Bitcoin developer Gregory Maxwell writes the next on Reddit:
There’s a design flaw within the Bitcoin protocol the place its doable for a 3rd celebration to take a sound transaction of yours and mutate it in a method which leaves it legitimate and functionally similar however with a distinct transaction ID. This enormously complicates writing right pockets software program, and it may be used abusively to invalidate lengthy chains of unconfirmed transactions that rely upon the non-mutant transaction (since transactions refer to one another by txid).
This situation arises from a number of sources, certainly one of them being OpenSSL’s willingness to simply accept and make sense of signatures with invalid encodings. A standard ECDSA signature encodes two massive integers, the encoding isn’t fixed size— if there are main zeros you’re imagined to drop them.
It’s straightforward to write down software program that assumes the signature can be a relentless size after which go away additional main zeros in them.
It is a very fascinating cautionary story, and is especially vital as a result of conditions like these are a part of the rationale why we’ve made sure design choices in our improvement philosophy. Particularly, the difficulty is that this: many individuals proceed to convey up the purpose that we’re in lots of locations unnecessarily reinventing the wheel, creating our personal serialization format, RLP, as an alternative of utilizing the present protobuf and we’re constructing an application-specific scripting language as an alternative of “simply utilizing Lua”. It is a very legitimate concern; not-invented-here syndrome is a commonly-used pejorative, so doing such in-house improvement does require justification.
And the cautionary story I quoted above gives exactly the proper instance of the justification that I’ll present. Exterior applied sciences, whether or not protobuf, Lua or OpenSSL, are superb, and have years of improvement behind them, however in lots of instances they had been by no means designed with the proper consensus, determinism and cryptographic integrity in thoughts that cryptocurrencies require. The OpenSSL scenario above is the proper instance; apart from cryptocurrencies, there actually isn’t any different conditions the place the truth that you possibly can take a sound signature and switch it into one other legitimate signature with a distinct hash is a big downside, and but right here it’s deadly. Certainly one of our core ideas in Ethereum is simplicity; the protocol must be so simple as doable, and the protocol mustn’t comprise any black packing containers. Each single characteristic of each single sub-protocol must be exactly 100% documented on the whitepaper or wiki, and applied utilizing that as a specification (ie. test-driven improvement). Doing this for an current software program bundle is arguably nearly as laborious as constructing a completely new bundle from scratch; in actual fact, it might even be tougher, since current software program packages usually have extra complexity than they should as a way to be feature-complete, whereas our alternate options don’t – learn the protobuf spec and examine it to the RLP spec to grasp what I imply.
Observe that the above precept has its limits. For instance, we’re actually not silly sufficient to begin inventing our personal hash algorithms, as an alternative utilizing the universally acclaimed and well-vetted SHA3, and for signatures we’re utilizing the identical outdated secp256k1 as Bitcoin, though we’re utilizing RLP to retailer the v,r,s triple (the v is an additional two bits for public key restoration functions) as an alternative of the OpenSSL buffer protocol. These sorts of conditions are those the place “simply utilizing X” is exactly the precise factor to do, as a result of X has a clear and well-understood interface and there aren’t any delicate variations between completely different implementations. The SHA3 of the empty string is c5d2460186…a470 in C++, in Python, and in Javascript; there’s no debate about it. In between these two extremes, it’s mainly a matter of discovering the precise steadiness.
