Malleability Exercise

This exercise is designed to help familiarize residents with possible sources of malleability that have been fixed with the gradual implementation of BIP62. It will also serve as a jumping off point for writing tests for Bitcoin Core, one of the best ways to get acquainted with the codebase.

Setup

Clone the Bitcoin Core repository and checkout the v0.18.0 tag.

Apply the following patch:

diff --git a/src/validation.cpp b/src/validation.cpp
index 0f860a4f1..e573be89b 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -1332,7 +1332,7 @@ void UpdateCoins(const CTransaction& tx, CCoinsViewCache& inputs, int nHeight)
 bool CScriptCheck::operator()() {
     const CScript &scriptSig = ptxTo->vin[nIn].scriptSig;
     const CScriptWitness *witness = &ptxTo->vin[nIn].scriptWitness;
-    return VerifyScript(scriptSig, m_tx_out.scriptPubKey, witness, nFlags, CachingTransactionSignatureChecker(ptxTo, nIn, m_tx_out.nValue, cacheStore, *txdata), &error);
+    return VerifyScript(scriptSig, m_tx_out.scriptPubKey, witness, fRequireStandard ? nFlags : nFlags & ~STANDARD_SCRIPT_VERIFY_FLAGS, CachingTransactionSignatureChecker(ptxTo, nIn, m_tx_out.nValue, cacheStore, *txdata), &error);
 }
 
 int GetSpendHeight(const CCoinsViewCache& inputs)

Compile Bitcoin Core:

./autogen.sh
./configure --disable-bench --disable-zmq --without-gui --without-libs --without-miniupnpc
make -j"$(($(nproc)+1))"

The exercise

Copy feature_malleability.py to test/functional/ and starting filling it in with your favourite malleation. There's some boilerplate code in the file to get you started and some comments you can fill out.

Some malleations that might work well for this exercise:

• extra stack item ignored by multisig (definitely works)
• malleability of failing signature
• inherent ECDSA signature malleability
• push operations in scriptSig of non-standard size type
• zero-padded number pushes
• non-push operations in scriptSig

Run ./test/functional/feature_malleability.py to test for success.

Note that some of these might require extra modifications to Bitcoin Core to work (e.g. making -acceptnonstdtxn=1 work). Since you've compiled once before, it'll be easy to do.

You might want to take a look at the modules I've imported at the top of the file for more convenience methods to use.

Debugging tips

Kalle's btcdeb might be very useful for this exercise, as it is a script debugger where you can turn standardness rules on or off.

If you find yourself needing to modify Bitcoin Core. A good resource is our Doxygen, however, do keep in mind that some of the functions are missing from Doxygen, so consider it a subset of the truth.