Expose mwixnet onion crypto for the swap server

The mwixnet swap server consumes the onion primitives from this crate
(mimblewimble/mwixnet re-imports them rather than vendoring its own). Master's
#726 mwixnet kept onion/crypto private and several helpers test-only — fine for
the wallet, but the server needs them.

- Make mwixnet::onion and onion::crypto public.
- Un-gate the crypto the server uses in production: secp::{commit, sign},
  dalek::{sign, option_dalek_pubkey_serde, dalek_sig_serde}; re-export RangeProof.
- Gate the test-only helpers (new_hop, test_util) behind a new `test-helpers`
  feature, off by default, so the server's tests can use them.

libwallet/Cargo.toml

@@ -10,6 +10,11 @@ exclude = ["**/*.grin", "**/*.grin2"]
 #build = "src/build/build.rs"
 edition = "2021"
 
+[features]
+# Exposes the mwixnet onion test helpers (test_util, new_hop) to downstream
+# crates such as the mwixnet swap server. Off by default; not part of normal builds.
+test-helpers = []
+
 [dependencies]
 blake2-rfc = "0.2"
 rand = "0.6"

libwallet/src/mwixnet/mod.rs

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 //! Onion modules for mxmixnet
-mod onion;
+pub mod onion;
 mod types;
 
 pub use onion::{

libwallet/src/mwixnet/onion/crypto/dalek.rs

@@ -68,7 +68,6 @@ impl AsRef<PublicKey> for DalekPublicKey {
 	}
 }
 
-#[cfg(test)]
 /// Serializes an Option<DalekPublicKey> to and from hex
 pub mod option_dalek_pubkey_serde {
 	use super::DalekPublicKey;
@@ -148,7 +147,6 @@ impl AsRef<Signature> for DalekSignature {
 }
 
 /// Serializes a DalekSignature to and from hex
-#[cfg(test)]
 pub mod dalek_sig_serde {
 	use super::DalekSignature;
 	use grin_util::ToHex;
@@ -176,7 +174,6 @@ pub mod dalek_sig_serde {
 
 /// Dalek signature sign wrapper
 // TODO: This is likely duplicated throughout crate, check
-#[cfg(test)]
 pub fn sign(sk: &SecretKey, message: &[u8]) -> Result<DalekSignature, DalekError> {
 	use ed25519_dalek::{Keypair, Signer};
 	let secret =

libwallet/src/mwixnet/onion/crypto/secp.rs

@@ -18,7 +18,7 @@ pub use grin_util::secp::{
 	self as secp256k1zkp,
 	constants::SECRET_KEY_SIZE,
 	key::{SecretKey, ZERO_KEY},
-	pedersen::Commitment,
+	pedersen::{Commitment, RangeProof},
 	rand::thread_rng,
 	ContextFlag, Secp256k1,
 };
@@ -47,7 +47,6 @@ pub fn read_secret_key<R: Reader>(reader: &mut R) -> Result<SecretKey, ser::Erro
 }
 
 /// Build a Pedersen Commitment using the provided value and blinding factor
-#[cfg(test)]
 pub fn commit(value: u64, blind: &SecretKey) -> Result<Commitment, secp256k1zkp::Error> {
 	let secp = Secp256k1::with_caps(ContextFlag::Commit);
 	let commit = secp.commit(value, blind.clone())?;
@@ -76,8 +75,6 @@ pub fn sub_value(commitment: &Commitment, value: u64) -> Result<Commitment, secp
 }
 
 /// Signs the message with the provided SecretKey
-#[cfg(test)]
-#[allow(dead_code)]
 pub fn sign(
 	sk: &SecretKey,
 	msg: &grin_util::secp::Message,

libwallet/src/mwixnet/onion/mod.rs

@@ -14,7 +14,7 @@
 
 //! Onion module definition
 
-mod crypto;
+pub mod crypto;
 pub mod onion;
 pub mod util;
 
@@ -48,7 +48,7 @@ pub struct Hop {
 }
 
 /// Crate a new hop
-#[cfg(test)]
+#[cfg(any(test, feature = "test-helpers"))]
 pub fn new_hop(
 	server_key: &SecretKey,
 	hop_excess: &SecretKey,
@@ -117,9 +117,9 @@ pub fn create_onion(
 	Ok(onion)
 }
 
-/// Internal tests
+/// Onion test helpers (random onions, commitments, proofs, keypairs).
 #[allow(missing_docs, dead_code)]
-#[cfg(test)]
+#[cfg(any(test, feature = "test-helpers"))]
 pub mod test_util {
 	use super::*;
 	use crypto::dalek::DalekPublicKey;