derive pad key for filler generation

Cargo.toml

@@ -6,6 +6,8 @@ edition = "2021"
 [dependencies]
 secp256k1 = { version = "0.28.0", features = ["serde"] }
 sha2 = "0.10.8"
+hmac = "0.12.1"
+thiserror = "1.0"
 
 [dev-dependencies]
 hex-literal = "0.4.1"

src/lib.rs

@@ -1,8 +1,15 @@
+use hmac::{Hmac, Mac as _};
 use secp256k1::{ecdh::SharedSecret, PublicKey, Scalar, Secp256k1, SecretKey, Signing};
 use sha2::{Digest as _, Sha256};
+use thiserror::Error;
 
 pub struct OnionPacket;
-pub struct Error;
+
+#[derive(Error, Debug)]
+pub enum SphinxError {
+    #[error("unknown sphinx error")]
+    Unknown,
+}
 
 /// Derives the ephemeral key for the next hop.
 ///
@@ -52,12 +59,18 @@ pub fn derive_hop_shared_secrets<C: Signing>(
         .collect()
 }
 
+pub fn derive_key(hmac_key: &[u8], shared_secret: &[u8]) -> [u8; 32] {
+    let mut mac = Hmac::<Sha256>::new_from_slice(hmac_key).expect("valid hmac key");
+    mac.update(shared_secret);
+    mac.finalize().into_bytes().into()
+}
+
 pub fn new_onion_packet(
     _payment_path: Vec<PublicKey>,
     _session_key: SecretKey,
     _hops_data: Vec<Vec<u8>>,
     _assoc_data: Vec<u8>,
-) -> Result<OnionPacket, Error> {
+) -> Result<OnionPacket, SphinxError> {
     Ok(OnionPacket)
 }
 
@@ -110,4 +123,21 @@ mod tests {
             assert_eq!(ss, expected_ss);
         }
     }
+
+    #[test]
+    fn test_derive_um_key() {
+        let shared_secret =
+            hex!("b5756b9b542727dbafc6765a49488b023a725d631af688fc031217e90770c328");
+        let expected_um_key = "4da7f2923edce6c2d85987d1d9fa6d88023e6c3a9c3d20f07d3b10b61a78d646";
+        let um_key = derive_key(b"um", &shared_secret);
+        assert_eq!(base16::encode_lower(&um_key), expected_um_key);
+    }
+
+    #[test]
+    fn test_derive_pad_key() {
+        let shared_secret = [0x41; 32];
+        let expected_pad_key = "70fa47d28edc4faf3e733ae0f4d2a12b8c5f09cbd74408eb7bc6ba2f1ebf88a2";
+        let pad_key = derive_key(b"pad", &shared_secret);
+        assert_eq!(base16::encode_lower(&pad_key), expected_pad_key);
+    }
 }