random.dart: RandomSource, the injectable source of every random value
of a writer; secureRandom, the default, Random.secure of the platform;
SeededRandomSource, deterministic, for tests and vectors only; and, for
the 16 slots of INNER_ACCESS_AGE, randomIndex, an integer drawn as
crypto/rand.Int draws it, and permute, as the permute of
capsule.Encrypt.
encryptOnG2 and wrapTlockStanza take the source of sigma, secureRandom
by default, so that a tlock stanza can be written again byte for byte;
ibe.dart no longer holds a Random.secure of its own.
The tests check the keystream, randomIndex and permute against the
vectors of Go, randomIndex at its bounds, the uniformity of permute, and
the CSPRNG on the VM: in dart test -p node there is no Random.secure.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
