This replace was written and offered by Litecoin MimbleWimble lead developer David Burkett.

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Safety Vulnerability

As shared on Twitter yesterday:

Kurt, a long-time GRIN group member, contacted Charlie and I to tell us of a vulnerability within the design for non-interactive transactions. Whereas the assault is tough to carry out in follow, it does permit for theft of funds if the circumstances line up excellent.

This assault is relatively technical, and obscure with out first studying all the crypto behind MWEB. Very informally, it really works like this:

1. Alice sends 2 cash to Bob:
   • coin 1 = 10 LTC
   • coin 2 = 20 LTC
2. Bob creates 2 transactions, 1 to Charlie, and one other again to Alice, and sends them at roughly the identical time:
   • tx1 = spend coin 1 to ship 8 LTCs to Alice (8 LTC Alice, 2 LTC Change)
   • tx2 = spend coin 2 to ship 15 LTCs to Charlie (15 LTC Charlie, 5 LTC Change)
3. Alice adjustments tx1 to spend coin 2 as a substitute, preserving the extra 10 LTCs for herself:
   • tx3 = spend coin 2 to ship 18 LTCs to Alice and a pair of LTC again to Bob as Change
   • tx1 & tx2 dropped and changed with tx3

There are a variety of the reason why this assault would fail in follow practically each time. However the penalties if it did succeed could be very severe, so it was apparent this was one thing we needed to forestall.

We’re very grateful for Kurt taking the time to check MWEB’s design, and for reaching out to share this assault with us. Because of the significance of the discovering, Charlie generously donated his personal cash to pay Kurt a well-deserved 0.15 BTC bounty.

The Repair

Contemplating the proximity to the deliberate launch date, panic began to set in. Luckily, I spotted there’s a comparatively easy repair for the assault that consists of introducing a brand new public key in every enter that forestalls reuse of enter signatures.

On the identical time we have been working by means of the main points of the assault & fixes, I used to be put in touch with some top-notch cryptographers who provided to do a safety audit of our design, which they have been contemplating to make use of as a place to begin for an additional challenge they have been engaged on.

The necessity for a extra formally documented design turned evident, so I spent the subsequent few weeks rewriting LIP-0004 right into a extra full and formally specified design, making minor tweaks alongside the best way to harden it the place I may. Clearly, I ought to’ve completed this from the start, as a result of we’ve had practically as many reviewers of LIP-0004 on this previous month as we now have for the earlier 1.5 years 🙂

Whereas I’d’ve cherished to have all of those eyes on the design way back, I’m thrilled about all the suggestions I’ve obtained.

Sadly, some adjustments do must be made to the code to now match the brand new design, which implies a number of extra extra weeks of dev work. Luckily, practically all the adjustments will probably be within the libmw subproject, which is extremely modularized and closely examined. That is nice information, because it means the adjustments ought to be simpler to make, take a look at, and most significantly, overview. This overview may be rigorously carried out by the opposite LTC builders, so I don’t imagine it’s essential to ship the adjustments again to the auditors. It will have an effect on launch date, however the delay ought to be minimal.

Launch Course of

I discussed final month that the discharge construct course of was time-consuming, and the scripts have been outdated, so I spent a while cleansing all the outdated scripts up, and creating a less complicated, extra automated construct course of. The construct scripts and verification keys are going to be maintained in a separate repo going ahead. Proper now, the brand new ltc-release-build is just below my private github account, but when it really works out nicely for the MWEB launch, we’ll get that moved to litecoin’s github org.

Timeline updates

• I’ve chosen to push the discharge to January to make sure we now have sufficient time to repair the vulnerability discovered. Hopefully that would be the final time 🤞. wenmweb.com is as soon as once more up-to-date.

• v0.21.1 any day now™ for actual this time™