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 social gathering to take a legitimate transaction of yours and mutate it in a means which leaves it legitimate and functionally equivalent however with a distinct transaction ID. This enormously complicates writing appropriate pockets software program, and it may be used abusively to invalidate lengthy chains of unconfirmed transactions that depend upon the non-mutant transaction (since transactions refer to one another by txid).
This situation arises from a number of sources, one among 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 alleged to drop them.
It’s straightforward to write down software program that assumes the signature shall be a relentless size after which depart additional main zeros in them.
This can be a very attention-grabbing cautionary story, and is especially essential as a result of conditions like these are a part of the rationale why we have now made sure design choices in our improvement philosophy. Particularly, the problem is that this: many individuals proceed to carry 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 prevailing protobuf and we’re constructing an application-specific scripting language as an alternative of “simply utilizing Lua”. This can be 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 supplies exactly the right 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 right consensus, determinism and cryptographic integrity in thoughts that cryptocurrencies require. The OpenSSL scenario above is the right instance; except for cryptocurrencies, there actually is not any different conditions the place the truth that you may take a legitimate signature and switch it into one other legitimate signature with a distinct hash is a major downside, and but right here it’s deadly. One in all our core ideas in Ethereum is simplicity; the protocol needs to be so simple as doable, and the protocol shouldn’t include 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 bundle is arguably nearly as onerous as constructing a wholly new bundle from scratch; the truth is, it might even be more durable, since current software program packages usually have extra complexity than they should with the intention to be feature-complete, whereas our options don’t – learn the protobuf spec and examine it to the RLP spec to grasp what I imply.
Word that the above precept has its limits. For instance, we’re definitely 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 previous 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 best factor to do, as a result of X has a clear and well-understood interface and there aren’t any refined 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 best steadiness.
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