Dr. Mark B Lundeberg, 2018 Feb 15
bitcoincash:qqy9myvyt7qffgye5a2mn2vn8ry95qm6asy40ptgx2
This security advisory notes a vulnerability in the common construction of cross-chain smart contracts (contracts between distinct cryptocurrencies) through hash locking. I focus on the primary use case in [atomic
Dr. Mark B. Lundeberg, 2018 September 6
bitcoincash:qqy9myvyt7qffgye5a2mn2vn8ry95qm6asy40ptgx2
A mechanism where a Bitcoin Cash payment is made to a personally identifying string (real name, email address, social media handle, etc.) instead of directly to a cryptographic key. The payment can only be claimed by the recipient if they generate a public key and get it certified by an identity verifier. This certification signature is confirmed in script via the new opcode OP_CHECKDATASIG.
Characteristics:
Mark Lundeberg 2018 Oct 17
Abstract: Both BCH post-forkday signatures and the BIP143 Segwit signatures are ostensibly designed to remove the 'quadratic hashing problem', however as I will show, they are still vulnerable for long scripts. Back-of-the-envelope calculations show that it will become a serious concern if the existing script limits are relaxed.
Dr. Mark B. Lundeberg, 2018 August 30
bitcoincash:qqy9myvyt7qffgye5a2mn2vn8ry95qm6asy40ptgx2
Since version 2.1, GnuPG is able to use the very same secp256k1 elliptic curve signature algorithm (ECDSA) as used in bitcoin. Quite soon Bitcoin Cash will add a new script opcode OP_CHECKDATASIG that is able to check signatures not just on the containing transaction, but also on arbitrary data. For fun, let's try to intersect the two signature systems and see what can be done!
It's quite common to see smart contract constructions like this:
OP_IF
<clause 1 conditions>
<pubkey1> OP_CHECKSIG
OP_ELSE
<clause 2 conditions>
<pubkey2> OP_CHECKSIG
OP_ENDIF
Last year saw the introduction of BIP158 and Neutrino wallet which use Golomb-coded sets to very efficiently encode which items are in bitcoin blocks.
The basic idea in BIP158 is that we have N items of interest (such as distinct scriptPubKeys) in a block, which we hash to integers in the interval [0,F-1] for some range F that is much larger than N. These are basically short hashes, but with an uneven number of bits. This set of hashes can be queried to check whether a candidate scriptPubKey might be in a block, by computing the hash for the candidate and checking if that integer is in the set. The false positive rate is approximately N/F, and can be therefore tuned by making F larger or smaller.
Golomb-Rice comes into play by providing a very size-efficient way of encoding the list of hashes (by sorting them and encoding just the differences with an entropy-optimized code), which is important for minimizing over
In my last gist, I described how the BIP158 block filters are technically not quite optimal, though in an inconsequential way since the false positive rate has been made so low.
In this document I'm going to describe how it's possible to significantly beat the bandwidth requirements of BIP158 while also achieving a lower overall false positive rate. The idea is to prepare multiple independent filters per block, each of which which has a higher false positive rate. Practically this can give roughly a factor of two improvement over BIP158, but at the cost of additional complexity.
The BIP158 false positive rate was chosen with a particular size of wallet in mind -- somewhere around 1000 items (addresses) to watch in each block. There is a trade off between the overall size of the filter (which must be do
Taproot is an interesting technology to enable multiparty privacy on a bitcoin. Currently, there is a problem with multiparty contracts in that they are obvious deviation from the most common script type (P2PKH), which hurts privacy. The script that gets used will typically indicate exactly what kind of protocol was at play. Also complex P2SH scripts take extra resources (transaction size and CPU cycles).
The basic idea with Taproot is that instead of P2SH where a script is committed by a hash, you can hide a script (or set of possible scripts) as a commitment within a normal-looking public key. Now there are two ways to spend from this public key:
To do #1, it means you need to know the private key, or, you have a set of signers who are able to produce a signature (that's where Schnor
A new sighash flag has been proposed (originally for Lightning, now for Eltoo) which redacts information about the spending inputs, most notably their transaction IDs.
This facility is both powerful and dangerous: it means that signatures intended for one transaction can be used on other transactions. It also makes up a very strong increase in the malleability of transactions. SIGHASH_NOINPUT allows for much more flexible off-chain smart contracts than nonmalleable transactions. Note that Eltoo requires SIGHASH_NOINPUT, for instance. It's not clear whether such a dangerous feature will be adopted on BCH any time soon.
In this gist I'm going to explain that CHECKDATASIG covenants let us write smart contracts that emulate SIGHASH_NOINPUT. This capability alone should in principle allow to build Lightning, Eltoo with bilaterally funded smart contracts on BCH (though, with some significant