# Tag Info

Accepted

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

A collision in any hash function gives a collision in a "squared" variant of the hash function. This is easy to see. If hash(x)==hash(y), then hashing the outputs ...
• 38.6k
Accepted

### Why does Birthday attack work only with random messages and not with chosen messages?

Birthday attacks do indeed work also for messages chosen by the attacker. For example, consider the case that I want to get a collision between a letter of recommendation and a letter that I was fired....
• 27.8k
Accepted

No, that is not correct. Assuming that the lock combinations are assigned randomly, and you have no apriori knowledge of the combination, you need 500 tries before you get a 50% chance of success. ...
• 147k

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

Strictly speaking, hashing twice might actually increase the chances of a collision. If there is a hash collision of two outputs of the hash function then any string that has that colliding hash will ...
• 279
Accepted

### What is a wide block cipher and why does it avoid birthday bound problems?

I don't think the term "wide block cipher" has a hard definition beyond has a larger block size than the current standard algorithm(s) which right now in most cases would equal to has a block ...
• 46k
Accepted

### Is HMAC prone to birthday attacks?

Is the same true for HMAC ? Yes, HMAC outputs are hashes of something so after $2^{n/2}$ you expect two to match. However, this alone does not help the attacker. The attacker cannot compute the MAC ...
• 32.1k
Accepted

### Why is a HMAC using a 32bit tag not prone to birthday attacks?

"Birthday attacks" relate to the mathematical phenomenon colloquially known as the birthday problem, which states that if you generate random value in a space of size $N$, you expect to hit your first ...

### Why does Birthday attack work only with random messages and not with chosen messages?

For example if there is a trade contract between two parties A and B ... why can A not generate a huge number of possible modified documents (for instance modifying space, dots, commas, etc. of D) and ...
• 147k
Accepted

### Do storage encryption systems care about size of data?

It's common for encryption algorithm specs to specify a hard limit of how much data you should encrypt with the same key. For example, consider the AES-XTS encryption algorithm, which was designed ...
• 14.5k
Accepted

### Are My Answers to This Hash Question Correct?

What needs to be memorized in applied science (physics, crypto) is not a set of formulas. It's, for a few of the simplest formulas studied: what the formula yields, for what inputs, the units for ...
• 141k
Accepted

### Security of Keccak/SHA3 against birthday attacks

Let us correct some of your numbers. The size of the capacity is twice the size of the expected security margin (against a birthday attack). This is the idea of flat sponge clain etc When using a ...
• 9,988