is to implement something similar to .NET's SecureString class, and be very careful to zero out any plaintext copies of your data as soon as you are done (don't forget to cleanup even when exceptions are thrown). A good way to do this with std::string and such is to use a custom allocator.
On Windows, if you use CryptProtectMemory (or RtlEncryptMemory for older systems), the encryption password is stored in non-pageable (kernel?) memory. In my testing, these functions are pretty darn fast, esp. taking into account the protection they are giving you.
On other systems, I like to use Blowfish since it's a good mix between speed and strength. In the latter case, you will have to randomly generate your own password (16+ bytes of entropy for Blowfish) at program startup. Unfortunately, there's not a whole lot you can do to protect that password without OS support, although you might use general obfuscation techniques to embed a hard-coded salt value into your executable that you can combine with the password (every little bit helps).
Overall, this strategy is only one part of a broader defense-in-depth approach. Also keep in mind that simple bugs such as buffer overflows and not sanitizing program input remain by far the most common attack vectors.