#8559: add Playfair cipher as a classical cryptosystem
----------------------------+-----------------------------------------------
Reporter: mvngu | Owner: mvngu
Type: enhancement | Status: new
Priority: major | Milestone: sage-4.3.4
Component: cryptography | Keywords:
Author: | Upstream: N/A
Reviewer: | Merged:
Work_issues: |
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From Alasdair !McAndrew:
{{{
Here is some code to implement the Playfair cipher. Maybe you could take
it and squeeze it into the Sage "Classical Cryptosystems" section - I
don't know how to do this.
cheers,
Alasdair
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# Playfair cipher
def makePF(word): #creates 5 x 5 Playfair array beginning with "word"
alph='ABCDEFGHIKLMNOPQRSTUVWXYZ'
pf=''
for ch in word:
if (ch<>"J") & (pf.find(ch)==-1): # ensures no letter is repeated
pf+=ch
for ch in alph:
if pf.find(ch)==-1: #only uses unused letters from alph
pf+=ch
PF=[[pf[5*i+j] for j in range(5)] for i in range(5)]
return PF
def pf_encrypt(di,PF): # encrypts a digraph di with a Playfair array PF
for i in range(5):
for j in range(5):
if PF[i][j]==di[0]:
i0=i
j0=j
if PF[i][j]==di[1]:
i1=i
j1=j
if (i0<>i1) & (j0<>j1):
return PF[i0][j1]+PF[i1][j0]
if (i0==i1) & (j0<>j1):
return PF[i0][(j0+1)%5]+PF[i1][(j1+1)%5]
if (i0<>i1) & (j0==j1):
return PF[(i0+1)%5][j0]+PF[(i1+1)%5][j1]
def insert(ch,str,j): # a helper function: inserts a character "ch" into
tmp='' # a string "str" at position j
for i in range(j):
tmp+=str[i]
tmp+=ch
for i in range(len(str)-j):
tmp+=str[i+j]
return tmp
def playfair(pl,word): # encrypts a plaintext "pl" with a Playfair cipher
PF=makePF(word) # using a keyword "word"
pl2=makeDG(pl)
tmp=''
for i in range(len(pl2)//2):
tmp+=pf_encrypt(pl2[2*i]+pl2[2*i+1],PF)
return tmp
def makeDG(str): # creates digraphs with different values from a string
"str"
tmp=str.replace('J','I') # replace all 'J's with 'I's
c=len(tmp)
i=0
while (c>0) & (2*i+1<len(tmp)):
if tmp[2*i]==tmp[2*i+1]:
tmp=insert("X",tmp,2*i+1)
c-=1
i+=1
else:
c-=2
i+=1
if len(tmp)%2==1:
tmp+='X'
return tmp
}}}
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/8559>
Sage <http://www.sagemath.org>
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