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Add combine_keys function to PublicKey #291

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Jun 8, 2021
Merged

Add combine_keys function to PublicKey #291

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40 changes: 38 additions & 2 deletions src/key.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,7 @@ pub const ONE_KEY: SecretKey = SecretKey([0, 0, 0, 0, 0, 0, 0, 0,

/// A Secp256k1 public key, used for verification of signatures
#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
#[repr(transparent)]
pub struct PublicKey(ffi::PublicKey);

impl fmt::LowerHex for PublicKey {
Expand Down Expand Up @@ -364,14 +365,26 @@ impl PublicKey {
/// the result would be the point at infinity, i.e. we are adding this point
/// to its own negation
pub fn combine(&self, other: &PublicKey) -> Result<PublicKey, Error> {
PublicKey::combine_keys(&[self, other])
}

/// Adds the keys in the provided slice together, returning the sum. Returns
/// an error if the result would be the point at infinity, i.e. we are adding
/// a point to its own negation
pub fn combine_keys(keys: &[&PublicKey]) -> Result<PublicKey, Error> {
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Post-merge note: PublicKey is a copy type and I am not sure why we need to have it here by reference

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IIRC the reason is because we need a &[&PublicKey] to transmute. It would also be possible to take a &[PublicKey] and do the conversion internally but I guess I preferred to keep the library code minimal. I don't have a strong opinion if people want to change it though.

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Yeah the C code takes pointer to a list of pointers, so we'd have to allocate a vector of pointers and we try to avoid allocations where possible

use core::mem::transmute;
use core::i32::MAX;

debug_assert!(keys.len() < MAX as usize);
unsafe {
let mut ret = ffi::PublicKey::new();
let ptrs = [self.as_c_ptr(), other.as_c_ptr()];
let ptrs : &[*const ffi::PublicKey] =
transmute::<&[&PublicKey], &[*const ffi::PublicKey]>(keys);
if ffi::secp256k1_ec_pubkey_combine(
ffi::secp256k1_context_no_precomp,
&mut ret,
ptrs.as_c_ptr(),
2
keys.len() as i32
) == 1
{
Ok(PublicKey(ret))
Expand Down Expand Up @@ -794,6 +807,29 @@ mod test {
assert_eq!(sum1.unwrap(), exp_sum);
}

#[test]
fn pubkey_combine_keys() {
let compressed1 = PublicKey::from_slice(
&hex!("0241cc121c419921942add6db6482fb36243faf83317c866d2a28d8c6d7089f7ba"),
).unwrap();
let compressed2 = PublicKey::from_slice(
&hex!("02e6642fd69bd211f93f7f1f36ca51a26a5290eb2dd1b0d8279a87bb0d480c8443"),
).unwrap();
let compressed3 = PublicKey::from_slice(
&hex!("03e74897d8644eb3e5b391ca2ab257aec2080f4d1a95cad57e454e47f021168eb0")
).unwrap();
let exp_sum = PublicKey::from_slice(
&hex!("0252d73a47f66cf341e5651542f0348f452b7c793af62a6d8bff75ade703a451ad"),
).unwrap();

let sum1 = PublicKey::combine_keys(&[&compressed1, &compressed2, &compressed3]);
assert!(sum1.is_ok());
let sum2 = PublicKey::combine_keys(&[&compressed1, &compressed2, &compressed3]);
assert!(sum2.is_ok());
assert_eq!(sum1, sum2);
assert_eq!(sum1.unwrap(), exp_sum);
}

#[test]
fn pubkey_equal() {
let pk1 = PublicKey::from_slice(
Expand Down