ethcore: fix pow difficulty validation (#9328)

* ethcore: fix pow difficulty validation

* ethcore: validate difficulty is not zero

* ethcore: add issue link to regression test

* ethcore: fix tests

* ethcore: move difficulty_to_boundary to ethash crate

* ethcore: reuse difficulty_to_boundary and boundary_to_difficulty

* ethcore: fix grumbles in difficulty_to_boundary_aux

ethash/src/lib.rs

@@ -16,10 +16,11 @@
 
 #![cfg_attr(feature = "benches", feature(test))]
 
-extern crate primal;
-extern crate parking_lot;
 extern crate either;
+extern crate ethereum_types;
 extern crate memmap;
+extern crate parking_lot;
+extern crate primal;
 
 #[macro_use]
 extern crate crunchy;
@@ -38,6 +39,7 @@ mod shared;
 pub use cache::{NodeCacheBuilder, OptimizeFor};
 pub use compute::{ProofOfWork, quick_get_difficulty, slow_hash_block_number};
 use compute::Light;
+use ethereum_types::{U256, U512};
 use keccak::H256;
 use parking_lot::Mutex;
 pub use seed_compute::SeedHashCompute;
@@ -136,6 +138,29 @@ impl EthashManager {
 	}
 }
 
+/// Convert an Ethash boundary to its original difficulty. Basically just `f(x) = 2^256 / x`.
+pub fn boundary_to_difficulty(boundary: &ethereum_types::H256) -> U256 {
+	difficulty_to_boundary_aux(&**boundary)
+}
+
+/// Convert an Ethash difficulty to the target boundary. Basically just `f(x) = 2^256 / x`.
+pub fn difficulty_to_boundary(difficulty: &U256) -> ethereum_types::H256 {
+	difficulty_to_boundary_aux(difficulty).into()
+}
+
+fn difficulty_to_boundary_aux<T: Into<U512>>(difficulty: T) -> ethereum_types::U256 {
+	let difficulty = difficulty.into();
+
+	assert!(!difficulty.is_zero());
+
+	if difficulty == U512::one() {
+		U256::max_value()
+	} else {
+		// difficulty > 1, so result should never overflow 256 bits
+		U256::from((U512::one() << 256) / difficulty)
+	}
+}
+
 #[test]
 fn test_lru() {
 	use tempdir::TempDir;
@@ -155,6 +180,43 @@ fn test_lru() {
 	assert_eq!(ethash.cache.lock().prev_epoch.unwrap(), 0);
 }
 
+#[test]
+fn test_difficulty_to_boundary() {
+	use ethereum_types::H256;
+	use std::str::FromStr;
+
+	assert_eq!(difficulty_to_boundary(&U256::from(1)), H256::from(U256::max_value()));
+	assert_eq!(difficulty_to_boundary(&U256::from(2)), H256::from_str("8000000000000000000000000000000000000000000000000000000000000000").unwrap());
+	assert_eq!(difficulty_to_boundary(&U256::from(4)), H256::from_str("4000000000000000000000000000000000000000000000000000000000000000").unwrap());
+	assert_eq!(difficulty_to_boundary(&U256::from(32)), H256::from_str("0800000000000000000000000000000000000000000000000000000000000000").unwrap());
+}
+
+#[test]
+fn test_difficulty_to_boundary_regression() {
+	use ethereum_types::H256;
+
+	// the last bit was originally being truncated when performing the conversion
+	// https://github.com/paritytech/parity-ethereum/issues/8397
+	for difficulty in 1..9 {
+		assert_eq!(U256::from(difficulty), boundary_to_difficulty(&difficulty_to_boundary(&difficulty.into())));
+		assert_eq!(H256::from(difficulty), difficulty_to_boundary(&boundary_to_difficulty(&difficulty.into())));
+		assert_eq!(U256::from(difficulty), boundary_to_difficulty(&boundary_to_difficulty(&difficulty.into()).into()));
+		assert_eq!(H256::from(difficulty), difficulty_to_boundary(&difficulty_to_boundary(&difficulty.into()).into()));
+	}
+}
+
+#[test]
+#[should_panic]
+fn test_difficulty_to_boundary_panics_on_zero() {
+	difficulty_to_boundary(&U256::from(0));
+}
+
+#[test]
+#[should_panic]
+fn test_boundary_to_difficulty_panics_on_zero() {
+	boundary_to_difficulty(&ethereum_types::H256::from(0));
+}
+
 #[cfg(feature = "benches")]
 mod benchmarks {
 	extern crate test;