Is there a 32 bit block cipher that is also a permutation over all 4bn elements?

yacrc

12/18/23, 10:21 PM

Purpose: I'd like to shuffle a file system's blocks without loosing space so I thought if I formatted the disk to have exactly 2^32=4bn sectors, then a secure cipher with 32 bit wide data blocks could map all sectors to new ones without wasting place, like a hash map would do due to its collision chances.

759

1 + 6

pseudo-random-permutation

Eugene Styer

12/18/23, 11:32 PM

See https://crypto.stackexchange.com/questions/8771/is-a-small-size-block-cipher-usable for the danger of a small block size

fgrieu

12/19/23, 6:21 AM

[Format Preserving Encryption](https://crypto.stackexchange.com/q/6098/555) techniques let you use the same technique regardless of the number of blocks, by effectively constructing a keyed permutation of any integer interval.

yacrc

12/19/23, 7:12 AM

The actual purpose will be to shuffle several backups as a protection against ransomware, because if the virus can't match all the copies, I might be able to keep a system alive even during active infection. Yet what if s.b doesn't store >10⁶⁴ data - which is immense? The matching complexity increases exponentially with the number of copies, I assume.

fgrieu

12/19/23, 7:28 AM

@yacr: I don't see how encryption or shuffling blocks can be an effective protection against cryptoransomware. The problem is making the data out of reach of the malware (at least, out of of write reach), so that it can't rewrite it encrypted.

yacrc

12/19/23, 7:52 AM

@fgrieu: suppose I created several copies of a disk partition, yet I encrypted both the data and the sector addresses with new keys for each copy. Then I should be able to simply access copies by knowing the keys, yet a cryptoransomware can't so easily find the matching data with billions of sectors in each copy. Unless all copies were damaged, I can periodically reconstruct the data that the virus encrypts.

fraxinus

12/19/23, 6:16 PM

The malware can simply damage your data no matter how it is encrypted.

Score:10

Crypto

forest

12/18/23, 10:40 PM

A block cipher with any given key is always a permutation. It will map all possible inputs to all possible outputs. If it didn't, and multiple inputs mapped to a single output, then it wouldn't be reversible.

Note that a 32-bit block size is not secure for encryption, as Eugene Styer pointed out in a comment. This is not likely to be an issue if you're only using it to "shuffle" $2^{32}$ sectors. For this, you could use Speck32/64, which is a block cipher with a 32-bit block and a 64-bit key. Again, it is not secure.

+ 0

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Is there a 32 bit block cipher that is also a permutation over all 4bn elements?

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.