Comment by thyristan
9 days ago
OTP can be useful especially for backups. Use a fast random number generator (real, not pseudo), write output to fill tape A. XOR the contents of tape A to your backup datastream and write result to Tape B. Store tape A and B in different locations.
But you have one copy of the key stream. It is not safe. You need at least two places to store at least two copies of the key stream. You cannot store it in non-friendly cloud (and this thread started from backing up government sensitive data into cloud owned by other country, possibly adversary one.
If you have two physically separate places which you could trust key stream, you could use them to backup non-encrypted (or "traditionally" encrypted) data itself, without any OTP.
You may want some redundancy because needing both tapes increases the risk to your backup. You could just backup more often. You could use 4 locations, so you have redundand keystreams and redundant backup streams. But in general, storing the key stream follows the same necessities as storing the backup or some traditional encryption keys for a backup. But in general, your backup already is a redundancy, and you will usually do multiple backups in time intervals, so it really isn't that bad.
Btw, you really really need a fresh keystream for each and every backup. You will have as many keystream tapes as you have backup tapes. Re-using the OTP keystream enables a lot of attacks on OTP, e.g. by a simple chosen plaintext an attacker can get the keystream from the backup stream and then decrypt other backup streams with it. XORing similar backup streams also gives the attacker an idea which bits might have changed.
And there is a difference to storing things unencrypted in two locations: If an attacker, like some evil maid, steals a tape in one location, you just immediately destroy its corresponding tape in the other location. That way, the stolen tape will forever be useless to the attacker. Only an attacker that can steal a pair of corresponding tapes in both locations before the theft is noticed could get at the plaintext.