Improve Message constructors

Observe:

- The word "hash" can be a verb or a noun, its usage in function names
  is therefore at times ambiguous.
- The function name `from_slice` gives no indication as to what the
  slice input is.

Improve Message constructors by doing:

- Add a constructor `Message::from_digest` that takes a 32 byte array as
  input.
- Rename `Message::from_slice` to `Message::from_digest_slice`
  (deprecate `from_slice` and add `from_digest_slice`)
- Improve the docs while we are at it.

examples/sign_verify.rs

@@ -11,7 +11,7 @@ fn verify<C: Verification>(
     pubkey: [u8; 33],
 ) -> Result<bool, Error> {
     let msg = sha256::Hash::hash(msg);
-    let msg = Message::from_slice(msg.as_ref())?;
+    let msg = Message::from_digest_slice(msg.as_ref())?;
     let sig = ecdsa::Signature::from_compact(&sig)?;
     let pubkey = PublicKey::from_slice(&pubkey)?;
 
@@ -24,7 +24,7 @@ fn sign<C: Signing>(
     seckey: [u8; 32],
 ) -> Result<ecdsa::Signature, Error> {
     let msg = sha256::Hash::hash(msg);
-    let msg = Message::from_slice(msg.as_ref())?;
+    let msg = Message::from_digest_slice(msg.as_ref())?;
     let seckey = SecretKey::from_slice(&seckey)?;
     Ok(secp.sign_ecdsa(&msg, &seckey))
 }

examples/sign_verify_recovery.rs

@@ -11,7 +11,7 @@ fn recover<C: Verification>(
     recovery_id: u8,
 ) -> Result<PublicKey, Error> {
     let msg = sha256::Hash::hash(msg);
-    let msg = Message::from_slice(msg.as_ref())?;
+    let msg = Message::from_digest_slice(msg.as_ref())?;
     let id = ecdsa::RecoveryId::from_i32(recovery_id as i32)?;
     let sig = ecdsa::RecoverableSignature::from_compact(&sig, id)?;
 
@@ -24,7 +24,7 @@ fn sign_recovery<C: Signing>(
     seckey: [u8; 32],
 ) -> Result<ecdsa::RecoverableSignature, Error> {
     let msg = sha256::Hash::hash(msg);
-    let msg = Message::from_slice(msg.as_ref())?;
+    let msg = Message::from_digest_slice(msg.as_ref())?;
     let seckey = SecretKey::from_slice(&seckey)?;
     Ok(secp.sign_ecdsa_recoverable(&msg, &seckey))
 }

no_std_test/src/main.rs

@@ -92,7 +92,7 @@ fn start(_argc: isize, _argv: *const *const u8) -> isize {
     secp.randomize(&mut FakeRng);
     let secret_key = SecretKey::new(&mut FakeRng);
     let public_key = PublicKey::from_secret_key(&secp, &secret_key);
-    let message = Message::from_slice(&[0xab; 32]).expect("32 bytes");
+    let message = Message::from_digest_slice(&[0xab; 32]).expect("32 bytes");
 
     let sig = secp.sign_ecdsa(&message, &secret_key);
     assert!(secp.verify_ecdsa(&message, &sig, &public_key).is_ok());
@@ -119,7 +119,7 @@ fn start(_argc: isize, _argv: *const *const u8) -> isize {
     {
         let secp_alloc = Secp256k1::new();
         let public_key = PublicKey::from_secret_key(&secp_alloc, &secret_key);
-        let message = Message::from_slice(&[0xab; 32]).expect("32 bytes");
+        let message = Message::from_digest_slice(&[0xab; 32]).expect("32 bytes");
 
         let sig = secp_alloc.sign_ecdsa(&message, &secret_key);
         assert!(secp_alloc.verify_ecdsa(&message, &sig, &public_key).is_ok());

src/ecdsa/mod.rs

@@ -371,11 +371,11 @@ impl<C: Verification> Secp256k1<C> {
     /// # let secp = Secp256k1::new();
     /// # let (secret_key, public_key) = secp.generate_keypair(&mut rand::thread_rng());
     /// #
-    /// let message = Message::from_slice(&[0xab; 32]).expect("32 bytes");
+    /// let message = Message::from_digest_slice(&[0xab; 32]).expect("32 bytes");
     /// let sig = secp.sign_ecdsa(&message, &secret_key);
     /// assert_eq!(secp.verify_ecdsa(&message, &sig, &public_key), Ok(()));
     ///
-    /// let message = Message::from_slice(&[0xcd; 32]).expect("32 bytes");
+    /// let message = Message::from_digest_slice(&[0xcd; 32]).expect("32 bytes");
     /// assert_eq!(secp.verify_ecdsa(&message, &sig, &public_key), Err(Error::IncorrectSignature));
     /// # }
     /// ```

src/ecdsa/recovery.rs

@@ -226,7 +226,7 @@ mod tests {
         let full = Secp256k1::new();
 
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
 
         // Try key generation
         let (sk, pk) = full.generate_keypair(&mut rand::thread_rng());
@@ -258,7 +258,7 @@ mod tests {
         s.randomize(&mut rand::thread_rng());
 
         let sk = SecretKey::from_slice(&ONE).unwrap();
-        let msg = Message::from_slice(&ONE).unwrap();
+        let msg = Message::from_digest_slice(&ONE).unwrap();
 
         let sig = s.sign_ecdsa_recoverable(&msg, &sk);
 
@@ -283,7 +283,7 @@ mod tests {
         s.randomize(&mut rand::thread_rng());
 
         let sk = SecretKey::from_slice(&ONE).unwrap();
-        let msg = Message::from_slice(&ONE).unwrap();
+        let msg = Message::from_digest_slice(&ONE).unwrap();
         let noncedata = [42u8; 32];
 
         let sig = s.sign_ecdsa_recoverable_with_noncedata(&msg, &sk, &noncedata);
@@ -307,7 +307,7 @@ mod tests {
         s.randomize(&mut rand::thread_rng());
 
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
 
         let (sk, pk) = s.generate_keypair(&mut rand::thread_rng());
 
@@ -315,7 +315,7 @@ mod tests {
         let sig = sigr.to_standard();
 
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
         assert_eq!(s.verify_ecdsa(&msg, &sig, &pk), Err(Error::IncorrectSignature));
 
         let recovered_key = s.recover_ecdsa(&msg, &sigr).unwrap();
@@ -329,7 +329,7 @@ mod tests {
         s.randomize(&mut rand::thread_rng());
 
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
 
         let (sk, pk) = s.generate_keypair(&mut rand::thread_rng());
 
@@ -345,7 +345,7 @@ mod tests {
         s.randomize(&mut rand::thread_rng());
 
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
 
         let noncedata = [42u8; 32];
 
@@ -362,7 +362,7 @@ mod tests {
         let mut s = Secp256k1::new();
         s.randomize(&mut rand::thread_rng());
 
-        let msg = Message::from_slice(&[0x55; 32]).unwrap();
+        let msg = Message::from_digest_slice(&[0x55; 32]).unwrap();
 
         // Zero is not a valid sig
         let sig = RecoverableSignature::from_compact(&[0; 64], RecoveryId(0)).unwrap();
@@ -433,7 +433,7 @@ mod benches {
     pub fn bench_recover(bh: &mut Bencher) {
         let s = Secp256k1::new();
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
         let (sk, _) = s.generate_keypair(&mut rand::thread_rng());
         let sig = s.sign_ecdsa_recoverable(&msg, &sk);
 

src/lib.rs

@@ -70,7 +70,7 @@
 //! let public_key = PublicKey::from_secret_key(&secp, &secret_key);
 //! // This is unsafe unless the supplied byte slice is the output of a cryptographic hash function.
 //! // See the above example for how to use this library together with `bitcoin-hashes-std`.
-//! let message = Message::from_slice(&[0xab; 32]).expect("32 bytes");
+//! let message = Message::from_digest_slice(&[0xab; 32]).expect("32 bytes");
 //!
 //! let sig = secp.sign_ecdsa(&message, &secret_key);
 //! assert!(secp.verify_ecdsa(&message, &sig, &public_key).is_ok());
@@ -93,7 +93,7 @@
 //!     0x3a, 0x17, 0x10, 0xc9, 0x62, 0x67, 0x90, 0x63,
 //! ]).expect("public keys must be 33 or 65 bytes, serialized according to SEC 2");
 //!
-//! let message = Message::from_slice(&[
+//! let message = Message::from_digest_slice(&[
 //!     0xaa, 0xdf, 0x7d, 0xe7, 0x82, 0x03, 0x4f, 0xbe,
 //!     0x3d, 0x3d, 0xb2, 0xcb, 0x13, 0xc0, 0xcd, 0x91,
 //!     0xbf, 0x41, 0xcb, 0x08, 0xfa, 0xc7, 0xbd, 0x61,
@@ -225,11 +225,41 @@ impl Message {
     /// the result of signing isn't a
     /// [secure signature](https://twitter.com/pwuille/status/1063582706288586752).
     #[inline]
-    pub fn from_slice(data: &[u8]) -> Result<Message, Error> {
+    #[deprecated(since = "0.28.0", note = "use from_digest_slice instead")]
-        match data.len() {
+    pub fn from_slice(digest: &[u8]) -> Result<Message, Error> {
+        Message::from_digest_slice(digest)
+    }
+
+    /// Creates a [`Message`] from a `digest`.
+    ///
+    /// **If you just want to sign an arbitrary message use `Message::from_hashed_data` instead.**
+    ///
+    /// The `digest` array has to be a cryptographically secure hash of the actual message that's
+    /// going to be signed. Otherwise the result of signing isn't a [secure signature].
+    ///
+    /// [secure signature]: https://twitter.com/pwuille/status/1063582706288586752
+    #[inline]
+    pub fn from_digest(digest: [u8; 32]) -> Message { Message(digest) }
+
+    /// Creates a [`Message`] from a 32 byte slice `digest`.
+    ///
+    /// **If you just want to sign an arbitrary message use `Message::from_hashed_data` instead.**
+    ///
+    /// The slice has to be 32 bytes long and be a cryptographically secure hash of the actual
+    /// message that's going to be signed. Otherwise the result of signing isn't a [secure
+    /// signature].
+    ///
+    /// # Errors
+    ///
+    /// If `digest` is not exactly 32 bytes long.
+    ///
+    /// [secure signature]: https://twitter.com/pwuille/status/1063582706288586752
+    #[inline]
+    pub fn from_digest_slice(digest: &[u8]) -> Result<Message, Error> {
+        match digest.len() {
             constants::MESSAGE_SIZE => {
                 let mut ret = [0u8; constants::MESSAGE_SIZE];
-                ret[..].copy_from_slice(data);
+                ret[..].copy_from_slice(digest);
                 Ok(Message(ret))
             }
             _ => Err(Error::InvalidMessage),
@@ -540,7 +570,7 @@ mod tests {
             Secp256k1 { ctx: ctx_vrfy, phantom: PhantomData };
 
         let (sk, pk) = full.generate_keypair(&mut rand::thread_rng());
-        let msg = Message::from_slice(&[2u8; 32]).unwrap();
+        let msg = Message::from_digest_slice(&[2u8; 32]).unwrap();
         // Try signing
         assert_eq!(sign.sign_ecdsa(&msg, &sk), full.sign_ecdsa(&msg, &sk));
         let sig = full.sign_ecdsa(&msg, &sk);
@@ -572,7 +602,7 @@ mod tests {
         let mut vrfy = unsafe { Secp256k1::from_raw_verification_only(ctx_vrfy.ctx) };
 
         let (sk, pk) = full.generate_keypair(&mut rand::thread_rng());
-        let msg = Message::from_slice(&[2u8; 32]).unwrap();
+        let msg = Message::from_digest_slice(&[2u8; 32]).unwrap();
         // Try signing
         assert_eq!(sign.sign_ecdsa(&msg, &sk), full.sign_ecdsa(&msg, &sk));
         let sig = full.sign_ecdsa(&msg, &sk);
@@ -618,7 +648,7 @@ mod tests {
         //        println!("{:?}", buf_ful[5]); // Can't even read the data thanks to the borrow checker.
 
         let (sk, pk) = full.generate_keypair(&mut rand::thread_rng());
-        let msg = Message::from_slice(&[2u8; 32]).unwrap();
+        let msg = Message::from_digest_slice(&[2u8; 32]).unwrap();
         // Try signing
         assert_eq!(sign.sign_ecdsa(&msg, &sk), full.sign_ecdsa(&msg, &sk));
         let sig = full.sign_ecdsa(&msg, &sk);
@@ -636,7 +666,7 @@ mod tests {
         let full = Secp256k1::new();
 
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
 
         // Try key generation
         let (sk, pk) = full.generate_keypair(&mut rand::thread_rng());
@@ -665,7 +695,7 @@ mod tests {
 
         for _ in 0..100 {
             let msg = crate::random_32_bytes(&mut rand::thread_rng());
-            let msg = Message::from_slice(&msg).unwrap();
+            let msg = Message::from_digest_slice(&msg).unwrap();
 
             let (sk, _) = s.generate_keypair(&mut rand::thread_rng());
             let sig1 = s.sign_ecdsa(&msg, &sk);
@@ -756,7 +786,7 @@ mod tests {
         let noncedata = [42u8; 32];
         for _ in 0..100 {
             let msg = crate::random_32_bytes(&mut rand::thread_rng());
-            let msg = Message::from_slice(&msg).unwrap();
+            let msg = Message::from_digest_slice(&msg).unwrap();
 
             let (sk, pk) = s.generate_keypair(&mut rand::thread_rng());
             let sig = s.sign_ecdsa(&msg, &sk);
@@ -803,7 +833,7 @@ mod tests {
         wild_msgs[1][0] -= 1;
 
         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()) {
+            for msg in wild_msgs.iter().map(|m| Message::from_digest_slice(&m[..]).unwrap()) {
                 let sig = s.sign_ecdsa(&msg, &key);
                 let low_r_sig = s.sign_ecdsa_low_r(&msg, &key);
                 let grind_r_sig = s.sign_ecdsa_grind_r(&msg, &key, 1);
@@ -822,14 +852,14 @@ mod tests {
         s.randomize(&mut rand::thread_rng());
 
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
 
         let (sk, pk) = s.generate_keypair(&mut rand::thread_rng());
 
         let sig = s.sign_ecdsa(&msg, &sk);
 
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
         assert_eq!(s.verify_ecdsa(&msg, &sig, &pk), Err(Error::IncorrectSignature));
     }
 
@@ -845,15 +875,15 @@ mod tests {
         );
 
         assert_eq!(
-            Message::from_slice(&[0; constants::MESSAGE_SIZE - 1]),
+            Message::from_digest_slice(&[0; constants::MESSAGE_SIZE - 1]),
             Err(Error::InvalidMessage)
         );
         assert_eq!(
-            Message::from_slice(&[0; constants::MESSAGE_SIZE + 1]),
+            Message::from_digest_slice(&[0; constants::MESSAGE_SIZE + 1]),
             Err(Error::InvalidMessage)
         );
-        assert!(Message::from_slice(&[0; constants::MESSAGE_SIZE]).is_ok());
+        assert!(Message::from_digest_slice(&[0; constants::MESSAGE_SIZE]).is_ok());
-        assert!(Message::from_slice(&[1; constants::MESSAGE_SIZE]).is_ok());
+        assert!(Message::from_digest_slice(&[1; constants::MESSAGE_SIZE]).is_ok());
     }
 
     #[test]
@@ -892,7 +922,7 @@ mod tests {
     fn test_noncedata() {
         let secp = Secp256k1::new();
         let msg = hex!("887d04bb1cf1b1554f1b268dfe62d13064ca67ae45348d50d1392ce2d13418ac");
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
         let noncedata = [42u8; 32];
         let sk =
             SecretKey::from_str("57f0148f94d13095cfda539d0da0d1541304b678d8b36e243980aab4e1b7cead")
@@ -919,7 +949,7 @@ mod tests {
         let secp = Secp256k1::new();
         let mut sig = ecdsa::Signature::from_der(&sig[..]).unwrap();
         let pk = PublicKey::from_slice(&pk[..]).unwrap();
-        let msg = Message::from_slice(&msg[..]).unwrap();
+        let msg = Message::from_digest_slice(&msg[..]).unwrap();
 
         // without normalization we expect this will fail
         assert_eq!(secp.verify_ecdsa(&msg, &sig, &pk), Err(Error::IncorrectSignature));
@@ -934,7 +964,7 @@ mod tests {
     fn test_low_r() {
         let secp = Secp256k1::new();
         let msg = hex!("887d04bb1cf1b1554f1b268dfe62d13064ca67ae45348d50d1392ce2d13418ac");
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
         let sk =
             SecretKey::from_str("57f0148f94d13095cfda539d0da0d1541304b678d8b36e243980aab4e1b7cead")
                 .unwrap();
@@ -952,7 +982,7 @@ mod tests {
     fn test_grind_r() {
         let secp = Secp256k1::new();
         let msg = hex!("ef2d5b9a7c61865a95941d0f04285420560df7e9d76890ac1b8867b12ce43167");
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
         let sk =
             SecretKey::from_str("848355d75fe1c354cf05539bb29b2015f1863065bcb6766b44d399ab95c3fa0b")
                 .unwrap();
@@ -972,7 +1002,7 @@ mod tests {
 
         let s = Secp256k1::new();
 
-        let msg = Message::from_slice(&[1; 32]).unwrap();
+        let msg = Message::from_digest_slice(&[1; 32]).unwrap();
         let sk = SecretKey::from_slice(&[2; 32]).unwrap();
         let sig = s.sign_ecdsa(&msg, &sk);
         static SIG_BYTES: [u8; 71] = [
@@ -1002,7 +1032,7 @@ mod tests {
         let sk_data = hex!("e6dd32f8761625f105c39a39f19370b3521d845a12456d60ce44debd0a362641");
         let sk = SecretKey::from_slice(&sk_data).unwrap();
         let msg_data = hex!("a4965ca63b7d8562736ceec36dfa5a11bf426eb65be8ea3f7a49ae363032da0d");
-        let msg = Message::from_slice(&msg_data).unwrap();
+        let msg = Message::from_digest_slice(&msg_data).unwrap();
 
         // Check usage as explicit parameter
         let pk = PublicKey::from_secret_key(SECP256K1, &sk);
@@ -1054,7 +1084,7 @@ mod benches {
     pub fn bench_sign_ecdsa(bh: &mut Bencher) {
         let s = Secp256k1::new();
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
         let (sk, _) = s.generate_keypair(&mut rand::thread_rng());
 
         bh.iter(|| {
@@ -1067,7 +1097,7 @@ mod benches {
     pub fn bench_verify_ecdsa(bh: &mut Bencher) {
         let s = Secp256k1::new();
         let msg = crate::random_32_bytes(&mut rand::thread_rng());
-        let msg = Message::from_slice(&msg).unwrap();
+        let msg = Message::from_digest_slice(&msg).unwrap();
         let (sk, pk) = s.generate_keypair(&mut rand::thread_rng());
         let sig = s.sign_ecdsa(&msg, &sk);
 

src/schnorr.rs

@@ -248,7 +248,7 @@ mod tests {
 
         for _ in 0..100 {
             let msg = crate::random_32_bytes(&mut rand::thread_rng());
-            let msg = Message::from_slice(&msg).unwrap();
+            let msg = Message::from_digest_slice(&msg).unwrap();
 
             let sig = sign(&secp, &msg, &kp, &mut rng);
 
@@ -263,7 +263,7 @@ mod tests {
         let secp = Secp256k1::new();
 
         let hex_msg = hex_32!("E48441762FB75010B2AA31A512B62B4148AA3FB08EB0765D76B252559064A614");
-        let msg = Message::from_slice(&hex_msg).unwrap();
+        let msg = Message::from_digest_slice(&hex_msg).unwrap();
         let sk = KeyPair::from_seckey_str(
             &secp,
             "688C77BC2D5AAFF5491CF309D4753B732135470D05B7B2CD21ADD0744FE97BEF",
@@ -285,7 +285,7 @@ mod tests {
         let secp = Secp256k1::new();
 
         let hex_msg = hex_32!("E48441762FB75010B2AA31A512B62B4148AA3FB08EB0765D76B252559064A614");
-        let msg = Message::from_slice(&hex_msg).unwrap();
+        let msg = Message::from_digest_slice(&hex_msg).unwrap();
         let sig = Signature::from_str("6470FD1303DDA4FDA717B9837153C24A6EAB377183FC438F939E0ED2B620E9EE5077C4A8B8DCA28963D772A94F5F0DDF598E1C47C137F91933274C7C3EDADCE8").unwrap();
         let pubkey = XOnlyPublicKey::from_str(
             "B33CC9EDC096D0A83416964BD3C6247B8FECD256E4EFA7870D2C854BDEB33390",
@@ -464,7 +464,7 @@ mod tests {
 
         let s = Secp256k1::new();
 
-        let msg = Message::from_slice(&[1; 32]).unwrap();
+        let msg = Message::from_digest_slice(&[1; 32]).unwrap();
         let keypair = KeyPair::from_seckey_slice(&s, &[2; 32]).unwrap();
         let aux = [3u8; 32];
         let sig = s.sign_schnorr_with_aux_rand(&msg, &keypair, &aux);