Earlier I wrote:
Plus just consider those really simple substitution codes where you make A = J, B = Z -- whatever random assignment. Also, you want to parse your messages into 5 letter chunks: ALSOY OUWAN TOPAR SEYOU RMESSA GESIN TO5LE TTERC HUNKS. You find this simple "club house" codes in books for little kids, with titles like 'I, Spy' and stuff. Of course this is nothing sophisticated, but it's a great opportunity to use Python string ops to: make the 5 letter uppercase chunks out of whatever plaintext input (could be interactive and first, then convert to file i/o); use the dictionary data structure to do the lookup/substition.
I've played around with the above, so far minus the 5-letter chunking, to develop a simple "clubhouse" substitution scheme in Python. Here's the kind of file it produces: RUFWVEUWB PLS VBJBL QBPWV PAU UFW RPTGBWV HWUFAGT RUWTG UL TGNV EULTNLBLT P LBY LPTNUL, EULEBNJBS NL CNHBWTQ PLS SBSNEPTBS TU TGB OWUOUVNTNUL TGPT PCC MBL PWB EWBPTBS BIFPC. LUY YB PWB BLAPABS NL P AWBPT ENJNC YPW, TBVTNLA YGBTGBW TGPT LPTNUL UW PLQ LPTNUL VU EULEBNJBS PLS VU SBSNEPTBS EPL CULA BLSFWB. YB PWB MBT UL P AWBPT HPTTCBRNBCS UR TGPT YPW. YB GPJB EUMB TU SBSNEPTB P OUWTNUL UR TGPT RNBCS PV P RNLPC WBVTNLA-OCPEB RUW TGUVB YGU GBWB APJB TGBNW CNJBV TGPT TGPT LPTNUL MNAGT CNJB. NT NV PCTUABTGBW RNTTNLA PLS OWUOBW TGPT YB VGUFCS SU TGNV. HFT NL P CPWABW VBLVB, YB EPLLUT SBSNEPTB, YB EPLLUT EULVBEWPTB, YB EPLLUT GPCCUY TGNV AWUFLS. TGB HWPJB MBL, CNJNLA PLS SBPS YGU VTWFAACBS GBWB GPJB EULVBEWPTBS NT RPW PHUJB UFW OUUW OUYBW TU PSS UW SBTWPET. TGB YUWCS YNCC CNTTCB LUTB LUW CULA WBMBMHBW YGPT YB VPQ GBWB, HFT NT EPL LBJBW RUWABT YGPT TGBQ SNS GBWB. ... along with the all-important key file, saved separately: D=Z Z=X Q=Y J=V ... and so on -- all 26 letters have their random substitutes. When you bring the key file and encrypted text back together, the original text (except capitalized). Notice: this primitive scheme keeps punctuation and whitespace as is, simply substitutes for the 26 uppercase alpha letters. But it's still a useful exercise for learning various aspects of Python, as well as starting to think like a cryptographer. For example, coming up with a substitution key is automatic and involves pseudo-randomly choosing from a grab-bag of alpha characters, removing the chosen letter, and choosing again, until all 26 have been chosen at pseudo-random. You then build a dictionary with this, i.e. pair 'A' with the first letter chose, 'B' with the next, and so on. There's no rule which says you can't pair a letter with itself -- could happen. Here's some code: import string, random def permute(): """ Randomly permute the uppercase alphabet by choosing its letters pseudo-randomly """ alphalist = list(string.uppercase) newlist = [] for i in range(len(alphalist)): randchar = random.choice(alphalist) alphalist.remove(randchar) newlist.append(randchar) return newlist def mkdict(): """ Pair uppercase alphabet with randomly permuted version of same """ tuples = zip(string.uppercase,permute()) codedict = {} for pair in tuples: codedict[pair[0]]=pair[1] return codedict The rest of clubhouse.py is about file i/o. You feed it filename.txt and get back filename.cpt and filename.key, the encrypted text and deciphering key. Then you reverse the process, getting filename.dcp (decrypted) back out. Looks like this in IDLE:
clubhouse.encrypt(r"./ocn/sample.txt") Writing to ./ocn/sample.cpt Saving key as ./ocn/sample.key
clubhouse.decrypt(r"./ocn/sample.cpt") Reading from ./ocn/sample.cpt Writing to ./ocn/sample.dcp Using key ./ocn/sample.key
FOURSCORE AND SEVEN YEARS AGO OUR FATHERS BROUGHT FORTH ON THIS CONTINENT A NEW NATION, CONCEIVED IN LIBERTY AND DEDICATED TO THE PROPOSITION THAT ALL MEN ARE CREATED EQUAL. NOW WE ARE ENGAGED IN A GREAT CIVIL WAR, TESTING WHETHER THAT NATION OR ANY NATION SO CONCEIVED AND SO DEDICATED CAN LONG ENDURE. WE ARE MET ON A GREAT BATTLEFIELD OF THAT WAR. WE HAVE COME TO DEDICATE A PORTION OF THAT FIELD AS A FINAL RESTING-PLACE FOR THOSE WHO HERE GAVE THEIR LIVES THAT THAT NATION MIGHT LIVE. IT IS ALTOGETHER FITTING AND PROPER THAT WE SHOULD DO THIS. BUT IN A LARGER SENSE, WE CANNOT DEDICATE, WE CANNOT CONSECRATE, WE CANNOT HALLOW THIS GROUND. THE BRAVE MEN, LIVING AND DEAD WHO STRUGGLED HERE HAVE CONSECRATED IT FAR ABOVE OUR POOR POWER TO ADD OR DETRACT. THE WORLD WILL LITTLE NOTE NOR LONG REMEMBER WHAT WE SAY HERE, BUT IT CAN NEVER FORGET WHAT THEY DID HERE. I've put the colorized (HTMLized) code for my clubhouse.py (version 1.0) at http://www.inetarena.com/~pdx4d/ocn/clubhouse.html with non-HTMLized (plaintext) source code at: http://www.inetarena.com/~pdx4d/ocn/python/clubhouse.py The html file has tie-backs to some of our posts in this thread on edu-sig (including this one). Kirby