Secure Memory Containers — Botan

# Botan

# Secure Memory Containers

A major concern with mixing modern multiuser OSes and cryptographic code is that at any time the code (including secret keys) could be swapped to disk, where it can later be read by an attacker. Botan stores almost everything (and especially anything sensitive) in memory buffers that a) clear out their contents when their destructors are called, and b) have easy plugins for various memory locking functions, such as the `mlock` call on many Unix systems.

Two of the allocation method used (“malloc” and “mmap”) don’t require any extra privileges on Unix, but locking memory does. At startup, each allocator type will attempt to allocate a few blocks (typically totaling 128k), so if you want, you can run your application `setuid` `root`, and then drop privileges immediately after creating your `LibraryInitializer`. If you end up using more than what’s been allocated, some of your sensitive data might end up being swappable, but that beats running as `root` all the time.

These classes should also be used within your own code for storing sensitive data. They are only meant for primitive data types (int, long, etc): if you want a container of higher level Botan objects, you can just use a `std::vector`, since these objects know how to clear themselves when they are destroyed. You cannot, however, have a `std::vector` (or any other container) of `Pipe` objects or filters, because these types have pointers to other filters, and implementing copy constructors for these types would be both hard and quite expensive (vectors of pointers to such objects is fine, though).

These types are not described in any great detail: for more information, consult the definitive sources~–~the header files `secmem.h` and `allocate.h`.

`SecureBuffer` is a simple array type, whose size is specified at compile time. It will automatically convert to a pointer of the appropriate type, and has a number of useful functions, including `clear()`, and `size_t` `size()`, which returns the length of the array. It is a template that takes as parameters a type, and a constant integer which is how long the array is (for example: `SecureBuffer<byte, 8> key;`).

`SecureVector` is a variable length array. Its size can be increased or decreased as need be, and it has a wide variety of functions useful for copying data into its buffer. Like `SecureBuffer`, it implements `clear` and `size`.

## Allocators

The containers described above get their memory from allocators. As a user of the library, you can add new allocator methods at run time for containers, including the ones used internally by the library, to use. The interface to this is in `allocate.h`. Code needing to allocate or deallocate memory calls `get_allocator`, which returns a pointer to an allocator object. This pointer should not be freed: the caller does not own the allocator (it is shared among multiple allocatore users, and uses a mutex to serialize access internally if necessary). It is possible to call `get_allocator` with a specific name to request a particular type of allocator, otherwise, a default allocator type is returned.

At start time, the only allocator known is a `Default_Allocator`, which just allocates memory using `malloc`, and zeroizes it when the memory is released. It is known by the name “malloc”. If you ask for another type of allocator (“locking” and “mmap” are currently used), and it is not available, some other allocator will be returned.

You can add in a new allocator type using `add_allocator_type`. This function takes a string and a pointer to an allocator. The string gives this allocator type a name to which it can be referred when one is requesting it with `get_allocator`. If an error occurs (such as the name being already registered), this function returns false. It will return true if the allocator was successfully registered. If you ask it to, `LibraryInitializer` will do this for you.

Finally, you can set the default allocator type that will be returned using the policy setting “default_alloc” to the name of any previously registered allocator.