How can I decode a file where each letter has been replaced with the letter 13 letters ahead of it in the alphabet?

Question

There's a text affected by ROT13, called rot.txt. ROT13 (rotate by 13 places) replaces a letter with the letter 13 letters after it in the alphabet. How do I write the command to view the actual text?

I tried:

cat rot.txt | tr 'A-Za-z' 'N-ZA-Mn-za-m'

but no success

Answer 1

Here is a way with an example

echo "hello world" | tr "$(echo -n {A..Z} {a..z} | tr -d ' ')" "$(echo -n {N..Z} {A..M} {n..z} {a..m} | tr -d ' ')"

Output:

uryyb jbeyq

Make it a function:

function rot13() {    
    cat | tr "$(echo -n {A..Z} {a..z} | tr -d ' ')" "$(echo -n {N..Z} {A..M} {n..z} {a..m} | tr -d ' ')" 
}

Usage:

➜ echo "hello world" | rot13 
uryyb jbeyq

Answer 2

The easiest way to decrypt is to use the same program used to encrypt the test; ROT13:

An offset of 13 allows the encryption to be reversible. The encryption and decryption method are identical. Applying 2 consecutive encryptions (2 shifts of 13) heads to find the original text.

The link also mentions that numbers can be encrypted by shifting ASCII 5 positions and punctuation characters can be encrypted by shifting ASCII 47 positions.

Answer 3

You can also achieve the same output by using the following piece of command:

tr '[a-z][A-Z]' '[n-za-m][N-ZA-M]'

or

tr '[a-zA-Z]' '[n-za-mN-ZA-M]'

Personally, I find this syntax a lot easier.

Answer 4

Just download the (now ancient, like some of us) rot13 command used in Unix back when there were wooden ships and iron men. You can find it here: https://sourceforge.net/projects/rot13/files/rot13/1.2.1/