chore: add h2c domain separated

moksha-core/src/dhke.rs

@@ -81,6 +81,32 @@ impl Dhke {
         point.unwrap()
     }
 
+    /// The point is generated by hashing the message with a domain separator and then
+    /// iteratively trying to compute a point from the hash. An increasing uint32 counter
+    /// (byte order little endian) is appended to the hash until a point is found that lies on the curve.
+
+    /// The chance of finding a valid point is 50% for every iteration. The maximum number of iterations
+    /// is 2**16. If no valid point is found after 2**16 iterations, a ValueError is raised (this should
+    /// never happen in practice).
+
+    /// The domain separator is b"Secp256k1_HashToCurve_Cashu_" or
+    /// bytes.fromhex("536563703235366b315f48617368546f43757276655f43617368755f").
+    fn hash_to_curve_domain_separated(message: &[u8]) -> Result<PublicKey, MokshaCoreError> {
+        let msg_to_hash = sha256::Hash::hash(&[b"Secp256k1_HashToCurve_Cashu_", message].concat());
+        (0..2u32.pow(16))
+            .map(|counter| sha256::Hash::hash(&[&msg_to_hash[..], &counter.to_le_bytes()].concat()))
+            .find_map(|hash| {
+                PublicKey::from_slice(
+                    &once(&0x02)
+                        .chain(hash.to_byte_array().iter())
+                        .cloned()
+                        .collect::<Vec<u8>>(),
+                )
+                .ok()
+            })
+            .ok_or(MokshaCoreError::NoValidPointFound)
+    }
+
     pub fn step1_alice(
         &self,
         secret_msg: impl Into<String>,
@@ -283,4 +309,30 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn test_hash_to_curve_domain_separated_zero() -> anyhow::Result<()> {
+        let input_str =
+            hex_to_string("0000000000000000000000000000000000000000000000000000000000000000");
+        let expected_result = "024cce997d3b518f739663b757deaec95bcd9473c30a14ac2fd04023a739d1a725";
+
+        let pk = Dhke::hash_to_curve_domain_separated(input_str.as_bytes())
+            .unwrap()
+            .to_string();
+        assert_eq!(pk, expected_result);
+        Ok(())
+    }
+
+    #[test]
+    fn test_hash_to_curve_domain_separated_one() -> anyhow::Result<()> {
+        let input_str =
+            hex_to_string("0000000000000000000000000000000000000000000000000000000000000001");
+        let expected_result = "022e7158e11c9506f1aa4248bf531298daa7febd6194f003edcd9b93ade6253acf";
+
+        let pk = Dhke::hash_to_curve_domain_separated(input_str.as_bytes())
+            .unwrap()
+            .to_string();
+        assert_eq!(pk, expected_result);
+        Ok(())
+    }
 }

moksha-core/src/error.rs

@@ -28,4 +28,7 @@ pub enum MokshaCoreError {
 
     #[error("Invalid token")]
     InvalidToken,
+
+    #[error("No valid point on curve secp256k1 found")]
+    NoValidPointFound,
 }