# Are transposition ciphers insecure?

Given plaintext: P[i], i = 1..N

Given stream of cryptographic numbers S[i, j], i = 1..N, S[i, j] in [1, j] generated by RC5 for example

We encrypt using Fisher-Yates shuffle algorithm:

for (int i = N, i > 1 ; i--) swap(P[i], S[i, i])


How to estimate complexity of breaking such chipher?

Is it lower than strength of S[i] generator?

How does it depend on value of N?

How does it depend on data we encrypt?

Unfortunately I was unable to find anything useful on topic.

P.S. Why not AES? Because chipertext is harder to detect using frequency analysis.

• What do you mean the last part abut AES? AES does not allow frequency analysis. – otus Jun 5 '16 at 5:45
• If we want to find out whether given file is encrypted or not - it does. – Sergey Alaev Jun 5 '16 at 6:03
• Thanks for correction, valid algo is en.wikipedia.org/wiki/Fisher–Yates_shuffle – Sergey Alaev Jun 5 '16 at 6:03
• What do you mean by finding out whether a file is encrypted? Can't you tell just by looking? – otus Jun 5 '16 at 6:17
• Well, if you know file type, you may look for known signatures. But what if you don't? e.g. for encrypted disk? – Sergey Alaev Jun 5 '16 at 6:21