Merge rust-bitcoin/rust-bitcoin#4046: hashes: Add `Copy`, `Clone`, and `Debug` imlps for `Hkdf`

da8b85ed7c Implement Debug for Hkdf (Tobin C. Harding)
85652359e8 hashes: Derive Copy and Clone for Hkdf (Tobin C. Harding)

Pull request description:

  Currently the `Hkdf` type does not derive any traits.

  Derive `Copy` and `Clone` and implement `Debug` based on secret obfuscation algo in `rust-secp` (in the `secret` module).

ACKs for top commit:
  apoelstra:
    ACK da8b85ed7cf34c0510c0b64c67477d3819bee369; successfully ran local tests
  Kixunil:
    ACK da8b85ed7c

Tree-SHA512: 8ae0e8857ea0e32ad5ef8f544979eeb9d530beb1b6f046ce28a286ca2231f8f696a9f4f8d9ea219d3389c4216d6b69766dbd96edbb27e7489803ac583bf3b200

hashes/src/hkdf/mod.rs

@@ -32,6 +32,7 @@ impl fmt::Display for MaxLengthError {
 impl std::error::Error for MaxLengthError {}
 
 /// HMAC-based Extract-and-Expand Key Derivation Function (HKDF).
+#[derive(Copy, Clone)]
 pub struct Hkdf<T: GeneralHash> {
     /// Pseudorandom key based on the extract step.
     prk: Hmac<T>,
@@ -105,6 +106,26 @@ where
     }
 }
 
+impl<T: GeneralHash> fmt::Debug for Hkdf<T> {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+        use crate::{sha256t, sha256t_tag};
+
+        struct Fingerprint([u8; 8]); // Print 16 hex characters as a fingerprint.
+
+        impl fmt::Debug for Fingerprint {
+            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { crate::debug_hex(&self.0, f) }
+        }
+
+        sha256t_tag! {
+            pub struct Tag = hash_str("bitcoin_hashes1DEBUG");
+        }
+
+        let hash = sha256t::Hash::<Tag>::hash(self.prk.as_ref());
+        let fingerprint = Fingerprint(core::array::from_fn(|i| hash.as_byte_array()[i]));
+        f.debug_tuple("Hkdf").field(&format_args!("#{:?}", fingerprint)).finish()
+    }
+}
+
 #[cfg(test)]
 #[cfg(feature = "alloc")]
 #[cfg(feature = "hex")]
@@ -192,4 +213,15 @@ mod tests {
             "3cb25f25faacd57a90434f64d0362f2a2d2d0a90cf1a5a4c5db02d56ecc4c5bf34007208d5b887185865"
         );
     }
+
+    #[test]
+    fn debug() {
+        let salt = Vec::from_hex("000102030405060708090a0b0c").unwrap();
+        let ikm = Vec::from_hex("0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b").unwrap();
+
+        let hkdf = Hkdf::<sha256::Hash>::new(&salt, &ikm);
+        let debug = alloc::format!("{:?}", hkdf);
+
+        assert_eq!(debug, "Hkdf(#ec7bd36ab2ed4045)");
+    }
 }