4 ms·

No, it won't. Just because it's encrypted doesn't mean it's totally random.

etskinner 3y ago
It does, actually. Most encryption schemes have the property of ciphertext indistinguishability: https://en.wikipedia.org/wiki/Ciphertext_indistinguishability https://en.wikipedia.org/wiki/Ciphertext_indistinguishabilit... If they didn't, they'd be vulnerable: https://en.wikipedia.org/wiki/Distinguishing_attack https://en.wikipedia.org/wiki/Distinguishing_attack However, something close to your point is true. If they compress the data and then encrypt, the ciphertext will indeed be smaller than the original data.
- Accujack 3y ago
  The encryption used by e.g. IBM tape drives is two layered - strong/slow and weak/fast. The Asymmetric key encryption used is the "strong" encryption, but it's far too slow to be used to encrypt the whole tape. So IBM encrypts the tape data with the weak/fast symmetric algorithm and then encrypts the key for that with the strong/slow asymmetric one. If you compress the symmetrically encrypted data, you will indeed gain some capacity. Not as much as you would compressing the raw data, but a visible amount, because the symmetric algorithm doesn't have the property of indistinguishability. That property is not needed for tape storage because anyone who has the tape can safely assume it contains a ciphertext. The tape has header information saying whether it's encrypted or not. If you read the links you posted, you will find out that it's not "most" cryptosystems, it's some. Some applications just don't need indistinguishability and LTO Tapes are one of them. All that said, IBM does compress first then encrypt to make the most of the data compression, but the resulting ciphertext isn't completely random.
ajb 3y ago
If your encryption is any good, it means that it's indistinguishable from random if you don't know the key. Which the tape drive won't.
rbanffy 3y ago
While you are technically correct - the best type of correct - the encrypted data should be indistinguishable from random data. If it’s not, you need better cryptography, as pointed out by the sibling comments.
- Accujack 3y ago
  Actually, it's not that simple. Read my response above. You can't really say indistinguishability makes one cryptosystem better than another because it prevents distinguishing attacks because for some applications it's both not needed and computationally expensive.

It does, actually. Most encryption schemes have the property of ciphertext indistinguishability: https://en.wikipedia.org/wiki/Ciphertext_indistinguishability https://en.wikipedia.org/wiki/Ciphertext_indistinguishabilit... If they didn't, they'd be vulnerable: https://en.wikipedia.org/wiki/Distinguishing_attack https://en.wikipedia.org/wiki/Distinguishing_attack However, something close to your point is true. If they compress the data and then encrypt, the ciphertext will indeed be smaller than the original data.

Accujack 3y ago
The encryption used by e.g. IBM tape drives is two layered - strong/slow and weak/fast. The Asymmetric key encryption used is the "strong" encryption, but it's far too slow to be used to encrypt the whole tape. So IBM encrypts the tape data with the weak/fast symmetric algorithm and then encrypts the key for that with the strong/slow asymmetric one. If you compress the symmetrically encrypted data, you will indeed gain some capacity. Not as much as you would compressing the raw data, but a visible amount, because the symmetric algorithm doesn't have the property of indistinguishability. That property is not needed for tape storage because anyone who has the tape can safely assume it contains a ciphertext. The tape has header information saying whether it's encrypted or not. If you read the links you posted, you will find out that it's not "most" cryptosystems, it's some. Some applications just don't need indistinguishability and LTO Tapes are one of them. All that said, IBM does compress first then encrypt to make the most of the data compression, but the resulting ciphertext isn't completely random.

ajb 3y ago

If your encryption is any good, it means that it's indistinguishable from random if you don't know the key. Which the tape drive won't.

rbanffy 3y ago

While you are technically correct - the best type of correct - the encrypted data should be indistinguishable from random data. If it’s not, you need better cryptography, as pointed out by the sibling comments.

Accujack 3y ago
Actually, it's not that simple. Read my response above. You can't really say indistinguishability makes one cryptosystem better than another because it prevents distinguishing attacks because for some applications it's both not needed and computationally expensive.