4 ms·

Wait what? I'm a confessed ignoramus but this does not make sense to me. Encrypted data is bits and bytes like everything else. You can compress it. (Maybe not

Wait what? I'm a confessed ignoramus but this does not make sense to me. Encrypted data is bits and bytes like everything else. You can compress it. (Maybe not as well as unencrypted text, but still.)

NovemberWhiskey 3y ago
>Encrypted data is bits and bytes like everything else. You can compress it. You can "compress" anything, if one of the modes of your compressor is simply to store the data "as is", but generally speaking encrypted data is indistinguishable from a random bit stream and cannot be compressed. If you think about it, one of the ways that compressors and attacks on encryption work is by looking for structure in the data. If you still have that after encryption, you don't have a good algorithm.
DiskoHexyl 3y ago
Encrypted data is highly random, and a compression ratio would be close to 1. So yeah- technically you can run a compression algorithm over it, but it won't help much
- MetaWhirledPeas 3y ago
  OK gotcha. Close to, but not exactly 1 because patterns appear even in randomness.
  - eichin 3y ago
    Because compression implementations generally have overhead, the output can actually be larger (similar to how compressing already-compressed data can produce a larger result.)
  - cesarb 3y ago
    Yes, but the times when you found these patterns (and managed to get a smaller output) will be matched by the times you found no patterns (and therefore the output was larger), such that the average compression ratio over a large enough amount of inputs will tend to 1.
cesarb 3y ago
> Encrypted data is bits and bytes like everything else. You can compress it. (Maybe not as well as unencrypted text, but still.) Due to the pigeonhole principle, to be able to make some inputs shorter you have to make some inputs longer. The way compression works is by making more likely inputs shorter, while making unlikely inputs longer. But for encrypted data, if your encryption method is good (that is, not ECB mode), all possible values are equally likely (unless you known the secret key).
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ivalm 3y ago
Big part is deduping. If they have a million images that are identical they can just store one and symlink the others.

NovemberWhiskey 3y ago

>Encrypted data is bits and bytes like everything else. You can compress it. You can "compress" anything, if one of the modes of your compressor is simply to store the data "as is", but generally speaking encrypted data is indistinguishable from a random bit stream and cannot be compressed. If you think about it, one of the ways that compressors and attacks on encryption work is by looking for structure in the data. If you still have that after encryption, you don't have a good algorithm.

DiskoHexyl 3y ago

Encrypted data is highly random, and a compression ratio would be close to 1. So yeah- technically you can run a compression algorithm over it, but it won't help much

MetaWhirledPeas 3y ago
OK gotcha. Close to, but not exactly 1 because patterns appear even in randomness.
- eichin 3y ago
  Because compression implementations generally have overhead, the output can actually be larger (similar to how compressing already-compressed data can produce a larger result.)
- cesarb 3y ago
  Yes, but the times when you found these patterns (and managed to get a smaller output) will be matched by the times you found no patterns (and therefore the output was larger), such that the average compression ratio over a large enough amount of inputs will tend to 1.

cesarb 3y ago

> Encrypted data is bits and bytes like everything else. You can compress it. (Maybe not as well as unencrypted text, but still.) Due to the pigeonhole principle, to be able to make some inputs shorter you have to make some inputs longer. The way compression works is by making more likely inputs shorter, while making unlikely inputs longer. But for encrypted data, if your encryption method is good (that is, not ECB mode), all possible values are equally likely (unless you known the secret key).

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ivalm 3y ago

Big part is deduping. If they have a million images that are identical they can just store one and symlink the others.