BABE signing and verification (#2270)

* Add basic BABE consensus type

* Update core/consensus/babe/slots/Cargo.toml

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Fix parameterization and run `rustfmt`

* Respond to review comments

* Update various Cargo.lock files

* Revert "Update various Cargo.lock files"

This reverts commit af53d7624752a744320e9cbb25749fdd8e6f46d2.

* `BabeSealSignature` → `BabeSeal`

* Move slot code to its own crate

This was highly non-trivial, due to cyclic dependencies.

* Remove redundancy between AuRa and BABE

Some of the code duplication was removed using a macro.

* Fix build error

* Avoid non-`#[doc(hidden)]` re-exports

Also, bump some library versions in `Cargo.toml`.

* Remove dead code in AuRa

* Remove impl_slot macro

It was more trouble than it was worth.

Also, delete useless dependencies on Serde.

* AuRa and BABE need different DB keys

* Bring back `aura::Network`, but deprecate it.

* Improve docs and add `slot_duration` inherent method

* Add docs to `substrate_consensus_aura::SlotDuration`

* Add missing documentation and #![forbid(missing_docs, unsafe_code)]

* Add a #![forbid(missing_docs)]

* Remove dependency of `test-runtime` on `slots`

* Update core/consensus/babe/src/lib.rs

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Remove wrongly added file

* Fix copyright notice

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Bump `impl_version` and `spec_version`

* Add more code to BABE

Most of it is copied from AuRa code, but at least the initial core is
there.

* Stuck on horrible compiler error message

* add missing files

* Spaces → tabs

* Simplify code

* Fix compilation

This involved fixing dependencies and adding a `Mutex`.

* More work on BABE

* Fix deprecation version

* Fix deprecation version; remove spurious carets

* Fix Cargo.toml

* Implement VRF signing logic

* The import queue code compiles, though it probably doesn’t work.

* Add VRF verification

* Update Cargo.lock

* Update dependencies

* Move test network to sr25519 authority keys

* Fix accidental build bustage

* Trying to get the tests to work

* Add logging messages and remove dead code

There seems to be a problem with the test network.  Since AuRa and BABE
are both affected, this is most likely due to the switch from ed25519 to
sr25519.

* Trying to get the tests to work

* Add logging messages and remove dead code

There seems to be a problem with the test network.  Since AuRa and BABE
are both affected, this is most likely due to the switch from ed25519 to
sr25519.

* Working testsuite at last!

The problem was with serialization and deserialization.  Normally,
those functions are generated automatically, but those for `BabeSeal`
had to be written manually.  The hand-written versions were not
correct, however, as shown by the decoder not being able to decode the
output of the encoder.

* Enable BabeSeal::Encode asserts in --release tests

* Bump runtime and dependency versions

* Fix wasm compilation

The wasm build was broken because of a typo in
`core/test-runtime/src/lib.rs`, and missing gates on the `std` feature
in `core/consensus/{aura,babe}/primitives/Cargo.toml`.  Additionally,
improve the quotation in the build scripts.

* Merge Cargo.lock

* Change expected JSON string

The test was also broken on `master`, so I suspect that the test was
incorrect.

* Responded to review

* Remove hard-coded threshold from production code

A hard-coded threshold is now only used in tests.

* Fix swapped doc comments

* Fix unused import warnings

* fix ci error

* fix typo

* Fix spacing in docs

* Minor changes suggested by @joepetrowski

on https://github.com/paritytech/substrate/pull/2372

* Remove unnecessary getters

* fix compile error

* Fix silly unused-variable error

* Improve documentation formatting

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Add issue links

* Revert excess verbosity and #![forbid(warnings)]

* Apply suggestions from code review

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Reformat some comments

* Threshold should depend on number of validators

Also, respond to code review

* Fix silly compilation errors

* Reduce logging verbosity

* Fix missing import

substrate/core/consensus/babe/primitives/src/lib.rs

@@ -29,31 +29,28 @@ pub const BABE_ENGINE_ID: ConsensusEngineId = [b'b', b'a', b'b', b'e'];
 /// Configuration data used by the BABE consensus engine.
 #[derive(Copy, Clone, Hash, PartialEq, Eq, Debug, Encode, Decode)]
 pub struct BabeConfiguration {
-	slot_duration: u64,
-	expected_block_time: u64,
-}
-
-impl BabeConfiguration {
-	/// Return the expected block time in milliseconds for BABE.  Currently,
-	/// only the value provided by this type at genesis will be used.
-	///
-	/// Dynamic expected block time may be supported in the future.
-	pub fn expected_block_time(&self) -> u64 {
-		self.expected_block_time
-	}
-
-	/// Return the slot duration in milliseconds for BABE.  Currently, only
+	/// The slot duration in milliseconds for BABE. Currently, only
 	/// the value provided by this type at genesis will be used.
 	///
 	/// Dynamic slot duration may be supported in the future.
-	pub fn slot_duration(&self) -> u64 {
-		self.slot_duration
-	}
+	pub slot_duration: u64,
+
+	/// The expected block time in milliseconds for BABE. Currently,
+	/// only the value provided by this type at genesis will be used.
+	///
+	/// Dynamic expected block time may be supported in the future.
+	pub expected_block_time: u64,
+
+	/// The maximum permitted VRF output, or *threshold*, for BABE. Currently,
+	/// only the value provided by this type at genesis will be used.
+	///
+	/// Dynamic thresholds may be supported in the future.
+	pub threshold: u64,
 }
 
 #[cfg(feature = "std")]
 impl slots::SlotData for BabeConfiguration {
-	/// Return the slot duration in milliseconds for BABE.  Currently, only
+	/// Return the slot duration in milliseconds for BABE. Currently, only
 	/// the value provided by this type at genesis will be used.
 	///
 	/// Dynamic slot duration may be supported in the future.
@@ -67,7 +64,7 @@ impl slots::SlotData for BabeConfiguration {
 decl_runtime_apis! {
 	/// API necessary for block authorship with BABE.
 	pub trait BabeApi {
-		/// Return the configuration for BABE.  Currently,
+		/// Return the configuration for BABE. Currently,
 		/// only the value provided by this type at genesis will be used.
 		///
 		/// Dynamic configuration may be supported in the future.