Implement a One-Time Pad

Question

Background

A one-time pad is a form of encryption that has been proven impossible to crack if used properly.

Encryption is performed by taking a plaintext (comprised of only letters A-Z) and generating a random string on the same length (also only letters). This string acts as the key. Each character in the plaintext is then paired up with the corresponding character in the key. The ciphertext is computed as follows: For each pair, both characters are converted to numbers (A=0, B=1, ... Z=25). The two numbers are added modulo 26. This number is the converted back into a character.

Decryption is exactly the opposite. The characters in the ciphertext and key are paired up and converted to numbers. The key is then subtracted from the ciphertext modulo 26, and the result is converted back into a character A-Z.

The Challenge

Your challenge is to write the shortest program possible that can both encrypt and decrypt a one-time pad.

On the first line of input (to STDIN), there will be either the word "ENCRYPT" or the word "DECRYPT".

If the word is encrypt, then the next line will be the plaintext. Your program should output two lines (to STDOUT), the first being the key, and the second being the ciphertext.

If the word is decrypt, your program will get two more line of input. The first line will be the key, and the second line will be the ciphertext. You program should output one line, which will be the plaintext that has been decrypted.

The plaintext, ciphertext, and key should always consist of uppercase letters A-Z. They will always be a single line and contain no whitespace.

The key should always be random. No large parts of it should repeat between runs, and there should be no patterns that can be found in the text.

Two simple examples:

ENCRYPT
HAPPYBIRTHDAY
>ABKJAQLRJESMG
>HBZYYRTICLVME

DECRYPT
ABKJAQLRJESMG
HBZYYRTICLVME
>HAPPYBIRTHDAY

The > represents which lines are output, so you don't have to print that symbol as output.

Answer 1

GolfScript, 53 chars

n%(0=2%{~.,[{26rand 65+}*]:K]}*zip{{-}*~)26%65+}%K]n*

This is a task for which GolfScript seems just about perfectly suited.

To keep the code short, I'm using the same code for both encryption and decryption: to decrypt, I subtract the key from the ciphertext, while for encryption, I first generate a random ciphertext and then subtract the plaintext from it. Even so, the extra code for implementing encryption mode takes just a little over half the length of the program.

De-golfed version with comments:

n %             # split input into an array of lines

# KEY GENERATION FOR ENCRYPTION MODE:
(               # extract the first line from the array
0 = 2 %         # check if the first char of that line is odd (E = 69)...
{               # ...and execute this block if it is:
    ~           # dump the remaining lines (of which the should be only one) on the stack
    . ,         # calculate the length of the last line...
    [ { 26 rand 65 + } * ]  # ...make an array of that many random letters...
    :K          # ...and assign it to K
    ]           # collect all the lines, including K, back into an array
} *

# ENCRYPTION / DECRYPTION ROUTINE:
zip             # transpose the array of 2 n-char strings into n 2-char strings...
{               # ...and execute this block for each 2-char string:
    {-} *       # subtract the second char code from the first
    ~ )         # negate the result (using the two's complement trick -x = ~x+1)
    26 % 65 +   # reduce modulo 26 and add 65 = A
} %

# OUTPUT:
K ] n*         # join the result and K (if defined) with a newline, stringifying them

Answer 2

Ruby (200 185)

sample runs + wc:

$ ruby onetimepad.rb
ENCODE
ANOTHERTESTINPUTZZZ
ZYCLGHDWLDASFUTHWKC
BPMIBXOXTPTQIVBMDPX
$ ruby onetimepad.rb
DECODE
ZYCLGHDWLDASFUTHWKC
BPMIBXOXTPTQIVBMDPX
ANOTHERTESTINPUTZZZ
$ wc onetimepad.rb
       4       7     185 onetimepad.rb

def f;gets.scan(/./).map{|b|b.ord-65};end
s=->a{a.map{|b|(b+65).chr}*''}
r=->b,a,o{s[a.zip(b).map{|a,b|(a.send o,b)%26}]}
puts(gets=~/^D/?r[f,f,:+]:[s[k=(p=f).map{rand 26}],r[k,p,:-]])

Answer 3

APL (Dyalog Unicode), 46 bytes

⎕A[26|{'N'∊⍞:⍉x,⍪(⎕A⍳w)+x←26?⍨⍴w←⍞⋄-⌿⎕A⍳↑⍞⍞}1]

Try it online!

The current full program is the result of a 2hr long session at The APL Orchard., with a lot of golfing input from Adám (-13 bytes!).

Requires ⎕IO←0 (0-indexing).

Explanation

⎕A[26|{'N'∊⍞:⍉x,⍪(⎕A⍳w)+x←26?⍨⍴w←⍞⋄-⌿⎕A⍳↑⍞⍞}1]
      {                                    }1  inner fn(called with junk param)
           ⍞                                   first input
       'N'∊                                    is 'N' present in it?
            :⍉x,⍪(⎕A⍳w)+x←26?⍨⍴w←⍞             if so, eNcrypt:
                               w←⍞              store message in w
                          26?⍨⍴                 generate 26 random numbers,
                        x←                      store in x
                 (⎕A⍳w)+                        add that to the indices in the alphabet
                ⍪                               table them
              x,                                join with the random numbers
             ⍉                                  transpose to get key and encrypted value
                                  ⋄-⌿⎕A⍳↑⍞⍞    else, decrypt:
                                         ⍞⍞     input key and value
                                        ↑       convert to matrix
                                     ⎕A⍳        get the indices of both in the alphabet
                                   -⌿           difference of all columns
   26|                                          mod the result of the inner fn.
  [                                          ] index the result of the program in 
⎕A                                             the capitalized alphabet.

Answer 4

Haskell, 203 characters

import Random
main=newStdGen>>=interact.(unlines.).(.lines).f.randomRs('A','Z')
f k['E':_,x]=[z const k x,z(e(+))k x]
f _[_,k,x]=[z(e(-))k x]
e(%)k x=toEnum$65+o x%o k`mod`26
o c=fromEnum c-65;z=zipWith

Example:

$ runghc OneTimePad.hs <<< $'ENCRYPT\nHELLOWORLD'
QMNQKGFZFD
XQYBYCTQQG
$ runghc OneTimePad.hs <<< $'DECRYPT\nQMNQKGFZFD\nXQYBYCTQQG'
HELLOWORLD

Answer 5

Perl, 220 171 characters

if(<>=~/D/){$_=<>;$w=<>;print chr((ord(substr$w,$i++,1)-ord$1)%26+65)while/(.)/g}else{$_=<>;$c.=chr((ord($1)-65+($i=rand(26)))%26+65),print chr$i+65while/(.)/g;print$/.$c}

Sample Run:

ENCRYPT
HELLO
CCTKK
JGEVY

DECRYPT
CCTKK
JGEVY
HELLO

Note: at least when I run it, "Press any key to continue..." is appended to the end of the last output. I hope this is ok, because it isn't part of the program. If not, I can make it so it appears on the next line.

This is my first real program in Perl, and my first golf ever, so I would really appreciate tips. Also, I found /(.)/g on the internet, but I have no idea how it works (is it a regular expression? I haven't learnt those yet). Can somebody explain it to me?

EDIT: Thanks to Ilmari Karonen for helping me out with the regexps, I used my new knowledge to save 7 characters!

Expanded, slightly legible version:

if(<>=~/D/){
    $_=<>;
    $w=<>;
    print chr((ord(substr$w,$i++,1)-ord$1)%26+65)while/(.)/g
}
else{
    $_=<>;
    $c.=chr((ord($1)-65+($i=rand(26)))%26+65),print chr$i+65while/(.)/g;
    print$/.$c
}

Answer 6

Python - 304 295

import random
r=raw_input
R=lambda s:range(len(s))
o=lambda c:ord(c)-65
j=''.join
if r()[0]=='D':
 s=r()
 d=r()
 print j(chr((o(s[i])-o(d[i]))%26+65)for i in R(s))
else:
 s=r()
 d=[random.randint(0,26)for i in R(s)]
 print j(chr((o(s[i])+d[i])%26+65)for i in R(s))
 print j(chr(n+65)for n in d)

I believe that this meets the specs exactly (Including the '>' at the start of the input prompts.) It does not validate input, so I think that it will just produce garbage output if you give it characters outside of [A-Z]. It also only checks the first letter of the input command. Anything starting with D will result in a decryption and anything else at all will result in an encryption.

Answer 7

C++ - 220 241 chars, 4 lines

#include<cstdlib>
#include<cstdio>
#define a scanf("%s"
char i,s[99],t[99];int main(){a,t);a,s);if(t[0]>68){for(;s[i];++i)s[i]=(s[i]+(t[i]=rand()%26+65))%26+65;puts(t);}else for(a,t);s[i];++i){s[i]=65+t[i]-s[i];if(s[i]<65)s[i]+=26;}puts(s);}

Edit 1- The MSVS standard library seems to include a lot of unnecessary files which meant that ios had all the includes that I needed but this didn't work with other compilers. Changed ios for the actual files that the functions needed appear in cstdlib and cstdio. Thanks to Ilmari Karonen for pointing this out.

Answer 8

Python - 270

import random
i=raw_input  
m=i()
a=i()
r=range(len(a))
o=ord
j=''.join
if m=='ENCRYPT':
  k=j(chr(65+random.randint(0,25)) for x in r)
  R=k+"\n"+j(chr((o(a[x])+o(k[x]))%26+65) for x in r)
elif m=='DECRYPT':
  k=i()
  R=j(chr((o(k[x])-o(a[x]))%26+65) for x in r)
print R

Sample output:

$ python onetimepad.py 
ENCRYPT
HELLOWORLD
UXCYNPXNNV
BBNJBLLEYY
$ python onetimepad.py 
DECRYPT
UXCYNPXNNV
BBNJBLLEYY
HELLOWORLD

Character count:

$ wc -c onetimepad.py 
270 onetimepad.py

Answer 9

J : 94 bytes

3 :0]1
c=:(26&|@)(&.(65-~a.&i.))
r=:1!:1@1:
((],:+c)[:u:65+[:?26$~#)@r`(r-c r)@.('D'={.)r 1
)

All necessary white space counted.

Commented version:

3 :0]1                                          NB. Make a function and call it
c=:(26&|@)(&.(65-~a.&i.))                       NB. Adverb for operating on the alphabet
                                                NB. (used for adding and subtracting the pad)
r=:1!:1@1:                                      NB. Read input line and decide (right to left)
((],:+c)[:u:65+[:?26$~#)@r   ` (r-c r)            @. ('D'={.)r 1
NB. Encryption (ger    0)    | Decryption (ger 1)| Agenda               
NB. pad,:(crypt=:plain + pad)| crypt - pad       | If D is first input, do (ger 1), else do (ger 0)
)

Answer 10

C# (445 416)

Forgot about Aggregate. Cut off a good bit.

Somewhat golfed:

namespace G {
using System;
using System.Linq;
using x = System.Console;
class P {
    static void Main() {
        string p = "", c = "", k = "";
        Random r = new Random();
        int i = 0;
        if (x.ReadLine()[0] == 'E') {
            p = x.ReadLine();
            k=p.Aggregate(k,(l,_)=>l+(char)r.Next(65,90));
            c=p.Aggregate(c,(m,l)=>m+(char)((l+k[i++])%26+65));
            x.WriteLine(k + "\n" + c);
        } else {
            k = x.ReadLine();
            c = x.ReadLine();
            p=c.Aggregate(p,(l,a)=>l+(char)((a-k[i++]+26)%26+65));
            x.WriteLine(p);
        }
    }
}

}

Golfed:

namespace G{using System;using System.Linq;using x=System.Console;class P{static void Main(){string p="",c="",k="";Random r=new Random();int i=0;if (x.ReadLine()[0]=='E'){p=x.ReadLine();k=p.Aggregate(k,(l,_)=>l+(char)r.Next(65,90));c=p.Aggregate(c,(m,l)=>m+(char)((l+k[i++])%26+65));x.WriteLine(k+"\n"+c);}else{k=x.ReadLine();c=x.ReadLine();p=c.Aggregate(p,(l,a)=>l+(char)((a-k[i++]+26)%26+65));x.WriteLine(p);}}}}

Answer 11

JavaScript 239

var F=String.fromCharCode
function R(l){var k='';while(l--)k+=F(~~(Math.random()*26)+65);return k}
function X(s,k,d){var o='',i=0,a,b,c
while(i<s.length)a=s.charCodeAt(i)-65,b=k.charCodeAt(i++)-65,c=d?26+(a-b):a+b,o+=F((c%26)+65)
return o}

Usage:

var str = "HELLOWORLD";
var key = R(str.length);
var enc = X(str, key, false);
console.log(enc);
console.log(X(enc,key, true));

Answer 12

C (159 + 11 for compiler flags)

Golfed:

d(a,b){return(a+b+26)%26+65;}a;char s[999],b,*c=s-1;main(){g;a=*s-69;g;while(*++c)a?b=-*c,*c=getchar():putchar(b=rand()%26+65),*c=d(*c,b);a||puts("");puts(s);}

Ungolfed:

d(a,b){
    //*a = (*a + b - 2*65 + 26) % 26 + 65; 
    return (a + b + 26) % 26 + 65;
}
a; char s[999], b, *c = s-1;
main(){
    gets(s);
    a = *s - 69; // -1 if decrypt 0 if encrypt
    gets(s);
    while(*++c){
        if(!a)
            putchar(b = rand() % 26 + 65); // 'A'
        else
            b = -*c, *c = getchar();
        *c = d(*c,b);
    }
    if(!a) puts("");
    puts(s);
}

Compile with -Dg=gets(s).

Example:

$./onetimepad
ENCRYPT
FOOBAR
>PHQGHU
>UVEHHL
$./onetimepad
DECRYPT
PHQGHU
UVEHHL
>FOOBAR

Answer 13

Ruby - 184 179 177 chars

def g;gets.scan(/./).map{|c|c.ord-65}end
m,=g
k=(s=g).map{rand 26}
m==4?(puts k.map{|c|(c+65).chr}*'';y=:+):(k,s=s,g)
puts s.zip(k).map{|c,o|(c.send(y||:-,o).to_i%26+65).chr}*''

Run it like this: $ ruby pad-lock.rb

Here is the ungolfed version if anyone is interested (it's not quite up to date with the golfed one though)

def prompt
    gets.scan(/./).map{ |c|c.ord - 65 }
end

mode = prompt[0]
operator = :-
secret = prompt
key = secret.map { |char| rand(26) }

if mode == 4 # the letter E, or ENCRYPT
    key.map { |char| print (char + 65).chr }
    puts
    operator = :+
else
    # make the old secret the new key,
    # and get a new secret (that has been encrypted)
    key, secret = secret, prompt
end

chars = secret.zip(key).map do |secret_char, key_char|

    # if mode == 4 (E) then add, otherwise subtract
    i = secret_char.send(operator, key_char).to_i

    ((i % 26) + 65).chr
end

puts chars.join("")