[Question]
  [
Write a program that takes two lines of input and uses the first as a key phrase to encrypt the second according to the Playfair encryption technique.
[Wikipedia describes Playfair encryption in some detail](http://en.wikipedia.org/wiki/Playfair_cipher), but to avoid any ambiguity, here's a brief summary:
**1. Generate a key table:**
Replace all occurrences of `J` in the key phrase with `I`, then strip all non-alphabet characters and repeated characters. Insert into a 5×5 encryption table, filling the remaining cells with the rest of the alphabet (except `J`; we don't like `J`).
Example:
```
                                        S T A C K
                                        O V E R F
Stack Overflow  -->  STACKOVERFLW  -->  L W B D G
                                        H I M N P
                                        Q U X Y Z
```
**2. Prepare the message to be encrypted**
Replace every `J` with an `I`, strip all non-alphabet characters and split into pairs, using an `X` to break any pairs that contain the same letter twice. If you end up with an odd number of letters, add `X` at the end. (Note: Numerals have to be spelt out in full — `ONE`, `TWO`, `THREE`, etc. — but you can assume this has already been done for you.)
Example:
```
In:
The cat crept into the crypt, crapped, and crept out again.
Out:
TH EC AT CR EP TI NT OT HE CR YP TC RA PX PE DA ND CR EP TO UT AG AI NX
```
**3. Encryption**
Encrypt each pair of letters in turn. If they are in different rows and columns of the key table, replace each with the letter from the same row in the column where the other letter is found (e.g., `VM`⇒`EI`, `LZ`⇒`GQ`). If they are in the same row (or column), choose the two characters immediately to the right (or below), wrapping around if necessary (e.g., `OE`⇒`VR`, `ZG`⇒`KP`).
Example:
```
In:
TH EC AT CR EP TI NT OT HE CR YP TC RA PX PE DA ND CR EP TO UT AG AI NX
Out:
SI RA CA RD FM VU IC VS MO RD ZN AK EC MZ MF BC YN RD FM SV TV KB TM MY
```
The string produced by this process is the encrypted message, which your program should output.
# Rules:
* The input text and key may be obtained from `stdin`, command line arguments or other such sources. Hard-coded input is not allowed.
* Your program must accept both upper and lower case text for the pass phrase and message.
* The encrypted output may be upper or lower case.
* Your program should accept key phrases of at least 64 characters in length, and message texts of at least 16 KB.
* You are not required to handle non-ASCII input.
* You may ignore the possibility of the letter pair `XX` occurring during encryption.
* There is no need to add whitespace to the output of the program.
* Your answer should include an example of a message, key phrase and encrypted output produced by your program.
* **This is a code golf challenge, so the answer with the shortest code (in bytes) will win.**
---
> 
> **NOTE:** Please remember that you only need to break consecutive letters if they appear **in the same pair**. So for example `MASSACHUSETTS` should be encrypted as `MA SX SA CH US ET TS` — the double `S` has to be split, but the double `T` doesn't.
> 
> 
> 
  
[Answer]
## ~~J~~ I\*, ~~536~~ ~~431~~ ~~417~~ ~~380~~ ~~263~~ ~~218~~ ~~203~~ ~~197~~ ~~186~~ 167
```
p=:4 :0
a=.u:65+9-.~i.26
,_2(5|,:~@|.@(=/)+$$,A.~5*1-0{=/)&.(5 5#:(~.n x,a)&i.)\(,'X'#~2|#)(({.,'X',}.)~1+2*1{&I._2{.\2=/\]) ::]^:_(n=:a(e.~#])'JI'charsub toupper)y
)
```
(with extensive suggestions from @algorithmshark)
example use:
```
   'Stack Overflow' p 'The cat crept into the crypt, crapped, and crept out again.'
SIRACARDFMVUICVSMORDZNAKECMZMFBCYNRDFMSVTVKBTMMY
```
splits input correctly:
```
   d=:(({.,'X',}.)~1+2*1{&I._2{.\2=/\]) ::]
   d^:_ 'MASSACHUSETTS'
MASXSACHUSETTS
```
---
\*replace every `J` with an `I`, right?
[Answer]
## Ruby, ~~461~~ ~~411~~ ~~366~~ ~~359~~ ~~352~~ ~~346~~ 330 characters
```
k,m=$<.map{|x|x.tr(?j,?i).upcase.tr('^A-Z','').chars}
t=[*((k&k)|[*?A..?Z]-[?J]).each_slice(5)]
m=(m*'').gsub(/(.)\1/,'\1X\1').chars
c=->n{[t.index{|r|k=r.index n},k]}
$><<(m.size%2<1?m:m+[?X]).each_slice(2).map{|p,q|a,b,d,e=*c[p],*c[q]
a==d ?[t[a][(b+1)%5],t[d][(e+1)%5]]:b==e ?[t[(a+1)%5][b],t[(d+1)%5][e]]:[t[a][e],t[d][b]]}*''
```
Thanks to @daniero for saving... err, a *lot* of bytes. \o/
Here's the ungolfed code:
```
key = gets.chomp
msg = gets.chomp
transform = ->str{
    str.gsub! 'j', 'i'
    str.upcase!
    str.gsub! /[^A-Z]/, ''
    str.split('')
}
# 1. Generate a key table
key = transform[key]
chars = key.uniq + ([*?A..?Z] - key - ['J'])
tbl = Array.new(5) {
    Array.new(5) {
        chars.shift
    }
}
# 2. Prepare the message
msg = transform[msg]
msg = msg.join('').gsub(/(.)\1/){ "#{$1}X#{$1}" }.split('')
msg = (msg.length % 2 == 0 ? msg : msg + ['X']).each_slice(2).to_a
# 3. Encryption
coords = ->chr{
    i = -1
    [tbl.index{|row| i = row.index chr}, i]
}
msg.map! do |c1, c2|
    c1, c2 = coords[c1], coords[c2]
    if c1[0] == c2[0]
        # same row
        [tbl[c1[0]][(c1[1] + 1) % 5], tbl[c2[0]][(c2[1] + 1) % 5]]
    elsif c1[1] == c2[1]
        # same column
        [tbl[(c1[0] + 1) % 5][c1[1]], tbl[(c2[0] + 1) % 5][c2[1]]]
    else
        # neither
        [tbl[c1[0]][c2[1]], tbl[c2[0]][c1[1]]]
    end
end
# Output!
puts msg.join
```
Here's some sample outputs:
```
llama@llama:...code/ruby/ppcg23276playfair$ printf 'Stack Overflow\nThe cat crept into the crypt, crapped, and crept out again.\n' | ./playfair.rb; printf 'This is a password!\nProgramming Puzzles and Code Golf is a Stack Exchange site.\n' | ./playfair.rb
SIRAVXRDFMVUUYVSBLRDZNYVECMZMFBCYNRDFMSVTVKBVBMY
WDDEDSXIXOQFBTUYVQFISQWGRPFBWMESATAHHGMBVEITQFFISHMI
```
[Answer]
# C: ~~495~~ ~~401~~ ~~355~~ 341 characters
~~It's just a rough sketch as of now. I should be able to shave off at least a hundred characters.~~
Goal accomplished: more than a hundred characters (154 as of now) have mysteriously vanished from the code.
```
p[25],l[96],a=1,b,c;o(x,y){putchar(p[x%5^y%5?x/5*5+(x/5^y/5?y:x+1)%5:(x+5)%25]);}main(){for(;a&&((a=(b=getchar())>31)||(b=65))||b++<90;c=0)for(b&=-33;b/65-b/91&&p[c]^b-(b==74);p[c++]||(p[--c]=b-(b==74),l[b]=c));for(;b=getchar(),b=b>31?b&-33:(c=88),b=b/65-b/91?a?a^b?(c*=c==88,b):(c=b,88):(a=b,0):0,a&b&&(o(a=l[a],b=l[b]),o(b,a),a=c),c^88;);}
```
With some pleasant whitespace:
```
p[25],l[96],a=1,b,c;
o(x,y){
    putchar(p[
        x%5^y%5
            ?x/5*5+(x/5^y/5?y:x+1)%5
            :(x+5)%25
    ]);
}
main(){
    for(;
        a&&(
            (a=(b=getchar())>31)||
            (b=65)
        )||b++<90;
        c=0
    )for(
        b&=-33;
        b/65-b/91&&
        p[c]^b-(b==74);
        p[c++]||(
            p[--c]=b-(b==74),
            l[b]=c
        )
    );
    for(;
        b=getchar(),
        b=b>31
            ?b&-33
            :(c=88),
        b=b/65-b/91
            ?a
                ?a^b
                    ?(c*=c==88,b)
                    :(c=b,88)
                :(a=b,0)
            :0,
        a&b&&(
            o(a=l[a],b=l[b]),
            o(b,a),
            a=c
        ),
        c^88;
    );
}
```
I wrote the first iteration of the program on the verge of falling asleep, so it had a lot of superfluous meaningless statements and such. Most of that is rectified, but there are quite a few areas where improvement is most definitely possible.
[Answer]
# Matlab - 458 chars
```
function p=pf(k,p)
k=[upper(k),65:90];k(k==74)=73;k(k<65|k>90)='';[~,i]=unique(k,'first');k=reshape(k(sort(i)),5,5);e=[k,k(:,1);k(1,:)];p=upper(p);p(p==74)=73;p(p<65|p>90)='';n=length(p);for i=1:2:n
if i<n&&p(i)==p(i+1)p=[p(1:i),88,p(i+1:end)];end
n=length(p);end
if mod(n,2)p=[p,88];n=n+1;end
for i=1:2:n [x,y]=find(k==p(i));[w,z]=find(k==p(i+1));p(i:i+1)=[k(w,y),k(x,z)];if x==w p(i:i+1)=[e(w,y+1),e(x,z+1)];end
if y==z p(i:i+1)=[e(x+1,z),e(w+1,y)];end
end
```
Some examples:
```
octave:180> pf('Stack Overflow', 'The cat crept into the crypt, crapped, and crept out again.')
ans = SIRACARDFMVUICVSMORDZNAKECMZMFBCYNRDFMSVTVKBTMMY
octave:181> pf('This is a password!','Programming Puzzles and Code Golf is a Stack Exchange site.')
ans = WDDEDSXIXOQFBTUYVQFISQWGRPFBWMESATAHHGMBVEITQFFISHMI
octave:182> pf('Matlab needs lambdas', 'Who thought elseif is good syntax?')
ans = XGQMFQPKQDSACDKGRIFPQNILDMTW
```
[Answer]
# Haskell - 711
Demo:
```
[timwolla@/data/workspace/haskell/PCG]ghc pcg-23276.hs
[1 of 1] Compiling Main             ( pcg-23276.hs, pcg-23276.o )
Linking pcg-23276 ...
[timwolla@/data/workspace/haskell/PCG]./pcg-23276 "Stack Overflow" "The cat crept into the crypt, crapped, and crept out again."
SIRACARDFMVUICVSMORDZNAKECMZMFBCYNRDFMSVTVKBTMMY
```
Code:
```
import Data.List
import Data.Char
import Data.Maybe
import System.Environment
main=do a<-getArgs
    putStrLn$concat$map(x (a!!0))$map(\x->if (length x)==1 then x++"X"else x)$s 2$concat$map(\x->if (length x)==1then x else intersperse 'X' x)$group$p (a!!1)
p=map(\x->if x=='J' then 'I' else x).filter(isUpper).map toUpper
k x=y++(delete 'J'$['A'..'Z']\\y)where y=nub$p x
u l m=(div i 5,mod i 5)where i=fromJust$elemIndex l$k m
x y z
    |a/=c&&b/=d=(e!!(a*5+d)):(e!!(c*5+b)):[]
    |a==c=(e!!(a*5+(mod(b+1)5))):(e!!(c*5+(mod(d+1)5))):[]
    |True=(e!!((5*(mod(a+1)5))+b)):(e!!((5*(mod(c+1)5))+d)):[]
    where
        o=u(z!!0)y
        t=u(z!!1)y
        a=fst o
        b=snd o
        c=fst t
        d=snd t
        e=k y
s _ []=[]
s n l=(take n l):(s n(drop n l))
```
Large version:
```
import Data.List
import Data.Char
import Data.Maybe
encryptAll key text = map (encrypt key) (transformValue text)
clean x = map (\x -> if x == 'J' then 'I' else x) $ filter (isUpper) $ map (toUpper) x
transformKey x = y ++ (delete 'J' $ ['A'..'Z'] \\ y)
    where y = nub (clean x)
transformValue x = map (\x -> if (length x) == 1 then x ++ "X" else x) $ split 2 $ concat $ map (\x -> if (length x) == 1 then x else intersperse 'X' x) $ group $ clean x
search letter key = (div index 5, mod index 5)
    where index = fromJust $ elemIndex letter $ transformKey key
encrypt key chars
    | rowA /= rowB && colA /= colB = (key' !! (rowA * 5 + colB)) : (key' !! (rowB * 5 + colA)) : []
    | rowA == rowB = (key' !! (rowA * 5 + ((colA + 1) `mod` 5))) : (key' !! (rowB * 5 + ((colB + 1) `mod` 5))) : []
    | otherwise = (key' !! ((5 * ((rowA + 1) `mod` 5)) + colA)) : (key' !! ((5 * ((rowB + 1) `mod` 5)) + colB)) : []
    where
        rowA = fst $ search (head chars) key
        colA = snd $ search (head chars) key
        rowB = fst $ search (last chars) key
        colB = snd $ search (last chars) key
        key' = transformKey key
-- http://stackoverflow.com/a/12876438/782822
split :: Int -> [a] -> [[a]]
split _ [] = []
split n l
  | n > 0 = (take n l) : (split n (drop n l))
  | otherwise = error "Negative n"
```
[Answer]
# Pyth - 111
Too late for competing, I just wanted to share. Here's 
[the encoder](https://pyth.herokuapp.com/?code=L%40G%3Arb0%5Cj%5CiJ%7By%2BwGKywWhZ%3DZh%2a2xlR%7BRcK2+1IhZ%3DKXZK%5Cx%3BM%40J%2BG%3F%21eH5%3F%21hH%3Fq4%25G5_4+1eHVcK2A%2CxJhNxJeN%3DZ-V.DH5.DG5pgGZpgH_RZ&input=Stack+Overflow%0AThe+cat+crept+into+the+crypt%2C+crapped%2C+and+crept+out+again.&debug=0)
and 
[decoder](https://pyth.herokuapp.com/?code=L%40G%3Arb0%5Cj%5CiJ%7By%2BwGKywWhZ%3DZh%2a2xlR%7BRcK2+1IhZ%3DKXZK%5Cx%3BM%40J%2BG%3F%21eH_5%3F%21hH%3F%21%25G5+4_1eHVcK2A%2CxJhNxJeN%3DZ-V.DH5.DG5pgGZpgH_RZ&input=Stack+Overflow%0Asiracardfmvuicvsmordznakecmzmfbcynrdfmsvtvkbtmmy&debug=0)
```
L@G:rb0\j\iJ{y+wGKywWhZ=Zh*2xlR{RcK2 1IhZ=KXZK\x;M@J+G?!eH5?!hH?q4%G5_4 1eHVcK2A,xJhNxJeN=Z-V.DH5.DG5pgGZpgH_RZ
```
**Explanation:**
```
L    b                              L defines common method y(b); 2 calls helps us saving two bytes
    r 0                             lowercase r(b,0)
   :   \j\i                         : replaces all occurrences of "j" with "i"
 @G                                 strips all non-alphabetic characters; G = pyth built-in alphabet
    w                               first input argument
   + G                              appends the alphabet (G)
  y                                 calls y(b)
 {                                  { makes set (removes duplicated characters)
J                                   assigns result to 'J' (KEY VARIABLE)
Kyw                                 assigns output from y(second input argument) to 'K' (TEXT VARIABLE)
WhZ                         ;       While (Z+1 != 0) <-> While (Z != -1) <-> While mismatched items found
             cK2                    list of K pairs.                    e.g. 'ABCCDDE' -> [AB, CC, DD, E]
         lR{R                       l length of { unique characters.    e.g. [2, 1, 1, 1]
        x       1                   1-length first index.               e.g. 1
     h*2                            *2+1 (Index in K)                   e.g. 3 'ABC CDDE'
   =Z                               Assigns to 'Z'
                  IhZ               if (Z != -1) <-> if (mismatched found)
                     =KXZK\x        X Inserts at Z index in K an 'x' and reassigns to 'K'  e.g. 'ABCXC...'
M                                   M defines function g(G, H) where G index H vector (INDEX CONVERSION)
     ?!eH                           if (same col)
         5                              then +5
         ?!hH                           else { if (same row)
             ?q4%G5                             then if (last col)
                   _4                               then -4
                      1                             else +1
                       eH                       else col
   +G                               index += increment
 @J                                 J[index]
VcK2                                V loops over cK2 list of K pairs
     ,xJhNxJeN                      x returns pair members index in J
    A                               A assigns G = xJhN, H = xJeN
                  .DH5              .D returns [row, col] = [i/5,i%5] of 5xn matrix from index of H
                      .DG5          idem. of G
                -V                  Subtracts vectors (RELATIVE POSITION)
              =Z                    Assigns to 'Z'
                          pgGZ          p prints g(G, Z) return value
                              pgH_RZ    p prints g(H, _RZ) return value, and _R changes signs of Z vector
```
**Sample Key/Message/Output:**
```
Stack Overflow
Gottfried Leibniz is famous for his slogan Calculemus, which means Let us calculate. He envisioned a formal language to reduce reasoning to calculation.
lfaukvvnrbbomwpmupkoexvqkovfimaqohflcmkcdsqwbxqtlintinbehcbovttksbtybsavmormwuthrhrbkevfxebqbspdxtbfsvfrwyarfrctrhmpwkrssbtybsvurh
```
[Answer]
# C, 516
Linefeeds added for improved ~~legibility~~ presentation. (Legibility went out the window, I'm afraid.)
```
#define Z(u,v) putchar(o[u]),putchar(o[v])
#define X while((Y=getchar())>31){Y&=223;if(Y==74)Y--;if(Y<65||Y>90
P,L,A,Y,f,a,i,r,c=512,o[25],d[2],*e=o;Q(){for(i=0;o[i]!=d[0];i++);i-=(f=i%5);
for(r=0;o[r]!=d[1];r++);r-=(a=r%5);if(f==a)Z(f+(i+5)%25,a+(r+5)%25);
else if(i==r)Z((f+1)%5+i,(a+1)%5+r);else Z(a+i,f+r);}main(){X||c&(A=1<<Y-65))continue;
c|=A;*e++=Y;}A=1;Y=65;for(P=0;P<25;P++){if(!(c&A))*e++=Y;
if(++Y==74)Y++,A+=A;A+=A;}L=0;X)continue;if(L&&Y==*d)d[1]=88,Q(),*d=Y;
else d[L]=Y,L=1-L;if(!L)Q();}if(L)d[1]=88,Q();}
```
**Example:**
```
$ ./pf
Playfair                                    
The quick brown fox jumps over the lazy dog
QMHNPEKSCBQVTPSVEPEFTQUGDOKGAYXFRTKV
```
[Answer]
# Python 3, ~~709~~ ~~705~~ ~~685~~ 664
Accepts input from stdin.
```
from string import ascii_uppercase as a
from itertools import product as d
import re
n=range
k=input()
p=input()
t=lambda x: x.upper().replace('J','I')
s=[]
for _ in t(k+a):
 if _ not in s and _ in a:
  s.append(_)
m=[s[i:i+5] for i in n(0,len(s),5)]
e={r[i]+r[j]:r[(i+1)%5]+r[(j+1)%5] for r in m for i,j in d(n(5),repeat=2) if i!=j}
e.update({c[i]+c[j]:c[(i+1)%5]+c[(j+1)%5] for c in zip(*m) for i,j in d(n(5),repeat=2) if i!=j})
e.update({m[i1][j1]+m[i2][j2]:m[i1][j2]+m[i2][j1] for i1,j1,i2,j2 in d(n(5),repeat=4) if i1!=i2 and j1!=j2})
l=re.findall(r'(.)(?:(?!\1)(.))?',''.join([_ for _ in t(p) if _ in a]))
print(''.join(e[a+(b if b else 'X')] for a,b in l))
```
Example:
```
mfukar@oxygen[/tmp]<>$ python playfair.py
Stack Overflow
The cat crept into the crypt, crapped, and crept out again.
SIRACARDFMVUICVSMORDZNAKECMZMFBCYNRDFMSVTVKBTMMY
```
[Answer]
## Python: 591 bytes
```
import sys
l=list
n=len
a=[sys.stdin.readline().upper().replace('J','I') for i in (1,2)]
b=l('ABCDEFGHIKLMNOPQRSTUVWXYZ')
def z(x):
    a=0
    if x in b:
        b.remove(x)
        a=1
    return a
c=l(filter(z,a[0]))+b
d=[x for x in a[1] if x in c]
e=1
while e<n(d):
    if d[e-1]==d[e]:
        d.insert(e,'X')
    e+=2
if n(d)%2>0:
    d+='X'
def y(i):
    z=c.index(d[i])
    return z/5,z%5
x=lambda i,j:c[(i%5)*5+(j%5)]
def w(i):
    e,f=y(i)
    g,h=y(i+1)
    if e==g:
        z=x(e,f+1)+x(g,h+1)
    elif f==h:
        z=x(e+1,f)+x(g+1,h)
    else:
        z=x(e,h)+x(g,f)
    print z,
e=0
while e<n(d):
    w(e)
    e+=2
print
```
This uses `stdin` to get the key and the message in that order. I hope it's not cheating to use a flat list to store the encryption matrix, because that made working with the matrix pretty simple. Here are some example runs:
```
>python playfair.py
Stack Overflow
The cat crept into the crypt, crapped, and crept out again.
SI RA CA RD FM VU IC VS MO RD ZN AK EC MZ MF BC YN RD FM SV TV KB TM MY
>python playfair.py
Stack Overflow
The quick red fox jumps over the lazy brown dog.
SI OX TU KS FR GR EQ UT NH OL ER VC MO BS QZ DE VL YN FL
```
[Answer]
## Java - 791
My first golf, so any criticism is welcome. Using Java because I shouldn't. It doesn't seem so bad; less than double the size of the current leader. I was expecting it to be bigger since it's, well, *Java* :) 
```
public class P{static String c(String s){return s.toUpperCase().replace('J','I').replaceAll("[^A-Z]","");}static int f(char[]a, char n){for(int i=0;i<a.length;i++)if(a[i]==n)return i;return -1;}public static void main(String[]a){int i=0,k,l;char j=0;String g=c(a[0]);char[]e,b,h=c(a[1]).toCharArray();b=new char[25];for(;j<g.length();j++)if(j==g.indexOf(g.charAt(j)))b[i++]=g.charAt(j);for(j=65;i<25;j++)if(f(b,j)<0&&j!=74)b[i++]=j;e=new char[h.length*2];for(i=0,j=0;j<h.length;){if(i%2>0&&h[j]==h[j-1])e[i++]=88;e[i++]=h[j++];}if(i%2>0)e[i++]=88;for(j=0;j<i;j+=2){k=f(b,e[j]);l=f(b,e[j+1]);if(k/5==l/5){e[j]=b[(k/5*5)+((k+1)%5)];e[j+1]=b[(l/5*5)+((l+1)%5)];}else if(k%5==l%5){e[j]=b[(k+5)%25];e[j+1]=b[(l+5)%25];}else{e[j]=b[(k/5*5)+(l%5)];e[j+1]=b[(l/5*5)+(k%5)];}}System.out.println(e);}}
```
With auto-format:
```
public class P {
    static String c(String s) {
        return s.toUpperCase().replace('J', 'I').replaceAll("[^A-Z]", "");
    }
    static int f(char[] a, char n) {
        for (int i = 0; i < a.length; i++)
            if (a[i] == n)
                return i;
        return -1;
    }
    public static void main(String[] a) {
        int i = 0, k, l;
        char j = 0;
        String g = c(a[0]);
        char[] e, b, h = c(a[1]).toCharArray();
        b = new char[25];
        for (; j < g.length(); j++)
            if (j == g.indexOf(g.charAt(j)))
                b[i++] = g.charAt(j);
        for (j = 65; i < 25; j++)
            if (f(b, j) < 0 && j != 74)
                b[i++] = j;
        e = new char[h.length * 2];
        for (i = 0, j = 0; j < h.length;) {
            if (i % 2 > 0 && h[j] == h[j - 1])
                e[i++] = 88;
            e[i++] = h[j++];
        }
        if (i % 2 > 0)
            e[i++] = 88;
        for (j = 0; j < i; j += 2) {
            k = f(b, e[j]);
            l = f(b, e[j + 1]);
            if (k / 5 == l / 5) {
                e[j] = b[(k / 5 * 5) + ((k + 1) % 5)];
                e[j + 1] = b[(l / 5 * 5) + ((l + 1) % 5)];
            } else if (k % 5 == l % 5) {
                e[j] = b[(k + 5) % 25];
                e[j + 1] = b[(l + 5) % 25];
            } else {
                e[j] = b[(k / 5 * 5) + (l % 5)];
                e[j + 1] = b[(l / 5 * 5) + (k % 5)];
            }
        }
        System.out.println(e);
    }
}
```
Sample output:
```
>java P "Stack Overflow" "The cat crept into the crypt, crapped, and crept out again."
SIRACARDFMVUICVSMORDZNAKECMZMFBCYNRDFMSVTVKBTMMY
>java P "Write a PlayFair encryption program" "Write a program that takes two lines of input and uses the first as a key phrase to encrypt the second according to the Playfair encryption technique."
RITEWFCPGMWPGEBLYTWYQTXWINOLMWVNLECAXRNBURZWXWQILEWUWYWNQTFLDINWWEMICOTPYRIKWZRMGCBPGUOGPUWOKYGIQILYPFAPTIWMDPFLETGCEWODOWDZTZ
```
[Answer]
# JS (node) - 528 466
```
k=n(2)+'ABCDEFGHIKLMNOPQRSTUVWXYZ',p=n(3),t=o=''
for(i=0;i<k.length;i++)if(!~t.indexOf(k[i]))t+=k[i]
for(i=0;i<p.length;){a=f(c=p[i++]),b=f(!(d=p[i])||c==d?'X':(i++,d))
if(a.x==b.x)a.y=(a.y+1)%5,b.y=(b.y+1)%5
else if(a.y==b.y)a.x=(a.x+1)%5,b.x=(b.x+1)%5
else a.x=b.x+(b.x=a.x,0)
o+=t[a.x+a.y*5]+t[b.x+b.y*5]}console.log(o)
function f(c){x=t.indexOf(c);return{x:x%5,y:x/5|0}}
function n(a){return process.argv[a].toUpperCase().replace(/[^A-Z]/g,'').replace(/J/g,'I')}
```
Sample output:
```
$ node playfair "Stack Overflow" "The cat crept into the crypt, crapped, and crept out again."
SIRACARDFMVUICVSMORDZNAKECMZMFBCYNRDFMSVTVKBTMMY
$ node playfair "Lorem ipsum" "dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat."
CRORSDAHGAMQKPXDOFQMQAMSBSSPUPBPTDMOAHURNRCRLUAULRGNMLCPLSKDSBSBSQQAHMIGRERYMQCROREMAGDTSZIMUHAIAQRQALSGLAHSLZRQPIETAPDXRPNMSFRYMEBPZGHARKIEMIOGROIGREPUHSUPAQIMUHAPUOYRPGRLLRCRKPXDUYAINZ
```
[Answer]
# PHP 582
```
<? list($i,$k,$v)=array_map(function($v){return str_split(preg_replace('#[^A-Z]#','',strtr(strtoupper($v),'J','I')));},$argv);@$i=array_flip;$k=$i($k)+$i(range('A','Z'));unset($k['J']);$k=array_keys($k);$q=$i($k);for($i=1;$i<count($v);$i+=2){if ($v[$i-1]==$v[$i]){array_splice($v,$i,0,'X');}}if(count($v)%2)$v[]='X';for($i=1;$i<count($v);$i+=2){$c=(int)($q[$v[$i-1]]/5);$d=$q[$v[$i-1]]%5;$e=(int)($q[$v[$i]]/5);$f=$q[$v[$i]]%5;if($c==$e){$d=($d+1)%5;$f=($f+1)%5;}elseif($d==$f){$c=($c+1)%5;$e=($e+1)%5;}else{$t=$f;$f=$d;$d=$t;}$v[$i-1]=$k[$c*5+$d];$v[$i]=$k[$e*5+$f];}echo join($v);
```
[Ungolfed](https://github.com/einacio/codegolfing/blob/master/playfair.php)  
[Decoder](https://github.com/einacio/codegolfing/blob/master/unplayfair.php)
outputs
```
$ php playfair.php "Stack Overflow" "The cat crept into the crypt, crapper, and crept out again."
SIRACARDFMVUICVSMORDZNAKECMZMFECYNRDFMSVTVKBTMMY
$ php playfair.php "This was codegolf?" "The full J answers is shorter than my preparation code :("
HIOKVGFHCMWTKZWSIYWIEPWAMWTCPNXQZKMOMEHSCPODEA
```
[Answer]
## Perl, 265
Very straightforward.
```
chomp(($k,$_)=map{uc=~y/A-Z//cdr=~y/J/I/r}<>."@{[A..Z]}",~~<>);1while$k=~s/((.).*)\2/$1/;while(/(.)((?=\1|$)|(.))/g){($a,$b,$c,$d)=map{$e=index$k,$_;5*int$e/5,$e%5}$1,$3||X;print substr$k,$_%25,1 for$a-$c?$b-$d?($a+$d,$c+$b):($a+5+$b,$c+5+$d):(++$b%5+$a,++$d%5+$c)}
```
Indented:
```
chomp(($k,$_)=map{uc=~y/A-Z//cdr=~y/J/I/r}<>."@{[A..Z]}",~~<>);
1while$k=~s/((.).*)\2/$1/;
while(/(.)((?=\1|$)|(.))/g){
    ($a,$b,$c,$d)=map{$e=index$k,$_;5*int$e/5,$e%5}$1,$3||X;
    print substr$k,$_%25,1 for
        $a-$c
            ?$b-$d
                ?($a+$d,$c+$b)
                :($a+5+$b,$c+5+$d)
            :(++$b%5+$a,++$d%5+$c)
}
```
[Answer]
# CoffeeScript - 610
Demo:
```
[timwolla@/data/workspace/js/PCG]coffee pcg-23276.coffee "Stack Overflow" "The cat crept into the crypt, crapped, and crept out again."
SIRACARDFMVUICVSMORDZNAKECMZMFBCYNRDFMSVTVKBTMMY
```
Code:
```
String::r=String::replace
_=(t,l)->
    for r in[0..4]
        for c in[0..4]
            return [r,c]if l is t[r][c]
K = {}
K[c]=c for c in (process.argv[2].toUpperCase().r(/J/g, 'I').r x=/([^A-Z])/g, '')
for i in[1..26]when i!=10
    c=String.fromCharCode 64+i
    K[c]=c
K=(c for c of K)
t=(K[s..s+4]for s in[0..24]by 5)
v=process.argv[3].toUpperCase().r(/J/g,'I').r(x,'').r(/(.)\1/g,'$1X$1').r /(..)/g, '$1 '
o=""
for p in v.trim().r(/\s([A-Z])$/, ' $1X').split /\s/
    [a,b]=p.split '';[d,f]=_ t,a;[e,g]=_ t,b
    o+=if d!=e&&f!=g
        t[d][g]+t[e][f]
    else if d==e
        t[d][++f%5]+t[e][++g%5]
    else
        t[++d%5][f]+t[++e%5][g]
console.log o
```
Ungolfed version:
```
search = (table, letter) ->
    for row in [0..4]
        for column in [0..4]
            return [ row, column ] if letter is table[row][column]
encrypt = (key, value) ->
    key = key.toUpperCase().replace(/J/g, 'I').replace /([^A-Z])/g, ''
    keyChars = {}
    keyChars[char] = char for char in key
    for i in [1..26] when i != 10
        char=String.fromCharCode 64 + i
        keyChars[char] = char
    keyChars = (char for char of keyChars)
    keyTable = (keyChars[start..start+4] for start in [0..24] by 5)
    value = value.toUpperCase().replace(/J/g, 'I').replace(/([^A-Z])/g, '').replace(/(.)\1/g, '$1X$1').replace /(..)/g, '$1 '
    pairs = value.trim().replace(/\s([A-Z])$/, ' $1X').split /\s/
    out = ""
    for pair in pairs
        [a,b] = pair.split ''
        [rowA, colA] = search keyTable, a
        [rowB, colB] = search keyTable, b
        if rowA!=rowB&&colA!=colB
            out += keyTable[rowA][colB]+keyTable[rowB][colA]
        else if rowA==rowB
            out += keyTable[rowA][++colA%5]+keyTable[rowB][++colB%5]
        else
            out += keyTable[++rowA%5][colA]+keyTable[++rowB%5][colB]
    out.replace /(..)/g, '$1 '
console.log encrypt process.argv[2], process.argv[3]
```
]