Add function to validate difficulty changes

The rule against difficulty adjustments changing by more than a factor of 4 can
be helpful for anti-DoS measures in contexts where we lack a full headers
chain, so expose this functionality separately and in the narrow case where we
only know the height, new value, and old value.

Includes fuzz test by Martin Zumsande.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/pow.h b/src/pow.h
index 1d802cd01e..44b9d673ef 100644
--- a/src/pow.h
+++ b/src/pow.h
@@ -20,4 +20,18 @@ unsigned int CalculateNextWorkRequired(const CBlockIndex* pindexLast, int64_t nF
 /** Check whether a block hash satisfies the proof-of-work requirement specified by nBits */
 bool CheckProofOfWork(uint256 hash, unsigned int nBits, const Consensus::Params&);
 
+/**
+ * Return false if the proof-of-work requirement specified by new_nbits at a
+ * given height is not possible, given the proof-of-work on the prior block as
+ * specified by old_nbits.
+ *
+ * This function only checks that the new value is within a factor of 4 of the
+ * old value for blocks at the difficulty adjustment interval, and otherwise
+ * requires the values to be the same.
+ *
+ * Always returns true on networks where min difficulty blocks are allowed,
+ * such as regtest/testnet.
+ */
+bool PermittedDifficultyTransition(const Consensus::Params& params, int64_t height, uint32_t old_nbits, uint32_t new_nbits);
+
 #endif // BITCOIN_POW_H