Currently `DeriveKey` is called every time that a secret must be encoded/decoded. Since this function is deterministic, its result can be cached to allow a 250x speedup (the original took less than half a microsecond, so this more of a micro-optimization...).
```
go test -bench=.
goos: linux
goarch: amd64
pkg: forgejo.org/modules/keying
cpu: Intel(R) Core(TM) Ultra 5 125H
BenchmarkExpandPRK-18 2071627 564.2 ns/op
BenchmarkExpandPRKOnce-18 541438192 2.206 ns/op
PASS
ok forgejo.org/modules/keying 2.369s
```
## Other changes
- Since the keys can be constructed once, it simplifies a bit the callsites (`keying.TOTP.Encrypt(...)` instead of `keying.DeriveKey(keying.ContextTOTP).Encrypt(...)`)
- All `Encrypt`/`Decrypt` calls will panic forever if called before `Init` has been called (current it panics as long as `Init` has not been called)
- Calling `Init` twice with different keys will trigger a panic (currently racy)
- Calling `Decrypt` with a short ciphertext does not panic anymore (like when calling with long-enough garbage)
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10114
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>
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