Aggsig signature subtraction function, rust side (#81)

* addition of ffi+api functions for aggsig partial sig subtract

* updated aggsig lib function to return more results based on underlying function

* update submodule to latest master

* clean up test result check

* bump version number

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 
 name = "grin_secp256k1zkp"
-version = "0.7.11"
+version = "0.7.12"
 authors = [ "Grin Developers <mimblewimble@lists.launchpad.net>",
             "Dawid Ciężarkiewicz <dpc@ucore.info>",
             "Andrew Poelstra <apoelstra@wpsoftware.net>" ]

src/aggsig.rs

@@ -258,6 +258,38 @@ pub fn add_signatures_single(
 	Ok(retsig)
 }
 
+/// Subtraction of partial signature from a signature
+/// Returns: Ok((Signature, None)) on success if the resulting signature has only one possibility
+///          Ok((Signature, Signature)) on success if the resulting signature could be one of either possiblity
+/// In:
+/// sig: completed signature from which to subtact a partial
+/// partial_sig: the partial signature to subtract
+pub fn subtract_partial_signature(
+	secp: &Secp256k1,
+	sig: &Signature,
+	partial_sig: &Signature,
+) -> Result<(Signature, Option<Signature>), Error> {
+	let mut ret_partsig = Signature::from(ffi::Signature::new());
+	let mut ret_partsig_alt = Signature::from(ffi::Signature::new());
+	let retval = unsafe {
+		ffi::secp256k1_aggsig_subtract_partial_signature(
+			secp.ctx,
+			ret_partsig.as_mut_ptr(),
+			ret_partsig_alt.as_mut_ptr(),
+			sig.as_ptr(),
+			partial_sig.as_ptr(),
+		)
+	};
+
+	match retval {
+		-1 => Err(Error::SigSubtractionFailure),
+		1 => Ok((ret_partsig, None)),
+		2 => Ok((ret_partsig, Some(ret_partsig_alt))),
+		_ => Err(Error::InvalidSignature)
+	}
+}
+
+
 /// Manages an instance of an aggsig multisig context, and provides all methods
 /// to act on that context
 #[derive(Clone, Debug)]
@@ -396,7 +428,8 @@ mod tests {
 		add_signatures_single, export_secnonce_single, sign_single, verify_single, verify_batch,
 		AggSigContext, Secp256k1,
 	};
-	use crate::ffi;
+	use crate::aggsig::subtract_partial_signature;
+use crate::ffi;
 	use crate::key::{PublicKey, SecretKey};
 	use rand::{thread_rng, Rng};
 	use crate::ContextFlag;
@@ -756,6 +789,14 @@ mod tests {
 				false,
 			);
 			assert!(result == true);
+
+			// Subtract sig1 from final sig
+			let (res_sig, res_sig_opt) = subtract_partial_signature(&secp, &final_sig, &sig1).unwrap();
+			assert!(res_sig == sig2 || res_sig_opt == Some(sig2));
+
+			// Subtract sig2 from final sig for good measure
+			let (res_sig, res_sig_opt) = subtract_partial_signature(&secp, &final_sig, &sig2).unwrap();
+			assert!(res_sig == sig1 || res_sig_opt == Some(sig1));
 		}
 	}
 }

src/ffi.rs

@@ -333,7 +333,15 @@ extern "C" {
                                                   num_sigs: size_t,
                                                   pubnonce_total: *const PublicKey)
                                                       -> c_int;
-    // EC
+
+    pub fn secp256k1_aggsig_subtract_partial_signature(cx: *const Context,
+                                                  ret_partsig: *mut Signature,
+                                                  ret_partsig_alt: *mut Signature,
+                                                  sig: *const Signature,
+                                                  part_sig: *const Signature)
+                                                      -> c_int;
+
+     // EC
     pub fn secp256k1_ec_seckey_verify(cx: *const Context,
                                       sk: *const c_uchar) -> c_int;
 

src/lib.rs

@@ -477,6 +477,8 @@ pub enum Error {
     InvalidRangeProof,
     /// Error creating partial signature
     PartialSigFailure,
+    /// Failure subtracting two signatures
+    SigSubtractionFailure,
 }
 
 impl Error {
@@ -493,6 +495,7 @@ impl Error {
             Error::IncorrectCommitSum => "secp: invalid pedersen commitment sum",
             Error::InvalidRangeProof => "secp: invalid range proof",
             Error::PartialSigFailure => "secp: partial sig (aggsig) failure",
+            Error::SigSubtractionFailure => "secp: subtraction (aggsig) did not result in any valid signatures",
         }
     }
 }