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 manner which leaves it legitimate and functionally an identical however with a unique transaction ID. This tremendously complicates writing appropriate pockets software program, and it may be used abusively to invalidate lengthy chains of unconfirmed transactions that rely on the non-mutant transaction (since transactions refer to one another by txid).
This subject arises from a number of sources, one in all them being OpenSSL’s willingness to simply accept and make sense of signatures with invalid encodings. A traditional ECDSA signature encodes two giant integers, the encoding isn’t fixed size— if there are main zeros you’re imagined to drop them.
It’s simple to write down software program that assumes the signature can be a relentless size after which go away further main zeros in them.
It is a very attention-grabbing cautionary story, and is especially essential as a result of conditions like these are a part of the explanation why we’ve got made sure design choices in our improvement philosophy. Particularly, the difficulty is that this: many individuals proceed to deliver up the purpose that we’re in lots of locations unnecessarily reinventing the wheel, creating our personal serialization format, RLP, as a substitute of utilizing the present protobuf and we’re constructing an application-specific scripting language as a substitute 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 circumstances they had been by no means designed with the proper consensus, determinism and cryptographic integrity in thoughts that cryptocurrencies require. The OpenSSL state of affairs above is the proper instance; other than cryptocurrencies, there actually is not 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 unique hash is a big drawback, and but right here it’s deadly. Considered one of our core ideas in Ethereum is simplicity; the protocol needs to be so simple as doable, and the protocol shouldn’t comprise any black packing containers. Each single characteristic of each single sub-protocol needs to be exactly 100% documented on the whitepaper or wiki, and carried out utilizing that as a specification (ie. test-driven improvement). Doing this for an current software program package deal is arguably virtually as arduous as constructing a wholly new package deal from scratch; in truth, it might even be more durable, since current software program packages typically have extra complexity than they should with a view to be feature-complete, whereas our options don’t – learn the protobuf spec and examine it to the RLP spec to know what I imply.
Notice that the above precept has its limits. For instance, we’re definitely not silly sufficient to start out inventing our personal hash algorithms, as a substitute 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 a substitute 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 are not any refined variations between totally 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.