instead of having a static one, as that could end up taking up a
lot of memory if your app keeps olm sessions hanging about.

As a way to dump the state of an olm session, ie. the chain indicies,
so we can debug why olm sessions break and get out of sync.

also make some params const where possible

Change interface to allow the app to get the private part of the
key and instantiate a decryption object from just the private part
of the key.

Changes the function generating a key from random bytes to be
initialising a key with a private key (because it's exactly the
same thing). Exports & imports private key parts as ArrayBuffer at
JS level rather than base64 assuming we are moving that way in
general.

...when generating a key in PkDecryption.

The pubkey is base64ed on the output, so will be longer.

Signed-off-by: Alexey Rusakov <ktirf@users.sf.net>

This disrupts building at least with Visual Studio.

Signed-off-by: Alexey Rusakov <ktirf@users.sf.net>

olm_import_inbound_group_session, which reads the format written by
olm_export_inbound_group_session to initialise a group session.

A pair of functions which allow you to export the megolm keys for an inbound
group session, so that an application can save/restore them.

make olm_pickle_* return the lengths of the base64-encoded pickles, rather than
the raw pickle. (From the application's POV, the format of the pickle is
opaque: it doesn't even know that it is base64-encoded. So returning the length
of the raw pickle is particularly unhelpful.)

All the other methods clear their random inputs. This one needs to do the same,
to reduce the risk of the randomness being used elsewhere and leaking key info.

Applications can use the index to detect replays of the same message.

Fixes a segfault when a group message had exactly the length of the mac +
signature.

Also tweak skipping of unknown tags to avoid an extra trip around the loop.

Some clients expect the session id to be globally unique,
so allowing the end devices to pick the session id will cause
problems.

Include the current ratchet index with the initial keys, this decreases
the risk that the client will supply the wrong index causing problems.

Sign the initial keys with the ratchet ed25519 key, this reduces the
risk of a client claiming a session that they didn't create.

Add ed25519 keys to the inbound and outbound sessions, and use them to sign and
verify megolm messages.

We just stuff the ed25519 public key in alongside the megolm session key (and
add a version byte), to save adding more boilerplate to the JS/python/etc
layers.

... so that I can use them from the group session bits.