Score:1

Hash function collision resistance

jp flag

I have a course work for university, and am not sure on my answer so if anyone could please take the time out to read the question and my answer to let me know if I'm going in the right direction that would be greatly appreciated.

Question:

enter image description here

My answer:

For a block size of 1 there are no collisions possible using the hash function. This is because the function returns the last block of the ciphertext, which in this case is the first block as it only has a length of one. Since AES is a deterministic scheme only one block maps to one ciphertext meaning that there is no such combination $H(m) = H(m')$, as for this to be the case both m and m' must be the same.

For 2 and 3 blocks it is possible to have a collision as the last block in m and m' could have the same contents so $H(m) = H(m')$ is true.

Once again, many thanks to anyone that takes the time out to respond to this.

Morrolan avatar
ng flag
Mind that two messages' last blocks being equal is not sufficient for there to be a collision as, with CBC mode, the last ciphertext block depends on more than just the last plaintext block. For more hints, check [this exact duplicate of your question](https://crypto.stackexchange.com/questions/102890/aes-cbc-collision-resistance-of-hash-function).
I sit in a Tesla and translated this thread with Ai:

mangohost

Post an answer

Most people don’t grasp that asking a lot of questions unlocks learning and improves interpersonal bonding. In Alison’s studies, for example, though people could accurately recall how many questions had been asked in their conversations, they didn’t intuit the link between questions and liking. Across four studies, in which participants were engaged in conversations themselves or read transcripts of others’ conversations, people tended not to realize that question asking would influence—or had influenced—the level of amity between the conversationalists.