Ensure derive_channel_keys doesn't panic if per-run seed is high

b04d1b8 changed the way we
calculate the `channel_keys_id` to include the 128-bit
`user_channel_id` as well, shifting the counter up four bytes and
the `starting_time_nanos` field up into the second four bytes.

In `derive_channel_keys` we hash the full `channel_keys_id` with an
HD-derived key from our master seed. Previously, that key was
derived with an index of the per-restart counter, re-calculated by
pulling the second four bytes out of the `user_channel_id`. Because
the `channel_keys_id` fields were shifted up four bytes, that is
now a reference to the `starting_time_nanos` value. This should be
fine, the derivation doesn't really add any value here, its all
being hashed anyway, except that derivation IDs must be below 2^31.
This implies that we panic if the user passes a
`starting_time_nanos` which has the high bit set. For those using
the nanosecond part of the current time this isn't an issue - the
value cannot exceed 1_000_000, which does not have the high bit
set, however, some users may use some other per-run seed.

Thus, here we simply drop the high bit from the seed, ensuring we
don't panic. Note that this is backwards compatible as it only
changes the key derivation in cases where we previously panicked.

Ideally we'd drop the derivation entirely, but that would break
backwards compatibility of key derivation.

Author: TheBlueMatt

lightning/src/chain/keysinterface.rs

@@ -1051,7 +1051,9 @@ impl KeysManager {
 		// We only seriously intend to rely on the channel_master_key for true secure
 		// entropy, everything else just ensures uniqueness. We rely on the unique_start (ie
 		// starting_time provided in the constructor) to be unique.
-		let child_privkey = self.channel_master_key.ckd_priv(&self.secp_ctx, ChildNumber::from_hardened_idx(chan_id as u32).expect("key space exhausted")).expect("Your RNG is busted");
+		let child_privkey = self.channel_master_key.ckd_priv(&self.secp_ctx,
+				ChildNumber::from_hardened_idx((chan_id as u32) % (1 << 31)).expect("key space exhausted")
+			).expect("Your RNG is busted");
 		unique_start.input(&child_privkey.private_key[..]);
 
 		let seed = Sha256::from_engine(unique_start).into_inner();