Make low-s verification mandatory for standard script verification

This change makes a node only accept transactions with low-s
signature encoding for relay and mining, but allows transactions
with high-s signature encoding in mined blocks (no blocks will
be rejected)

Pros:
- If deployed by all miners, this will eliminate this particular
  malleability attack.
- There is no impact on consensus

Cons:
- Wallets that do not implement low-s signature encoding will
  see their transactions be rejected by growing numbers of peers
  and ultimately not be able to get any transaction mined.

Follow ups:
- Eventually, this verification needs to be confirmed through a
  consensus rule (enforcement of BIP62)

1 files changed, 2 insertions, 1 deletions

diff --git a/src/script/standard.h b/src/script/standard.h
index a8b0acc98..e4203f377 100644
--- a/src/script/standard.h
+++ b/src/script/standard.h
@@ -50,7 +50,8 @@ static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY
                                                          SCRIPT_VERIFY_MINIMALDATA |
                                                          SCRIPT_VERIFY_NULLDUMMY |
                                                          SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS |
-                                                         SCRIPT_VERIFY_CLEANSTACK;
+                                                         SCRIPT_VERIFY_CLEANSTACK |
+                                                         SCRIPT_VERIFY_LOW_S;
 
 /** For convenience, standard but not mandatory verify flags. */
 static const unsigned int STANDARD_NOT_MANDATORY_VERIFY_FLAGS = STANDARD_SCRIPT_VERIFY_FLAGS & ~MANDATORY_SCRIPT_VERIFY_FLAGS;