logoalt Hacker News

SideQuarkyesterday at 11:18 PM0 repliesview on HN

> The nice thing about a secure XOF is that it doesn’t matter if the entropy given to the XOF is less than perfect. If an XOF is given 10 different sources of entropy, and only one of them is secure, the XOF will remain secure

Secure is not a binary state; not understanding basic information theory and how entropy evolves is fatal.

This right here shows 100% why no one should trust you or your code. You have a seriously fundamental misunderstanding of entropy or what the Bernstein blog post (and it is a blog post, even if it’s Bernstein) states.

His post states, correctly, that a hash is as secure as its weakest entropy source. Adding more bad ones does not strengthen it. He doesn’t say you can ignore the entropy per source, and simply hope one “is secure,” whatever the heck that even means.

Entropy of a source is a number, often in units of bits/sec (or nats or Harley’s or some rate for differential entropy). It’s most definitely doesn’t even make sense to say “one is secure”. That’s a nonsensical phrase.

If you do not correctly know the entropy bit rate of all your inputs, and very importantly cross correlations, you cannot know if you have enough entropy accumulated for an operation, which is then used up; you cannot make downstream claims about security. That you’re so incredibly lax and naive and state the opposite of reality shows the lack of crypto skill. This type of misunderstanding is why there’s still groups hiring people like me to break systems: tons are implemented very poorly, leave holes from poor entropy, timing attacks, power attacks, glitch attacks, etc. depending on the system.

The very least anyone designing such things should know about information theory is a solid understanding of the book by Cover, then stack on top significant knowledge about the physical systems used under the software and have detailed models for them.

This is why people should be skeptical about this stack.