utils: drop datum_secure_strequals

[1ma]

The DATUM gateway already depends on libsodium, and it has a constant-time comparison function. This other PR sounds like the sort of thing that we shouldn't be even thinking about in this repo.

Tested in my own DATUM deployment.

[wizkid057]

If I recall the reasoning correctly, the libsodium function does not work correctly and as intended when comparing different length values, and no such function exists within libsodium.

[luke-jr]

Right, sodium_memcmp can only compare memory of equal length, which doesn't work for things like passwords where the strings have potentially different lengths.

[1ma]

Oof, good catch. Pretty rough that they don't mention this in their docs.

Comparing passwords in constant time is very common, but in DG there's a bit of friction because it has the secret in cleartext. Usually a backend would store hashes of passwords and hash the incoming password before comparing both.

Still, I think we can use their pwhash API. DG would simply store the result of crypto_pwhash_str() in the datum_config struct, then in datum_api.c we would call crypto_pwhash_str_verify.

Let me try this approach before giving up.

[1ma]

This length check before calling sodium_memcmp should not leak any information because the length of CSRF tokens is already public (64 bytes).