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Implement low r signing #259

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merged 1 commit into from Dec 22, 2020
Merged

Implement low r signing #259

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124 changes: 124 additions & 0 deletions src/lib.rs
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Expand Up @@ -660,6 +660,34 @@ impl<C: Context> Secp256k1<C> {

}

fn der_length_check(sig: &ffi::Signature, max_len: usize) -> bool {
let mut ser_ret = [0; 72];
let mut len: usize = ser_ret.len();
unsafe {
let err = ffi::secp256k1_ecdsa_signature_serialize_der(
ffi::secp256k1_context_no_precomp,
ser_ret.as_mut_c_ptr(),
&mut len,
sig,
);
debug_assert!(err == 1);
}
len <= max_len
}

fn compact_sig_has_zero_first_bit(sig: &ffi::Signature) -> bool {
let mut compact = [0; 64];
unsafe {
let err = ffi::secp256k1_ecdsa_signature_serialize_compact(
ffi::secp256k1_context_no_precomp,
compact.as_mut_c_ptr(),
sig,
);
debug_assert!(err == 1);
}
compact[0] < 0x80
}

impl<C: Signing> Secp256k1<C> {

/// Constructs a signature for `msg` using the secret key `sk` and RFC6979 nonce
Expand All @@ -678,6 +706,59 @@ impl<C: Signing> Secp256k1<C> {
}
}

fn sign_grind_with_check(
&self, msg: &Message,
sk: &key::SecretKey,
check: impl Fn(&ffi::Signature) -> bool) -> Signature {
let mut entropy_p : *const ffi::types::c_void = ptr::null();
let mut counter : u32 = 0;
let mut extra_entropy = [0u8; 32];
loop {
unsafe {
let mut ret = ffi::Signature::new();
// We can assume the return value because it's not possible to construct
// an invalid signature from a valid `Message` and `SecretKey`
assert_eq!(ffi::secp256k1_ecdsa_sign(self.ctx, &mut ret, msg.as_c_ptr(),
sk.as_c_ptr(), ffi::secp256k1_nonce_function_rfc6979,
entropy_p), 1);
if check(&ret) {
return Signature::from(ret);
}

counter += 1;
// From 1.32 can use `to_le_bytes` instead
let le_counter = counter.to_le();
let le_counter_bytes : [u8; 4] = mem::transmute(le_counter);
for (i, b) in le_counter_bytes.iter().enumerate() {
extra_entropy[i] = *b;
}

entropy_p = extra_entropy.as_ptr() as *const ffi::types::c_void;
}
}
}

/// Constructs a signature for `msg` using the secret key `sk`, RFC6979 nonce
/// and "grinds" the nonce by passing extra entropy if necessary to produce
/// a signature that is less than 71 - bytes_to_grund bytes. The number
/// of signing operation performed by this function is exponential in the
/// number of bytes grinded.
/// Requires a signing capable context.
pub fn sign_grind_r(&self, msg: &Message, sk: &key::SecretKey, bytes_to_grind: usize) -> Signature {
let len_check = |s : &ffi::Signature| der_length_check(s, 71 - bytes_to_grind);
return self.sign_grind_with_check(msg, sk, len_check);
}

/// Constructs a signature for `msg` using the secret key `sk`, RFC6979 nonce
/// and "grinds" the nonce by passing extra entropy if necessary to produce
/// a signature that is less than 71 bytes and compatible with the low r
/// signature implementation of bitcoin core. In average, this function
/// will perform two signing operations.
/// Requires a signing capable context.
pub fn sign_low_r(&self, msg: &Message, sk: &key::SecretKey) -> Signature {
return self.sign_grind_with_check(msg, sk, compact_sig_has_zero_first_bit)
}

/// Generates a random keypair. Convenience function for `key::SecretKey::new`
/// and `key::PublicKey::from_secret_key`; call those functions directly for
/// batch key generation. Requires a signing-capable context. Requires compilation
Expand Down Expand Up @@ -1003,6 +1084,18 @@ mod tests {
let (sk, pk) = s.generate_keypair(&mut thread_rng());
let sig = s.sign(&msg, &sk);
assert_eq!(s.verify(&msg, &sig, &pk), Ok(()));
let low_r_sig = s.sign_low_r(&msg, &sk);
assert_eq!(s.verify(&msg, &low_r_sig, &pk), Ok(()));
let grind_r_sig = s.sign_grind_r(&msg, &sk, 1);
assert_eq!(s.verify(&msg, &grind_r_sig, &pk), Ok(()));
let compact = sig.serialize_compact();
if compact[0] < 0x80 {
assert_eq!(sig, low_r_sig);
} else {
assert_ne!(sig, low_r_sig);
}
assert!(super::compact_sig_has_zero_first_bit(&low_r_sig.0));
assert!(super::der_length_check(&grind_r_sig.0, 70));
}
}

Expand All @@ -1029,8 +1122,12 @@ mod tests {
for key in wild_keys.iter().map(|k| SecretKey::from_slice(&k[..]).unwrap()) {
for msg in wild_msgs.iter().map(|m| Message::from_slice(&m[..]).unwrap()) {
let sig = s.sign(&msg, &key);
let low_r_sig = s.sign_low_r(&msg, &key);
let grind_r_sig = s.sign_grind_r(&msg, &key, 1);
let pk = PublicKey::from_secret_key(&s, &key);
assert_eq!(s.verify(&msg, &sig, &pk), Ok(()));
assert_eq!(s.verify(&msg, &low_r_sig, &pk), Ok(()));
assert_eq!(s.verify(&msg, &grind_r_sig, &pk), Ok(()));
}
}
}
Expand Down Expand Up @@ -1090,6 +1187,33 @@ mod tests {
assert_eq!(secp.verify(&msg, &sig, &pk), Ok(()));
}

#[test]
fn test_low_r() {
let secp = Secp256k1::new();
let msg = hex!("887d04bb1cf1b1554f1b268dfe62d13064ca67ae45348d50d1392ce2d13418ac");
let msg = Message::from_slice(&msg).unwrap();
let sk = SecretKey::from_str("57f0148f94d13095cfda539d0da0d1541304b678d8b36e243980aab4e1b7cead").unwrap();
let expected_sig = hex!("047dd4d049db02b430d24c41c7925b2725bcd5a85393513bdec04b4dc363632b1054d0180094122b380f4cfa391e6296244da773173e78fc745c1b9c79f7b713");
let expected_sig = Signature::from_compact(&expected_sig).unwrap();

let sig = secp.sign_low_r(&msg, &sk);

assert_eq!(expected_sig, sig);
}

#[test]
fn test_grind_r() {
let secp = Secp256k1::new();
let msg = hex!("ef2d5b9a7c61865a95941d0f04285420560df7e9d76890ac1b8867b12ce43167");
let msg = Message::from_slice(&msg).unwrap();
let sk = SecretKey::from_str("848355d75fe1c354cf05539bb29b2015f1863065bcb6766b44d399ab95c3fa0b").unwrap();
let expected_sig = Signature::from_str("304302202ffc447100d518c8ba643d11f3e6a83a8640488e7d2537b1954b942408be6ea3021f26e1248dd1e52160c3a38af9769d91a1a806cab5f9d508c103464d3c02d6e1").unwrap();

let sig = secp.sign_grind_r(&msg, &sk, 2);

assert_eq!(expected_sig, sig);
}

#[cfg(feature = "serde")]
#[test]
fn test_signature_serde() {
Expand Down