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Comment by jre | Hacker News Reader

OP here.

erikb is spot on in the sibling comment. This hasn't been expert-reviewed, hasn't been audited so I'm pretty confident there is a bug somewhere that I don't know about.

It's educational in the sense that I tried as best a I could to implement the various algorithmic parts (mining, validating blocks & transactions, etc...).

I originally used MD5 because I thought I would do more exploration regarding difficulty and MD5 is faster to compute than SHA. In the end, I didn't do that exploration, so I could easily replace MD5 with SHA. I'll update the notebook to use SHA, but I'm still not gonna remove the warning :)

I'll also try to point out more explicitly which parts I think are not secure.

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> I'll also try to point out more explicitly which parts I think are not secure.

Things I've noticed:

* Use of floating point arithmetic.

* Non-reproducible serialization in verify_transaction can produce slightly different, but equivalent JSON, which leads to rejecting transactions if produced JSON is platform-dependent (e.g. CRLFs, spaces vs tabs).

* Miners can perform DoS by creating a pair of blocks referencing each other (recursive call in verify_block is made before any sanity checks or hash checks, so they can modify block's ancestor without worrying about changing its hash).

* mine method can loop forever due to integer overflow.

* Miners can put in block a transaction with output sum greater than input sum - only place where it is checked is in compute_fee and no path from verify_block leads there.

Those are all very good points I didn't think about, thanks for these.

I'll fix the two bugs with verify_block and the possibility for a miner to inject invalid a output > input transaction.

I'll add a note for the 3 others.

For deterministic serialization (~canonicalization), you can use sort_keys=True or serialize OrderedDicts. For deseialization, you'd need object_pairs_hook=collections.OrderedDict.

Most current blockchains sign a binary representation with fixed length fields. In terms of JSON, JSON-LD is for graphs and it can be canonicalized. Blockcerts and Chainpoint are JSON-LD specs:

> Blockcerts uses the Verifiable Claims MerkleProof2017 signature format, which is based on Chainpoint 2.0.

https://github.com/blockchain-certificates/cert-verifier-js/...

FYI, dicts are now ordered by default as of Python 3.6.

That's an implementation detail, and shouldn't be relied upon. If you want an ordered dictionary, you should use collections.OrderedDict.

True, but if you need both key ordering and performance, dict is the better option.

It's now the spec for 3.6+.

> #python news: @gvanrossum just pronounced that dicts are now guaranteed to retain insertion order. This is the end of a long journey.

https://twitter.com/raymondh/status/941709626545864704

More here: https://www.reddit.com/r/Python/comments/7jyluw/dict_knownor...

OrderedDicts are backwards-compatible and are guaranteed to maintain order after deletion.

For what it's worth, MD5 is perfectly safe for use as a proof of work algorithm. You just don't want to use it for authentication / data integrity

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