how to make transposition cipher more secure

[nelson0078]

how to make transposition cipher more secure ? by adding more than one round of transposition under the same key?

please advise.

[Moschops]

No, that doesn't make it any more secure.

Imagine a round of transposition that goes like this:
A becomes B
B becomes C
C becomes D
...
Z becomes A

So applying it twice does this:
A becomes C
B becomes D
C becomes E
...
Z becomes B


This is exactly the same as a single round of transposition that does this:
A becomes C
B becomes D
C becomes E
...
Z becomes B

Similarly, applying two rounds of transposition with two different keys also doesn't make it any more secure; it's the same as a single round with a particular key.

[nelson0078]

So how to put into words ?

[Moschops]

I thought I already did explain it using words.

Addendum; I see I've actually used a substitution cipher as my example of why repeated rounds don't add security, but a similar argument applies to transpositions.

Basically, a transposition cipher shuffles the letters around. Shuffling them twice is exactly the same as having shuffled them once in a different way, and adds no security. If you used two different transposition methods, I think that can increase security, but to be honest in this day and age substitution and transposition ciphers are basically toys, and shouldn't be used if you need actual security. Fun to play with and code though.

[nelson0078]

Thanks it helps a lot :)