3.2 Design principle: Secure default algorithms

A basic design principle of this library is that its default algorithms are cryptographically secure at the time of this writing. We will change the default algorithms if an attack on them becomes known, and replace them by new defaults that are deemed appropriate at that time.

This may mean, for example, that where sha256 is currently the default algorithm, blake2s256 or some other algorithm may become the default in the future.

To preserve interoperability and compatibility and at the same time allow us to transparently update default algorithms of this library, the following conventions are used:

1. If an explicit algorithm is specified as an option, then that algorithm is used.
2. If no algorithm is specified, then a cryptographically secure algorithm is used.
3. If an option that normally specifies an algorithm is present, and a logical variable appears instead of a concrete algorithm, then that variable is unified with the secure default value.

This allows application programmers to inspect which algorithm was actually used, and store it for later reference.

For example:

?- crypto_data_hash(test, Hash, [algorithm(A)]).
Hash = '9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08',
A = sha256.

This shows that at the time of this writing, sha256 was deemed sufficiently secure, and was used as default algorithm for hashing.

You therefore must not rely on which concrete algorithm is being used by default. However, you can rely on the fact that the default algorithms are secure. In other words, if they are not secure, then this is a mistake in this library, and we ask you to please report such a situation as an urgent security issue.