Add support for BIP340 signtures where the messages are not exactly 32 bytes

k256/src/schnorr.rs

@@ -288,6 +288,69 @@ mod tests {
         }
     }
 
+    #[test]
+    fn bip340_ext_sign_vectors() {
+        // Test indexes 15-18 from https://github.com/bitcoin/bips/blob/master/bip-0340/test-vectors.csv
+        //
+        // These tests all use the same key and aux
+        let sk = SigningKey::from_bytes(&hex!(
+            "0340034003400340034003400340034003400340034003400340034003400340"
+        ))
+        .unwrap();
+
+        let aux_rand = [0u8; 32];
+
+        struct Bip340ExtTest {
+            index: usize,
+            msg: alloc::vec::Vec<u8>,
+            signature: [u8; 64],
+        }
+
+        let bip340_ext_sign_vectors = [
+            Bip340ExtTest {
+                index: 15,
+                msg: vec![],
+                signature: hex!(
+                   "71535DB165ECD9FBBC046E5FFAEA61186BB6AD436732FCCC25291A55895464CF
+                    6069CE26BF03466228F19A3A62DB8A649F2D560FAC652827D1AF0574E427AB63"
+                )
+            },
+            Bip340ExtTest {
+                index: 16,
+                msg: hex!("11").to_vec(),
+                signature: hex!("08A20A0AFEF64124649232E0693C583AB1B9934AE63B4C3511F3AE1134C6A303EA3173BFEA6683BD101FA5AA5DBC1996FE7CACFC5A577D33EC14564CEC2BACBF")
+            },
+            Bip340ExtTest {
+                index: 17,
+                msg: hex!("0102030405060708090A0B0C0D0E0F1011").to_vec(),
+                signature: hex!("5130F39A4059B43BC7CAC09A19ECE52B5D8699D1A71E3C52DA9AFDB6B50AC370C4A482B77BF960F8681540E25B6771ECE1E5A37FD80E5A51897C5566A97EA5A5"),
+            },
+            Bip340ExtTest {
+                index: 18,
+                msg: vec![0x99; 100],
+                signature: hex!("403B12B0D8555A344175EA7EC746566303321E5DBFA8BE6F091635163ECA79A8585ED3E3170807E7C03B720FC54C7B23897FCBA0E9D0B4A06894CFD249F22367"),
+            },
+        ];
+
+        for vector in bip340_ext_sign_vectors {
+            let sig = sk
+                .sign_prehash_with_aux_rand(&vector.msg, &aux_rand)
+                .unwrap_or_else(|_| {
+                    panic!(
+                        "low-level Schnorr signing failure for index {}",
+                        vector.index
+                    )
+                });
+
+            assert_eq!(
+                vector.signature,
+                sig.to_bytes(),
+                "wrong signature for index {}",
+                vector.index
+            );
+        }
+    }
+
     /// Verification test vector
     struct VerifyVector {
         /// Index of test case

k256/src/schnorr/signing.rs

@@ -78,7 +78,7 @@ impl SigningKey {
     /// The preferred interfaces are the [`Signer`] or [`RandomizedSigner`] traits.
     pub fn sign_prehash_with_aux_rand(
         &self,
-        msg_digest: &[u8; 32],
+        msg_digest: &[u8],
         aux_rand: &[u8; 32],
     ) -> Result<Signature> {
         let mut t = tagged_hash(AUX_TAG).chain_update(aux_rand).finalize();
@@ -164,14 +164,13 @@ where
     D: Digest + FixedOutput<OutputSize = U32>,
 {
     fn try_sign_digest(&self, digest: D) -> Result<Signature> {
-        self.sign_prehash_with_aux_rand(&digest.finalize_fixed().into(), &Default::default())
+        self.sign_prehash_with_aux_rand(&digest.finalize_fixed(), &Default::default())
     }
 }
 
 impl PrehashSigner<Signature> for SigningKey {
     fn sign_prehash(&self, prehash: &[u8]) -> Result<Signature> {
-        let prehash = prehash.try_into().map_err(|_| Error::new())?;
-        self.sign_prehash_with_aux_rand(&prehash, &Default::default())
+        self.sign_prehash_with_aux_rand(prehash, &Default::default())
     }
 }
 
@@ -186,7 +185,7 @@ where
     ) -> Result<Signature> {
         let mut aux_rand = [0u8; 32];
         rng.fill_bytes(&mut aux_rand);
-        self.sign_prehash_with_aux_rand(&digest.finalize_fixed().into(), &aux_rand)
+        self.sign_prehash_with_aux_rand(&digest.finalize_fixed(), &aux_rand)
     }
 }
 
@@ -202,12 +201,10 @@ impl RandomizedPrehashSigner<Signature> for SigningKey {
         rng: &mut impl CryptoRngCore,
         prehash: &[u8],
     ) -> Result<Signature> {
-        let prehash = prehash.try_into().map_err(|_| Error::new())?;
-
         let mut aux_rand = [0u8; 32];
         rng.fill_bytes(&mut aux_rand);
 
-        self.sign_prehash_with_aux_rand(&prehash, &aux_rand)
+        self.sign_prehash_with_aux_rand(prehash, &aux_rand)
     }
 }
 

k256/src/schnorr/verifying.rs

@@ -64,7 +64,6 @@ impl PrehashVerifier<Signature> for VerifyingKey {
         prehash: &[u8],
         signature: &Signature,
     ) -> core::result::Result<(), Error> {
-        let prehash: [u8; 32] = prehash.try_into().map_err(|_| Error::new())?;
         let (r, s) = signature.split();
 
         let e = <Scalar as Reduce<U256>>::reduce_bytes(