Add test to ensure old share compatibility

src/qos_crypto/src/shamir.rs

@@ -30,6 +30,7 @@ mod test {
 	use rand::prelude::SliceRandom;
 
 	use super::*;
+
 	#[test]
 	fn make_and_reconstruct_shares() {
 		let secret = b"this is a crazy secret";
@@ -65,4 +66,39 @@ mod test {
 			assert_eq!(secret.to_vec(), reconstructed);
 		}
 	}
+
+	#[test]
+	fn can_reconstruct_from_old_shares() {
+		// This test if fundamental to ensure updates to the Shamir Secret
+		// Sharing logic can be made safely. Here we hardcode shares that were
+		// created with the oldest version of this logic, and ensure that we can
+		// reconstruct. If this test starts failing please do _not_ ignore it,
+		// it's telling you the current quorum key shares will become invalid
+		// when combined!
+		let shares = [
+			qos_hex::decode("0116b5873b04714bd159f8ff59613f9e79cbb2e7a526")
+				.unwrap(),
+			qos_hex::decode("029bfa75d3977e39d90650792e76d1476722da43fed0")
+				.unwrap(),
+			qos_hex::decode("03e036d28be67b172833c1f2037b8bf96d8c0bd63e82")
+				.unwrap(),
+		];
+
+		// Setting is 2-out-of-3. Let's try 3 ways.
+		let reconstructed1 =
+			shares_reconstruct(vec![shares[0].clone(), shares[1].clone()])
+				.unwrap();
+		let reconstructed2 =
+			shares_reconstruct(vec![shares[1].clone(), shares[2].clone()])
+				.unwrap();
+		let reconstructed3 =
+			shares_reconstruct(vec![shares[0].clone(), shares[2].clone()])
+				.unwrap();
+
+		// Regardless of the combination we should get the same secret
+		let expected_secret = b"my cute little secret";
+		assert_eq!(reconstructed1, expected_secret);
+		assert_eq!(reconstructed2, expected_secret);
+		assert_eq!(reconstructed3, expected_secret);
+	}
 }