This week’s newsletter announces a security upgrade for C-Lightning, describes a paper and additional research into wallets that accidentally revealed their private keys, and lists some notable code changes in popular Bitcoin infrastructure projects.

Action items

  • Upgrade to C-Lightning 0.6.3: this release fixes a remote DoS vulnerability that could be used to crash C-Lightning nodes and potentially steal money. See the notable code changes section below for details. This release also includes other less critical bug fixes and new features.


  • Weak signature nonces discovered: a preprint paper by researchers Joachim Breitner and Nadia Heninger describes how they discovered hundreds of Bitcoin private keys by looking for signatures generated using nonces with less than the expected entropy of 256 bits. Independent code archaeology by Gregory Maxwell indicates that the main culprit was probably the BitPay Bitcore software which introduced a bug around July 2014 and released a fix about a month later. (Note: BitPay Bitcore is unrelated to Bitcoin Core.) From there, the bug propagated to software such as BitPay Copay that depended upon Bitcore. About 97% of the faulty signatures found in the paper are compatible with Maxwell’s Copay hypothesis, and the paper provides plausible explanations for most of the remaining 3% of signatures, indicating that users of modern wallets are probably safe provided they do not continue to use addresses whose bitcoins they spent using earlier vulnerable programs.

    If you ever used an affected version of Bitcore (0.1.28 to 0.1.35), Copay (0.4.1 to 0.4.3), or other vulnerable software, you should create a new wallet file, send all of your funds from the old wallet file to an address in the new wallet, and discontinue use of the previous wallet file. When designing software that signs Bitcoin transactions, you should prefer to use peer-reviewed implementations that generate signature nonces deterministically, such as libsecp256k1 which implements RFC6979.

    The fast analysis method employed by the authors of the paper took advantage of users who engaged in address reuse, but even keys for addresses that have not been reused are vulnerable to attack if the nonce generation is biased or too small. This can be either through using the same method for keys that were used multiple times (e.g. for Replace-By-Fee) or through simply brute-forcing using the baby-step giant-step or Pollard’s Rho methods.

Notable code changes

Notable code changes this week in Bitcoin Core, LND, C-Lightning, Eclair, and libsecp256k1.

  • Bitcoin Core #15039 disables nLockTime-based anti-fee-sniping if the most recent block seen by the node had a timestamp eight or more hours ago. Anti-fee-sniping attempts to equalize the advantages between honest miners who simply extend the block chain and dishonest miners who create chain forks in an attempt to steal fees from honest miners. However, when using anti-fee-sniping, nodes that have been offline for a while don’t know what block is at the tip of the chain and so they could create multiple transactions offline that would all use the same very old nLockTime value, linking those transactions together in block chain analysis. This merge fixes the problem by disabling the feature if a node is offline for too long.

  • C-Lightning #2214 fixes a remote crash bug which could lead to loss of funds. All users are advised to upgrade to 0.6.3 to get a fix for this issue.

    The vulnerability allowed a peer to crash your C-Lightning node by trying to get you to accept a payment with a smaller timelock than your node allows. If a crashed node remains shutdown for too long, it’s possible for an attacker to steal from it if they previously opened a channel with that node. Note, though, that the attacker must risk their own money to attempt the attack, and so nodes can pretend to be offline in order to take money from any attackers—which is hoped to be enough of a risk to discourage most attacks.

  • C-Lightning #2230 updates the listpeers RPC’s “channel” output to include a private flag indicating whether the channel is being announced to peers on not.

  • C-Lightning #2244 disables plugins by default but adds a configuration option --enable-plugins to enable them at startup. Plugins may be re-enabled by default for a future release when the entire plugin API has been implemented.

  • Eclair #797 changes the way payment routes are calculated. Previously, routes were calculated from the spender to the receiver; now they’re calculated from the receiver to the spender. This fixes a problem where the node was miscalculating fees.