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### How does hashing twice protect against birthday attacks?

The bitcoin wiki says: Bitcoin is using two hash iterations (denoted SHA256^2 ie "SHA256 function squared") and the reason for this relates to a partial attack on the smaller but related SHA1 hash. ...
2k views

### How many hex digits do I need to compare when manually checking hash functions?

I sometimes run sha256sum on large files after transferring them from one place to another, and will just skim the hash output to verify it's correct. But, I usually just look at the first/last 5 or 6 ...
7k views

### Are there any well-known examples of SHA-256 collisions?

The popularity of SHA-256 as a hashing algorithm, along with the fact that it has 2256 buckets to choose from leads me to believe that collisions do exist but are quite rare. Are there any well-...
10k views

### Computational requirements for breaking SHA-256?

Let's define "breaking" a hash function $H$ as being threefold (corresponding to the main properties of a cryptographic hash function): preimage attacks to get $m$ knowing $H(m)$ second-preimage ...