If someone were to find a Proof of Work collision, could they pin the tail of the tangle to the head?

Question

If someone were to find a Transaction Proof of Work collision, would it be possible to pin the tail of the tangle to the head and create an infinite loop?

IE. Create a self-referential tangle segment that would cause a permanent network failure?

This question assumes that a version of the IRI checks both the Hash and equality of all transactions. IE. Doesn't consider them duplicate unless they actually are.

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This is a fairly theoretical question. Admittedly, it is unlikely. However, there is a non zero chance of a hash collision. Another issue might be, what would happen if one node received the one Tx1 at the same time as another node received Tx2 with the hash collision?

To stimulate the reader's imagination, here are some links that I find interesting:

• How would Git handle a SHA-1 collision on a blob? https://stackoverflow.com/questions/9392365/how-would-git-handle-a-sha-1-collision-on-a-blob

• Why does the IPFS project have multihashes? https://www.reddit.com/r/ipfs/comments/61r38j/hash_collision_resolution/

• How does the bitcoin network recover in case of a hash collision in the block hash? https://bitcoin.stackexchange.com/questions/38384/how-does-the-bitcoin-network-recover-in-case-of-a-hash-collision-in-the-block-ha

• Google's 'Announcing the first SHA1 collision' https://security.googleblog.com/2017/02/announcing-first-sha1-collision.html

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Answer 1

If the transaction hash is already in the local tangle, any new transaction with same hash will be ignored (immediately dropped by the IRI).

So, no : you can't transform the acyclic graph in a graph with this strategy. (or transform the "almost acyclic graph" -according cfb comment- into a graph)

Here is the relevant code

public boolean store(Tangle tangle) throws Exception {
    if (hash.equals(Hash.NULL_HASH) || exists(tangle, hash)) {
        return false;  //IRI don't save a tx with 999..9 hash
                       ///nor a tx already in the DB
    }
    ...
}