Switch to bitcoin's version of HKDF

* Our implementation of HKDF was pushed down into bitcoin, but has
only been published in bitcoin_hashes, not in a new version of bitcoin
yet. Once bitcoin is updated we can depend on the re-exported HKDF
there.

protocol/Cargo.toml

@@ -12,7 +12,7 @@ rust-version = "1.63.0"
 default = ["std"]
 async = ["std", "futures/std"]
 tokio = ["std", "tokio/io-util"]
-std = ["alloc", "bitcoin/std", "chacha20-poly1305/std", "rand/std", "rand/std_rng"]
+std = ["alloc", "bitcoin/std", "bitcoin_hashes/std", "chacha20-poly1305/std", "rand/std", "rand/std_rng"]
 alloc = ["chacha20-poly1305/alloc"]
 
 [dependencies]
@@ -21,6 +21,9 @@ futures = { version = "0.3.30", default-features = false, optional = true }
 tokio = { version = ">=1.37.0, <1.39.0", default-features = false, optional = true }
 rand = { version = "0.8.0", default-features = false }
 bitcoin = { version = "0.32.4", default-features = false }
+# Depending on hashes directly for HKDF, can drop this and 
+# use the re-exported version in bitcoin > 0.32.*.
+bitcoin_hashes = { version ="0.15.0", default-features = false }
 chacha20-poly1305 = { version = "0.1.1", default-features = false }
 
 [dev-dependencies]

protocol/src/lib.rs

@@ -24,7 +24,6 @@ extern crate alloc;
 extern crate std;
 
 mod fschacha20poly1305;
-mod hkdf;
 #[cfg(feature = "std")]
 pub mod serde;
 
@@ -37,14 +36,12 @@ use alloc::vec;
 #[cfg(feature = "alloc")]
 use alloc::vec::Vec;
 
-use bitcoin::{
-    hashes::sha256,
-    secp256k1::{
-        self,
-        ellswift::{ElligatorSwift, ElligatorSwiftParty},
-        PublicKey, Secp256k1, SecretKey, Signing,
-    },
+use bitcoin::secp256k1::{
+    self,
+    ellswift::{ElligatorSwift, ElligatorSwiftParty},
+    PublicKey, Secp256k1, SecretKey, Signing,
 };
+use bitcoin_hashes::{hkdf, sha256};
 use fschacha20poly1305::{FSChaCha20, FSChaCha20Poly1305};
 // Default to the futures-rs traits, but can overwrite with more specific
 // tokio implemenations for easier caller integration.