Topic: FRAGMENTED BACKUPS VULNERABILITY!! IF YOU USE THEM, READ THIS!! (Read 44127 times)

I see what you mean, toggling secureprint cycles the fragments. I'll look into it.

Not sure if this is worth even worth a mention, but I was playing around with fragmented backups to test and found something.

I figured I would try to make a backup with a mix of everything. First paper, w/o secureprint, then paper w secureprint, then file w/o secureprint and finally file with securerint. As this was only a test, I didn't take a very close look other than seeing the wallet name (obviously) and the secureprint code staying the same after toggling the latter on/off. After saving and printing the backups, I deleted the wallet and tried to restore them. When I did, the message "If any of the fragments require a SecurePrint code, you will only have to enter it once" (emphasis mine) made me confident the mix would not be a problem.

When I realized none of the backups fragment matched what was stated on the paper/file I realized something was wrong and went back to create a new one. Toggling secureprint on/off shows that this creates a completely new set. However, 3/4 fragmented backups I made have the same fragment-ID (not the 4th), and all 4 obviously the same wallet ID (this was with 0.96.3).

This was just me playing around, and I would have been a lot more careful before depositing anything. Still, there could perhaps be a warning that:
1. you can't mix and match secureprint
2. each time you toggle on/off you create a new set of fragments.

With 0.96.3.99, none of the fragments got the same ID, but the SecurePrint code stayed the same through each toggle.

Based on the initial text of the previous SSS being deterministic, I'm guessing this wasn't a problem before this fix was made?

It's more cautious the cycle the wallets.

as long as the paper fragments have been secure, if i destroy the paper fragment backups while retaining a copy of the full original root seed, i should be ok, right?

If you are using constant time AES and ECDSA, the only time to perform a timing attack is at restore time, which only applies to SSS in this case (assuming it's not in constant time).


1) You have assume any skilled/well equipped attacker will always get access to your watching only wallet (think law enforcement raiding your home).

2) This means an attacker will know not only of all your coins on chain, but also have access to all the script hash preimage. Therefor, if you used multisig, they will know of this as well.

3) On the other hand, SSS allows you to split your backups without reflecting that on chain, i.e. you could have all your coins in single sig scripts and there is no way for the attacked to know this just with your WO wallet.

I doubt that side-channel attacks are at all relevant for paper backups.  That normally requires statistical data on the timing or the amount of power it takes to do the operation.  It might be a worry that a compromized computer can gain information from a hardware wallet through side channels, but not when restoring a wallet.

My guess is that the main problem with SSS is that it is one of the few cryptographic operations that are simple enough that people implement it themselves, instead of getting it from a peer-reviewed and well-debugged library.


SSS does not just have a plausible deniability edge over multisig scripts (I am sure it does, although I cannot see how).  It solves a different problem: that of safely keeping backups.  Multisig of course gives the same security to your backup, but it also makes spending from the wallet on a day-to-day basis significantly more cumbersome.  And in times with high pressure on the blockchain, it also adds to the fee.

Well said . couldn't hurt for more peer review regardless.

You should cycle the wallets if you're going to bother printing new fragments. Better safe that sorry. Who knows where sha256 will be 30 years from now.


The restoration code is the same, but the fragment IDs are not deterministic anymore, so it's possible the GUI will refuse to proceed further with newly generated fragments until you comment out that check from the Python code. In other words, it can work if you're willing to deal with a little Python.

That's a way to look at the security of the fragment. There is no code allowing to go from 1 fragment to your secret atm, and doing this off of the second version of the SSS scheme is non trivial, just so we are clear.

I'll consider it 1 of 1 for now. Thanks.

I'd rather you don't confuse the 2 for the sake of convenience, but that's a way to look at it.

Is it fine if I don't care 1 sheet can now recover my Bitcoin? And just keep them as a paper backup? There's no actual leak right?

I think you're reading too much into gmax's words. His general concern with SSS implementations is that they are basically all done over prime fields, which the easier implementation, but introduces side channel attacks cause it relies on bignum operations.

A GF(256) implementation would be in constant time, but that's harder to pull and therefor brings in question whether it's worth the effort at all, considering multisig covers a lot of what SSS does. I argued that SSS still has an edge over multisig scripts (plausible deniability), therefor I'll consider implementing SSS over GF(256) for the new wallets.

Since Greg Has concerns about every SSS version he has come across -

https://np.reddit.com/r/Bitcoin/comments/72dfy1/armory_wallet_fragmented_backups_may_be/dnho2w6/

I think we should be highly skeptical about entropy wallets until more peer review is done as well.

The SSS implementation in Armory is completely unrelated to that. It also predates that by a few years.

Entropy , used by many to create SSS fragments for cold storage uses a similar scheme -

https://github.com/mycelium-com/entropy

Specification -

https://github.com/cetuscetus/btctool/blob/bip/bip-xxxx.mediawiki

I'm don't know what you are referring to.

Does this effect SSS with mycelium entropy?

This is false. Where did this idea come from?


What does "probably relatively benign" mean in cryptographic terms?


Why is this?

I had misunderstood your post.  My bad.


Mine has a header line, and two data lines.  I am not completely pwned then, but only has a slightly reduced security.  Ah, well.



Oh my god!  It must be one of the worst security f*ckups in a bitcoin wallet!