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2

It'll be the same situation that NY City suffered some days ago: when you have little variability on your data, i.e., they have a fixed-small size, it'll always be fast to brute force. You don't say how long your number is, so I'll assume it can range from 0 - 10,000,000,000 (so, a unique number for each human being on Earth today, plus some spare). You ...

3

If the plaintext format is indeed as you describe, then you're out of luck: the insertion of the newlines and the consequent shifting of the plaintext records is enough to disrupt any structure in the ciphertext. If the plaintext were longer, say, 8 records, then it could work, but with just 7 records there's no way to switch the first and last record ...

-4

Well, DES is weak and since you can guess a plaintext you can use a rainbow table to crack the key and then of course decrypt and re-encrypt the message. Apart from that I don't think you can re-order the blocks to change the first and last line, if you really want to do this specific operation.

3

$ROT(n)$ can be thought of as a character based stream cipher. It works because addition - the encryption method used - is commutative, i.e. $ROT(x, ROT(y, m)) = ROT(y, ROT(x, m))$. Another well known commutative function is $XOR$. It is used by the one time pad, but - more practically - also for block ciphers in streaming mode. So you can encrypt using AES ...

4

I'd treat your input as one 192 bit input instead of thinking about 5 separate inputs. If you don't need security, you can always reduce the number of rounds of cryptographic primitives. If you merely need statistically random output, 20% of the usual number of rounds should be fine with many hashes. A few suggestions: SipHash has good performance for ...

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