Before the introducee sends her ACK,
she derives a master key from the ephemeral shared secret as before.
Two nonces and a MAC key are then derived from the master key.
The local introducee signs one of the nonces and calculates a MAC
over her own identity public key, ephemeral public key,
transport properties and timestamp.
The local introducee includes the signature and MAC in her ACK.
On receiving the remote introducee's ACK,
the local introducee verifies the signature and MAC.
Should the verification fail, an ABORT is sent to the introducer and
the remote introducee that was added as inactive is deleted again.
1. The things we're really trying to protect - contact identities,
message contents, etc - can't be erased from memory because they're
encapsulated inside objects we don't control.
2. Long-term secrets can't be protected by erasing them from memory
because they're stored in the database and the database key has to be
held in memory whenever the app's running.
3. If the runtime uses a compacting garbage collector then we have no
way to ensure an object is erased from memory.
4. Trying to erase secrets from memory makes the code more complex.
Conclusion: Let's not try to protect secrets from an attacker who can
read arbitrary memory locations.
Note that this is only the generator part of Fortuna, not the
accumulator. The generator requires a seed, which is provided by a
platform-specific implementation of SeedProvider. On Linux the
implementation reads the seed from /dev/urandom.
