Sassafras primitives (#1249)

* Introduce Sassafras primitives

* Keystore workaround

* Fix doc

* Use  in keystore

* Improve bandersnatch vrf docs

* Apply review suggestions

* Update README

* Docs improvement

* Docs fix

substrate/primitives/consensus/sassafras/src/vrf.rs

@@ -0,0 +1,104 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Utilities related to VRF input, output and signatures.
+
+use crate::{Randomness, TicketBody, TicketId};
+use scale_codec::Encode;
+use sp_consensus_slots::Slot;
+use sp_std::vec::Vec;
+
+pub use sp_core::bandersnatch::{
+	ring_vrf::{RingContext, RingProver, RingVerifier, RingVrfSignature},
+	vrf::{VrfInput, VrfOutput, VrfSignData, VrfSignature},
+};
+
+fn vrf_input_from_data(
+	domain: &[u8],
+	data: impl IntoIterator<Item = impl AsRef<[u8]>>,
+) -> VrfInput {
+	let buf = data.into_iter().fold(Vec::new(), |mut buf, item| {
+		let bytes = item.as_ref();
+		buf.extend_from_slice(bytes);
+		let len = u8::try_from(bytes.len()).expect("private function with well known inputs; qed");
+		buf.push(len);
+		buf
+	});
+	VrfInput::new(domain, buf)
+}
+
+/// VRF input to claim slot ownership during block production.
+pub fn slot_claim_input(randomness: &Randomness, slot: Slot, epoch: u64) -> VrfInput {
+	vrf_input_from_data(
+		b"sassafras-claim-v1.0",
+		[randomness.as_slice(), &slot.to_le_bytes(), &epoch.to_le_bytes()],
+	)
+}
+
+/// Signing-data to claim slot ownership during block production.
+pub fn slot_claim_sign_data(randomness: &Randomness, slot: Slot, epoch: u64) -> VrfSignData {
+	let input = slot_claim_input(randomness, slot, epoch);
+	VrfSignData::new_unchecked(
+		b"sassafras-slot-claim-transcript-v1.0",
+		Option::<&[u8]>::None,
+		Some(input),
+	)
+}
+
+/// VRF input to generate the ticket id.
+pub fn ticket_id_input(randomness: &Randomness, attempt: u32, epoch: u64) -> VrfInput {
+	vrf_input_from_data(
+		b"sassafras-ticket-v1.0",
+		[randomness.as_slice(), &attempt.to_le_bytes(), &epoch.to_le_bytes()],
+	)
+}
+
+/// VRF input to generate the revealed key.
+pub fn revealed_key_input(randomness: &Randomness, attempt: u32, epoch: u64) -> VrfInput {
+	vrf_input_from_data(
+		b"sassafras-revealed-v1.0",
+		[randomness.as_slice(), &attempt.to_le_bytes(), &epoch.to_le_bytes()],
+	)
+}
+
+/// Data to be signed via ring-vrf.
+pub fn ticket_body_sign_data(ticket_body: &TicketBody, ticket_id_input: VrfInput) -> VrfSignData {
+	VrfSignData::new_unchecked(
+		b"sassafras-ticket-body-transcript-v1.0",
+		Some(ticket_body.encode().as_slice()),
+		Some(ticket_id_input),
+	)
+}
+
+/// Make ticket-id from the given VRF input and output.
+///
+/// Input should have been obtained via [`ticket_id_input`].
+/// Output should have been obtained from the input directly using the vrf secret key
+/// or from the vrf signature outputs.
+pub fn make_ticket_id(input: &VrfInput, output: &VrfOutput) -> TicketId {
+	let bytes = output.make_bytes::<16>(b"ticket-id", input);
+	u128::from_le_bytes(bytes)
+}
+
+/// Make revealed key seed from a given VRF input and ouput.
+///
+/// Input should have been obtained via [`revealed_key_input`].
+/// Output should have been obtained from the input directly using the vrf secret key
+/// or from the vrf signature outputs.
+pub fn make_revealed_key_seed(input: &VrfInput, output: &VrfOutput) -> [u8; 32] {
+	output.make_bytes::<32>(b"revealed-seed", input)
+}