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// -*- mode: rust; -*-
//
// This file is part of schnorrkel.
// Copyright (c) 2019 Web 3 Foundation
// See LICENSE for licensing information.
//
// Authors:
// - jeffrey Burdges <jeff@web3.foundation>
//! ### Schnorr signature creation and verification, including batch verification.
use core::fmt::{Debug};
use curve25519_dalek::constants;
use curve25519_dalek::ristretto::{CompressedRistretto,RistrettoPoint};
use curve25519_dalek::scalar::Scalar;
use super::*;
use crate::context::{SigningTranscript,SigningContext};
// === Actual signature type === //
/// The length of a curve25519 EdDSA `Signature`, in bytes.
pub const SIGNATURE_LENGTH: usize = 64;
/// A Ristretto Schnorr signature "detached" from the signed message.
///
/// These cannot be converted to any Ed25519 signature because they hash
/// curve points in the Ristretto encoding.
#[allow(non_snake_case)]
#[derive(Clone, Copy, Eq, PartialEq)]
pub struct Signature {
/// `R` is a `RistrettoPoint`, formed by using an hash function with
/// 512-bits output to produce the digest of:
///
/// - the nonce half of the `SecretKey`, and
/// - the message to be signed.
///
/// This digest is then interpreted as a `Scalar` and reduced into an
/// element in ℤ/lℤ. The scalar is then multiplied by the distinguished
/// basepoint to produce `R`, and `RistrettoPoint`.
pub (crate) R: CompressedRistretto,
/// `s` is a `Scalar`, formed by using an hash function with 512-bits output
/// to produce the digest of:
///
/// - the `r` portion of this `Signature`,
/// - the `PublicKey` which should be used to verify this `Signature`, and
/// - the message to be signed.
///
/// This digest is then interpreted as a `Scalar` and reduced into an
/// element in ℤ/lℤ.
pub (crate) s: Scalar,
}
impl Debug for Signature {
fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
write!(f, "Signature( R: {:?}, s: {:?} )", &self.R, &self.s)
}
}
fn check_scalar(bytes: [u8; 32]) -> SignatureResult<Scalar> {
// Since this is only used in signature deserialisation (i.e. upon
// verification), we can do a "succeed fast" trick by checking that the most
// significant 4 bits are unset. If they are unset, we can succeed fast
// because we are guaranteed that the scalar is fully reduced. However, if
// the 4th most significant bit is set, we must do the full reduction check,
// as the order of the basepoint is roughly a 2^(252.5) bit number.
//
// This succeed-fast trick should succeed for roughly half of all scalars.
if bytes[31] & 240 == 0 {
return Ok(Scalar::from_bits(bytes))
}
Scalar::from_canonical_bytes(bytes).ok_or(SignatureError::ScalarFormatError)
}
impl Signature {
const DESCRIPTION : &'static str = "A 64 byte Ristretto Schnorr signature";
/*
const DESCRIPTION_LONG : &'static str =
"A 64 byte Ristretto Schnorr signature, similar to an ed25519 \
signature as specified in RFC8032, except the Ristretto point \
compression is used for the curve point in the first 32 bytes";
*/
/// Convert this `Signature` to a byte array.
#[inline]
pub fn to_bytes(&self) -> [u8; SIGNATURE_LENGTH] {
let mut bytes: [u8; SIGNATURE_LENGTH] = [0u8; SIGNATURE_LENGTH];
bytes[..32].copy_from_slice(&self.R.as_bytes()[..]);
bytes[32..].copy_from_slice(&self.s.as_bytes()[..]);
bytes[63] |= 128;
bytes
}
/// Construct a `Signature` from a slice of bytes.
///
/// We distinguish schnorrkell signatures from ed25519 signatures
/// by setting the high bit of byte 31. We return an error if
/// this marker remains unset because otherwise schnorrkel
/// signatures would be indistinguishable from ed25519 signatures.
/// We cannot always distinguish between schnorrkel and ed25519
/// public keys either, so without this market bit we could not
/// do batch verification in systems that support precisely
/// ed25519 and schnorrkel.
///
/// We cannot distinguish amongst different `SigningTranscript`
/// types using these markey bits, but protocol should not need
/// two different transcript types.
#[inline]
pub fn from_bytes(bytes: &[u8]) -> SignatureResult<Signature> {
if bytes.len() != SIGNATURE_LENGTH {
return Err(SignatureError::BytesLengthError {
name: "Signature",
description: Signature::DESCRIPTION,
length: SIGNATURE_LENGTH
});
}
let mut lower: [u8; 32] = [0u8; 32];
let mut upper: [u8; 32] = [0u8; 32];
lower.copy_from_slice(&bytes[..32]);
upper.copy_from_slice(&bytes[32..]);
if upper[31] & 128 == 0 {
return Err(SignatureError::NotMarkedSchnorrkel);
}
upper[31] &= 127;
Ok(Signature{ R: CompressedRistretto(lower), s: check_scalar(upper) ? })
}
/// Depricated construction of a `Signature` from a slice of bytes
/// without checking the bit distinguishing from ed25519. Deprecated.
#[inline]
pub fn from_bytes_not_distinguished_from_ed25519(bytes: &[u8]) -> SignatureResult<Signature> {
if bytes.len() != SIGNATURE_LENGTH {
return Err(SignatureError::BytesLengthError {
name: "Signature",
description: Signature::DESCRIPTION,
length: SIGNATURE_LENGTH
});
}
let mut bytes0: [u8; SIGNATURE_LENGTH] = [0u8; SIGNATURE_LENGTH];
bytes0.copy_from_slice(bytes);
bytes0[63] |= 128;
Signature::from_bytes(&bytes0[..])
}
}
serde_boilerplate!(Signature);
// === Implement signing and verification operations on key types === //
impl SecretKey {
/// Sign a transcript with this `SecretKey`.
///
/// Requires a `SigningTranscript`, normally created from a
/// `SigningContext` and a message, as well as the public key
/// correspodning to `self`. Returns a Schnorr signature.
///
/// We employ a randomized nonce here, but also incorporate the
/// transcript like in a derandomized scheme, but only after first
/// extending the transcript by the public key. As a result, there
/// should be no attacks even if both the random number generator
/// fails and the function gets called with the wrong public key.
#[allow(non_snake_case)]
pub fn sign<T: SigningTranscript>(&self, mut t: T, public_key: &PublicKey) -> Signature
{
t.proto_name(b"Schnorr-sig");
t.commit_point(b"sign:pk",public_key.as_compressed());
let mut r = t.witness_scalar(b"signing",&[&self.nonce]); // context, message, A/public_key
let R = (&r * &constants::RISTRETTO_BASEPOINT_TABLE).compress();
t.commit_point(b"sign:R",&R);
let k: Scalar = t.challenge_scalar(b"sign:c"); // context, message, A/public_key, R=rG
let s: Scalar = &(&k * &self.key) + &r;
::zeroize::Zeroize::zeroize(&mut r);
Signature{ R, s }
}
/// Sign a message with this `SecretKey`, but doublecheck the result.
pub fn sign_doublecheck<T>(&self, t: T, public_key: &PublicKey) -> SignatureResult<Signature>
where T: SigningTranscript+Clone
{
let sig = self.sign(t.clone(),public_key);
let sig = Signature::from_bytes(& sig.to_bytes()) ?;
PublicKey::from_bytes(& public_key.to_bytes()) ?
.verify(t,&sig).map(|()| sig)
}
/// Sign a message with this `SecretKey`.
pub fn sign_simple(&self, ctx: &[u8], msg: &[u8], public_key: &PublicKey) -> Signature
{
let t = SigningContext::new(ctx).bytes(msg);
self.sign(t,public_key)
}
/// Sign a message with this `SecretKey`, but doublecheck the result.
pub fn sign_simple_doublecheck(&self, ctx: &[u8], msg: &[u8], public_key: &PublicKey)
-> SignatureResult<Signature>
{
let t = SigningContext::new(ctx).bytes(msg);
let sig = self.sign(t,public_key);
let sig = Signature::from_bytes(& sig.to_bytes()) ?;
PublicKey::from_bytes(& public_key.to_bytes()) ?
.verify_simple(ctx,msg,&sig).map(|()| sig)
}
}
impl PublicKey {
/// Verify a signature by this public key on a transcript.
///
/// Requires a `SigningTranscript`, normally created from a
/// `SigningContext` and a message, as well as the signature
/// to be verified.
#[allow(non_snake_case)]
pub fn verify<T: SigningTranscript>(&self, mut t: T, signature: &Signature)
-> SignatureResult<()>
{
let A: &RistrettoPoint = self.as_point();
t.proto_name(b"Schnorr-sig");
t.commit_point(b"sign:pk",self.as_compressed());
t.commit_point(b"sign:R",&signature.R);
let k: Scalar = t.challenge_scalar(b"sign:c"); // context, message, A/public_key, R=rG
let R = RistrettoPoint::vartime_double_scalar_mul_basepoint(&k, &(-A), &signature.s);
if R.compress() == signature.R { Ok(()) } else { Err(SignatureError::EquationFalse) }
}
/// Verify a signature by this public key on a message.
pub fn verify_simple(&self, ctx: &[u8], msg: &[u8], signature: &Signature)
-> SignatureResult<()>
{
let t = SigningContext::new(ctx).bytes(msg);
self.verify(t,signature)
}
/// A temporary verification routine for use in transitioning substrate testnets only.
#[cfg(feature = "preaudit_deprecated")]
#[allow(non_snake_case)]
pub fn verify_simple_preaudit_deprecated(&self, ctx: &'static [u8], msg: &[u8], sig: &[u8])
-> SignatureResult<()>
{
let t = SigningContext::new(ctx).bytes(msg);
if let Ok(signature) = Signature::from_bytes(sig) {
return self.verify(t,&signature);
}
let signature = Signature::from_bytes_not_distinguished_from_ed25519(sig) ?;
let mut t = merlin::Transcript::new(ctx);
t.append_message(b"sign-bytes", msg);
let A: &RistrettoPoint = self.as_point();
t.proto_name(b"Schnorr-sig");
t.commit_point(b"pk",self.as_compressed());
t.commit_point(b"no",&signature.R);
let k: Scalar = t.challenge_scalar(b""); // context, message, A/public_key, R=rG
let R = RistrettoPoint::vartime_double_scalar_mul_basepoint(&k, &(-A), &signature.s);
if R.compress() == signature.R { Ok(()) } else { Err(SignatureError::EquationFalse) }
}
}
impl Keypair {
/// Sign a transcript with this keypair's secret key.
///
/// Requires a `SigningTranscript`, normally created from a
/// `SigningContext` and a message. Returns a Schnorr signature.
///
/// # Examples
///
/// Internally, we manage signature transcripts using a 128 bit secure
/// STROBE construction based on Keccak, which itself is extremly fast
/// and secure. You might however influence performance or security
/// by prehashing your message, like
///
/// ```
/// use schnorrkel::{Signature,Keypair};
/// use rand::prelude::*; // ThreadRng,thread_rng
/// use sha3::Shake128;
/// use sha3::digest::{Input};
///
/// # #[cfg(all(feature = "std"))]
/// # fn main() {
/// let mut csprng: ThreadRng = thread_rng();
/// let keypair: Keypair = Keypair::generate_with(&mut csprng);
/// let message: &[u8] = b"All I want is to pet all of the dogs.";
///
/// // Create a hash digest object and feed it the message:
/// let prehashed = Shake128::default().chain(message);
/// # }
/// #
/// # #[cfg(any(not(feature = "std")))]
/// # fn main() { }
/// ```
///
/// We require a "context" string for all signatures, which should
/// be chosen judiciously for your project. It should represent the
/// role the signature plays in your application. If you use the
/// context in two purposes, and the same key, then a signature for
/// one purpose can be substituted for the other.
///
/// ```
/// # use schnorrkel::{Keypair,Signature,signing_context};
/// # use rand::prelude::*; // ThreadRng,thread_rng
/// # use sha3::digest::Input;
/// #
/// # #[cfg(all(feature = "std"))]
/// # fn main() {
/// # let mut csprng: ThreadRng = thread_rng();
/// # let keypair: Keypair = Keypair::generate_with(&mut csprng);
/// # let message: &[u8] = b"All I want is to pet all of the dogs.";
/// # let prehashed = ::sha3::Shake256::default().chain(message);
/// #
/// let ctx = signing_context(b"My Signing Context");
///
/// let sig: Signature = keypair.sign(ctx.xof(prehashed));
/// # }
/// #
/// # #[cfg(any(not(feature = "std")))]
/// # fn main() { }
/// ```
///
// lol [terrible_idea]: https://github.com/isislovecruft/scripts/blob/master/gpgkey2bc.py
pub fn sign<T: SigningTranscript>(&self, t: T) -> Signature
{
self.secret.sign(t, &self.public)
}
/// Sign a message with this keypair's secret key.
pub fn sign_simple(&self, ctx: &[u8], msg: &[u8]) -> Signature
{
self.secret.sign_simple(ctx, msg, &self.public)
}
/// Verify a signature by keypair's public key on a transcript.
///
/// Requires a `SigningTranscript`, normally created from a
/// `SigningContext` and a message, as well as the signature
/// to be verified.
///
/// # Examples
///
/// ```
/// use schnorrkel::{Keypair,Signature,signing_context};
/// use rand::prelude::*; // ThreadRng,thread_rng
///
/// # fn main() {
/// let mut csprng: ThreadRng = thread_rng();
/// let keypair: Keypair = Keypair::generate_with(&mut csprng);
/// let message: &[u8] = b"All I want is to pet all of the dogs.";
///
/// let ctx = signing_context(b"Some context string");
///
/// let sig: Signature = keypair.sign(ctx.bytes(message));
///
/// assert!( keypair.public.verify(ctx.bytes(message), &sig).is_ok() );
/// # }
/// ```
pub fn verify<T: SigningTranscript>(&self, t: T, signature: &Signature) -> SignatureResult<()>
{
self.public.verify(t, signature)
}
/// Verify a signature by keypair's public key on a message.
pub fn verify_simple(&self, ctx: &[u8], msg: &[u8], signature: &Signature) -> SignatureResult<()>
{
self.public.verify_simple(ctx, msg, signature)
}
/// Sign a message with this `SecretKey`, but doublecheck the result.
pub fn sign_doublecheck<T>(&self, t: T) -> SignatureResult<Signature>
where T: SigningTranscript+Clone
{
let sig = self.sign(t.clone());
let sig = Signature::from_bytes(& sig.to_bytes()) ?;
PublicKey::from_bytes(& self.public.to_bytes()) ?
.verify(t,&sig).map(|()| sig)
}
/// Sign a message with this `SecretKey`, but doublecheck the result.
pub fn sign_simple_doublecheck(&self, ctx: &[u8], msg: &[u8])
-> SignatureResult<Signature>
{
let t = SigningContext::new(ctx).bytes(msg);
let sig = self.sign(t);
let sig = Signature::from_bytes(& sig.to_bytes()) ?;
PublicKey::from_bytes(& self.public.to_bytes()) ?
.verify_simple(ctx,msg,&sig).map(|()| sig)
}
}
#[cfg(test)]
mod test {
use sha3::Shake128;
use curve25519_dalek::digest::{Input};
use super::super::*;
#[test]
fn sign_verify_bytes() {
let good_sig: Signature;
let bad_sig: Signature;
let ctx = signing_context(b"good");
let good: &[u8] = "test message".as_bytes();
let bad: &[u8] = "wrong message".as_bytes();
// #[cfg(feature = "getrandom")]
let mut csprng = ::rand_core::OsRng;
let keypair = Keypair::generate_with(&mut csprng);
good_sig = keypair.sign(ctx.bytes(&good));
bad_sig = keypair.sign(ctx.bytes(&bad));
let good_sig = Signature::from_bytes(&good_sig.to_bytes()[..]).unwrap();
let bad_sig = Signature::from_bytes(&bad_sig.to_bytes()[..]).unwrap();
assert!(keypair.verify(ctx.bytes(&good), &good_sig).is_ok(),
"Verification of a valid signature failed!");
assert!(!keypair.verify(ctx.bytes(&good), &bad_sig).is_ok(),
"Verification of a signature on a different message passed!");
assert!(!keypair.verify(ctx.bytes(&bad), &good_sig).is_ok(),
"Verification of a signature on a different message passed!");
assert!(!keypair.verify(signing_context(b"bad").bytes(&good), &good_sig).is_ok(),
"Verification of a signature on a different message passed!");
}
#[test]
fn sign_verify_xof() {
let good_sig: Signature;
let bad_sig: Signature;
let ctx = signing_context(b"testing testing 1 2 3");
let good: &[u8] = b"test message";
let bad: &[u8] = b"wrong message";
let prehashed_good: Shake128 = Shake128::default().chain(good);
let prehashed_bad: Shake128 = Shake128::default().chain(bad);
// You may verify that `Shake128: Copy` is possible, making these clones below correct.
// #[cfg(feature = "getrandom")]
let mut csprng = ::rand_core::OsRng;
let keypair = Keypair::generate_with(&mut csprng);
good_sig = keypair.sign(ctx.xof(prehashed_good.clone()));
bad_sig = keypair.sign(ctx.xof(prehashed_bad.clone()));
let good_sig_d = Signature::from_bytes(&good_sig.to_bytes()[..]).unwrap();
let bad_sig_d = Signature::from_bytes(&bad_sig.to_bytes()[..]).unwrap();
assert_eq!(good_sig, good_sig_d);
assert_eq!(bad_sig, bad_sig_d);
assert!(keypair.verify(ctx.xof(prehashed_good.clone()), &good_sig).is_ok(),
"Verification of a valid signature failed!");
assert!(! keypair.verify(ctx.xof(prehashed_good.clone()), &bad_sig).is_ok(),
"Verification of a signature on a different message passed!");
assert!(! keypair.verify(ctx.xof(prehashed_bad.clone()), &good_sig).is_ok(),
"Verification of a signature on a different message passed!");
assert!(! keypair.verify(signing_context(b"oops").xof(prehashed_good), &good_sig).is_ok(),
"Verification of a signature on a different message passed!");
}
#[cfg(feature = "preaudit_deprecated")]
#[test]
fn can_verify_know_preaudit_deprecated_message() {
use hex_literal::hex;
const SIGNING_CTX : &'static [u8] = b"substrate";
let message = b"Verifying that I am the owner of 5G9hQLdsKQswNPgB499DeA5PkFBbgkLPJWkkS6FAM6xGQ8xD. Hash: 221455a3\n";
let public = hex!("b4bfa1f7a5166695eb75299fd1c4c03ea212871c342f2c5dfea0902b2c246918");
let public = PublicKey::from_bytes(&public[..]).unwrap();
let signature = hex!("5a9755f069939f45d96aaf125cf5ce7ba1db998686f87f2fb3cbdea922078741a73891ba265f70c31436e18a9acd14d189d73c12317ab6c313285cd938453202");
assert!( public.verify_simple_preaudit_deprecated(SIGNING_CTX,message,&signature[..]).is_ok() );
}
}