# [bitcoin-dev] Compact Block Relay BIP

Peter R peter_r at gmx.com
Mon May 9 23:37:00 UTC 2016

```Greg Maxwell wrote:

> What are you talking about? You seem profoundly confused here...
>
> I obtain some txouts. I write a transaction spending them in malleable
> form (e.g. sighash single and an op_return output).. then grind the
> extra output to produce different hashes.  After doing this 2^32 times
> I am likely to find two which share the same initial 8 bytes of txid.

[9 May 16 @ 4:30 PDT]

I’m trying to understand the collision attack that you're explaining to Tom Zander.

Mathematica is telling me that if I generated 2^32 random transactions, that the chances that the initial 64-bits on one of the pairs of transactions is about 40%.  So I am following you up to this point.  Indeed, there is a good chance that a pair of transactions from a set of 2^32 will have a collision in the first 64 bits.

But how do you actually find that pair from within your large set?  The only way I can think of is to check if the first 64-bits is equal for every possible pair until I find it.  How many possible pairs are there?

It is a standard result that there are

m! / [n! (m-n)!]

ways of picking n numbers from a set of m numbers, so there are

(2^32)! / [2! (2^32 - 2)!] ~ 2^63

possible pairs in a set of 2^32 transactions.  So wouldn’t you have to perform approximately 2^63 comparisons in order to identify which pair of transactions are the two that collide?

Perhaps I made an error or there is a faster way to scan your set to find the collision.  Happy to be corrected…

Best regards,
Peter

```

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