Cryptography with Liberty Basic

102: Classical Cryptography: DES

By Onur Alver (CryptoMan) ( CryptoMan )

- CryptoMan

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INTRODUCTION

We will continue in our series of articles about Cryptography from where we left of at our first article. We have talked about the basics of cryptography and historical cryptography which has still some influences today. modern cryptography. You can see those these simple and seemingly complex ideas is are finding their way into various articles, code snippets or and sample code as ‘good’ 'good’ or ‘sufficient’ cryptography. Unfortunately, those things are only such simple methods may serve as good entertainment but not are really cryptography. So, let’s continue our study of cryptography and understand what is good cryptography.

In our first article we have focused on substitution technique. Remember that substitution technique is a system where the letters of plaintext (unciphered data) are replaced by other letters or numbers or symbols. This is like substituting X for A, K for B, etc. or XORing plaintext with another character which is effectively moving moves the alphabet by as many characters of the ASCII value of the character it is XOR’ed with.

Now, let’s continue with the transposition . technique.

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TRANSPOSITION

Transposition is changing the position of letters in a predetermined way. For example example, you can divide the text into two halves by taking every other character and assigning that character to left half or right half as shown below:

PLAINTEXT :

 PLAINTEXT: ATTACKENEMYONAUGUST22AT 330GMT --> 
 LEFT : ATCEEYNUUT2T30M ---> --> 
 RIGHT: TAKNMOAGS2A03GT ---> TRANSPOSED :ATCEEYNUUT2T30MTAKNMOAGS2A03GT Ofcourse, 
 TRANSPOSED:ATCEEYNUUT2T30MTAKNMOAGS2A03GT 

Of course, you can invent more complex division schemes than the simple example given above and what you above. What is usually do done after transposition is to pass the resultant transposed text from a number of substitution functions which will translate characters into other characters in following a predetermined schedule. On the The following example example, coded in Liberty Basic we will first use the above language, uses the simple left - right transposition followed by a polyalphabetic substitution.

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 REM TRANSPOSITION + SUBSTITUTION EXAMPLE 
 REM FIRST TRANSPOSED THEN POLYALPHABETICALLY SUBSTITUTED 
 REM AND EVERYTHING DONE IN REVERSE TO OBTAIN ORIGINAL MESSAGE. 
 POLYDEPTH=5:DIM CRYPTKEY$(POLYDEPTH) 
 PLAINKEY$= "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ:. " 
 CRYPTKEY$(1)="LNOP4KQ6RSTFVEWZ:M.01X2H3G5A789BY CUDIJ" 
 CRYPTKEY$(2)="4KQ6RSTFVEWL7IJ89BYCU DNOPZ:M.01X2H3G5A" 
 CRYPTKEY$(3)="59B YCUDIJLA78NOP4KQ6RSTFVEWZ:M.01X2H3G" 
 CRYPTKEY$(4)="VEWZ:M.01X2H3LNOP4KQ6RSTFG5A789BY CUDIJ" 
 CRYPTKEY$(5)="4KQL7IJ89BYCU DNO6RSTFVEWPZ:M.01X2H3G5A" 
 PLAINTEXT$="ATTACK AT 23:45 25 JUNE 2001" 
 TRANSPOSEDTEXT$="" 
 LEFTSIDE$="" 
 RIGHTSIDE$="" 
 FOR I=1 TO INT(LEN(PLAINTEXT$)) STEP 2 
 LEFTSIDE$ =LEFTSIDE$+MID$(PLAINTEXT$,I,1) 
 RIGHTSIDE$=RIGHTSIDE$+MID$(PLAINTEXT$,I+1,1) 
 NEXT I 
 TRANSPOSEDTEXT$=LEFTSIDE$+RIGHTSIDE$ 
 CIPHERTEXT$="" 
 FOR I=1 TO LEN(TRANSPOSEDTEXT$) 
 FOR J=1 TO LEN(PLAINKEY$) 
 CurrentChar$=UPPER$(MID$(TRANSPOSEDTEXT$,I,1)) 
 CurrentPos$ =MID$(PLAINKEY$,J,1) 
 IF CurrentChar$=CurrentPos$ THEN 
 CIPHERTEXT$ = CIPHERTEXT$ + MID$( CRYPTKEY$( (I MOD POLYDEPTH)+1), J, 1) 
 EXIT FOR 
 END IF 
 NEXT J 
 NEXT I 
 DECIPHEREDTEXT$="" 
 FOR I=1 TO LEN(CIPHERTEXT$) 
 FOR J=1 TO LEN(CRYPTKEY$(1)) 
 CurrentChar$=UPPER$(MID$(CIPHERTEXT$,I,1)) 
 CurrentPos$ =MID$(CRYPTKEY$( (I MOD POLYDEPTH)+1),J,1) 
 IF CurrentChar$=CurrentPos$ THEN 
 DECIPHEREDTEXT$ = DECIPHEREDTEXT$ + MID$( PLAINKEY$, J, 1) 
 EXIT FOR 
 END IF 
 NEXT J 
 NEXT I 
 ‘AT THIS STAGE WHAT IS DECIPHERED IS TRANSPOSED TEXT 
 ‘TO FIND CLEARTEXT WE MUST UN-TRANSPOSE IT 
 SKIP=INT(LEN(PLAINTEXT$)/2) 
 CLR$="" 
 FOR I=1 TO SKIP 
 CLR$=CLR$+MID$(DECIPHEREDTEXT$,I,1)+MID$(DECIPHEREDTEXT$,SKIP+I,1) 
 NEXT I 
 PRINT "PLAIN TEXT :";PLAINTEXT$ 
 PRINT "TRANSPOSED TEXT:";TRANSPOSEDTEXT$ 
 PRINT "CIPHER TEXT :";CIPHERTEXT$ 
 PRINT "DECIPHERED :";CLR$ 

---

The result of the execution of this code is: results in:

 PLAIN TEXT :ATTACK TEXT.....:ATTACK AT 23:45 25 JUNE 2001 
 TRANSPOSED TEXT:ATC T2:52 UE20TAKA 34 5JN 01 
 CIPHER TEXT :W:3A8QHMQJ0NW48W62APRGMSHA5E DECIPHERED :ATTACK TEXT....:W:3A8QHMQJ0NW48W62APRGMSHA5E 
 DECIPHERED.....:ATTACK AT 23:45 25 JUNE 2001 

So, you must be getting the idea by now. By creating more complex transposition and substitution formulas you can build very good crypto systems. The key idea is that your formula and design must be public so everyone can build the necessary software according to your algorithm, algorithm. This allows others to test, analyze and critize it for possible critique your encryption to identify weaknesses and suggest improvements. What must be secret is the is the KEY to your encryption.

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ENCRYPTION KEY

As a good physical key, an the encryption key should be unique and should only open a specific lock or or, in our case case, decrypt the cipher text. This software equivalent of a physical key is a string of bits, say 64 bits. This bits.This to say a chain of ones and zeroes like this:
1100011110010110000010111110000100001111111100110000010101101110

And you You can visualize the ones as the teeth and the zeroes as the rod of the key.

 ______ 
 [__ ]1100011110010110000010111110000100001111111100110000010101101110 
 [ ( ) ]||---||||--|-||-----|-|||||----|----||||||||--||-----|-|-||-|||- 
 [ (__) ]|| |||| | || | ||||| | |||||||| || | | || ||| 
 [______] 

These ones and zeroes will instruct into our algorithm how we want to transpose and substitute each bit on our data data. This is done instead of making a simple schedule of take taking every other character apart. What you do instead, To explain more fully, you take a block of data, usually 8 bytes, convert it into 64 bits and according to a the complex schedule as (as instructed by ones and zeroes in your key key), mix all the bits successively with several iterations in such a way that at the end nobody should be able to
form the original 64 bit order by looking at it or working at it without actually knowing the key.

The more bits you have in your key the more complex it gets. becomes. The idea is to make it so complex and costly in computer time and effort that it must be unfeasible to find it at the solution in a reasonable timeframe, that reasonable time like frame being a few hours, days, weeks or years. Number The number of bits in the key is known as the key length. length
.
However, you must not confuse the key lengths of block ciphers like DES, 3DES, DES , 3DES , IDEA or AES with public key ciphers like RSA, RSA , Diffie - Helman, etc. A 128 bit 3DES ( Triple DES ) key has equivalent strength to the 2048 bit RSA. RSA key. So, if someone claims that he has a super 512 bit cipher based on state of the RSA which admittedly is much better than strong 128 bit ciphers, you should take this as an entertainment. This is like comparing apples and oranges. Key size should be considered in the context of algorithm. As a general rule rule, 112 bit to 256 bit block ciphers can be considered pretty fairly strong and unbreakable. Number theoric public key alogrithms like RSA must have 1024 or more bits. In fact for public key lengths, estimated secure key lengths are given by years. Currently it must be equal to or more than 1024 bits but by bits. By the year 2010 2010, you should raise it that standard to 2048 bits and beyond. We will discuss public key cryptography and RSA on in the next article.

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DES : DATA ENCRYPTION STANDARD

The most widely used encryption system in the world is DES: Data Encryption Standard Standard. The DES system was adopted by the U.S. government as the standard for encryption in 1977 by US government. 1977. DES is was designed by IBM engineers for in 1973, in response to a call in 1973 for from the US government request for requesting an encryption system for all unclasssified government data. It The DES system is based on an earlier cipher system called LUCIFER and was the only system acceptable by the US National Security Agency (NSA). Originally designed as a 128-bit system, under the advise of NSA it was reduced to 56-bits. 56-bits following the advice of NSA.

There has been significant controversy over this reduction which NSA claims that it was made due to hardware design reasons while oppenents limitations. Oppenents say this significantly reduced the strength of the algorithm to a level so that it any encryption can be easily broken by NSA. NSA . Nevertheless, DES has been widely adopted by many sectors but mainly by sectors, chiefly the financial sector which has decided to use it for the protection of PIN numbers on ATMs and EFT/POS terminals. It DES is also used for encrypt encrypting sensitive data (ECB mode) on host to host communications, checking for message integrity with a special mode of DES called cipher block chaining. chaining (CBC mode).

DES is a block cipher operating on 64 bit data blocks (8 bytes) using 56 bit keys implementing succesfully the following concepts:

Diffusion: Diffusion : the statistical structure of cleartext data is mapped into a long range statistics of ciphertext. This means that every clear to cipher text transformation affects significantly following transformations making statistical frequency analysis efforts unfeasible

Confusion: Confusion : makes the relationship between ciphertext and the key very complex to make discovery of the key extremely difficult.

Avalanche Effect: Effect : assures big changes in ciphertext with even 1 bit change in the key.

The DES algorithm has 16 rounds with a specific S-box scheduling substitutions, shifting and shuffling bits with permutations with a resultant avalanche effect so huge that it the statistical relation between the cleartext and ciphertext becomes nearly impossible at the end. to decipher.

DES is effectively employing employs the techniques of substitutions and transpositions in a very systematic and foolproof way.

DES design have has classified secrets such as the nature of S-boxes. Why are those numbers are used ? used? These questions were never answered but only said that those are indeed very good numbers, and that any arbitrary change of these number could result in significant security losses. This is to say, these numbers are so designed that they are optimizing optimize the bit shuffling in such a way as to make sure no trace of original data can be deduced from the output without knowing the key.

DES is a symetric symmetric cipher , so using the same key is used for both encryption and decryption. To decrypt For decryption, the algorithm is executed in reverse order to obtain the cleartext . Therefore, the encyrptor and decryptor must share a common secret key. key .

The Algorithm is a public key, but key is is secret. If the key become became known than all of the encrypted data can be decrypted.
This brings following significant problems:
- o Key Generation -
o Key Distribution -
o Key Exchange -
o Key Protection -
o Key Verification

We will discuss these problems on in the next article before talking about article, as well as discussing how public key algorithms like RSA which is are solving these problems.

I believe this is a sufficient overview of DES in these for an introductory articles. article. Those readers who want to go deeper can find many articles on Internet or as well as several books in their bookstores. So, let’s go directly into an example implementation of DES written purely in Liberty Basic. Basic .

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 REM 
 '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' 
 'DES ENCRYPTION / DECRYPTION DEMO SOFTWARE WRITTEN PURELY IN LIBERTY BASIC 
 'Copyright(c) 2006, Verisoft CryptoMan 
 'www.verisoft.com onur@verisoft.com 
 '---------------------------------------------------------------------------------------------- 
 'This software and source code is provided for educational private use and may not be used 
 'for commercial purposes without express written consent of VERISOFT. 
 '---------------------------------------------------------------------------------------------- 
 'LIABILITY CLAUSE: 
 'Verisoft will not accept any liabilities or claims due to use, non-use, misuse of this software 
 'or damages to property or life under any circumstances, direct or indirect. Use it at your own 
 'risk. There are no claims to performance, correctness and fitness of this software. 
 'Use of or exportation of cryptographic software to certain countries; and disclosure thereof 
 'may violate local laws and regulations. 
 '----------------------------------------------------------------------------------------------- 
 ' LESSON 1: : THIS EXAMPLE ONLY ENCRYPTS AND DECRYPTS PLAINTEXT FILES 
 ' DES : IT IS NOT FAST, IT IS NOT PERFECT, IT IS NOT COMPLETE ! 
 ' SYMETRIC CRYPTO : IT ONLY SHOWS, HOW REAL ANSI/ISO DATA ENCRYPTION STANDARD 
 ' : WORKS: SINGLE DES, 56-BIT KEYS, ECB MODE. 
 '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' 
 'WARNING: ' WARNING: IF YOU ENCRYPT AND IMPORTANT FILE, AND THEN ERASE IT, AND THEN FORGET 
 ' THE KEY ' AND YOU HAVE ONLY ENCRYPTED FILE; YOU WILL BE IN TROUBLE. PLEASE, DON'T 
 ' CALL ' US OR ANYONE. IF YOU HAVE GOOD FRIENDS AT NSA, THEY MAY DO SOMETHING BUT 
 ' NOT US. SORRY. DON'T ENCRYPT YOUR IMPORTANT FILES AND FORGET YOUR KEY !!!!!!!! 
 '_______________________________________________________________________________________________ '_________________________________________________________________________________________ 
 GLOBAL INITIALTR$,FINALTR$,SWAP$,KEYTR1$,KEYTR2$,ETR$,PTR$,CR$,LF$ 
 CALL Initialize 
 
 NoMainWin 
 WindowWidth = 800 
 WindowHeight = 500 
 UpperLeftX=int((DisplayWidth-WindowWidth)/2) 
 UpperLeftY=int((DisplayHeight-WindowHeight)/2) 
 
 statictext #main.statictext6, "Source File", 15, 2, 108, 20 
 statictext #main.statictext7, "Destination File", 15, 57, 192, 20 
 statictext #main.statictext8, "Encryption Key", 15, 112, 200, 20 
 textbox #main.source, 15, 24, 280, 25 
 button #main.browseSource, "Browse", [browseSource], UL, 230, 2, 60, 20 
 textbox #main.destination, 15, 80, 280, 25 
 button #main.browseDestination, "Browse", [browseDestination], UL, 230, 57, 60, 20 
 textbox #main.key, 15, 132, 110, 25 
 radiobutton #main.selectDecrypt, "Decrypt Source to Destination", [selectDecrypt], [reset], 15, 192, 280, 25 
 radiobutton #main.selectEncrypt, "Encrypt Source to Destination", [selectEncrypt], [reset], 15, 172, 280, 25 
 button #main.go, "Go", [go], UL, 10, 222, 85, 25 
 texteditor #main.tesrc, 300,10, 490, 200 'The handle for our texteditor is #window.te 
 texteditor #main.tedes, 300,230,490, 200 'The handle for our texteditor is #window.te 
 graphicbox #main.gb, 800, 1, 10, 10 
 
 open "DES Crypto Demo V1.00"+space$(80)+"Verisoft(c)2006, www.verisoft.com " for window as #main 
 print #main, "font Courier 10" 
 print #main, "trapclose [quit]" 
 
 MODE = 1 
 #main.selectEncrypt, "set" 
 wait 
 
 [browseSource] 'Search for source file 
 filedialog "Select source file:","*.*",INFILE$ 
 if INFILE$ <> "" then #main.source, INFILE$ 
 wait 
 
 [browseDestination] 'Search for destination file 
 filedialog "Select destination file:","*.*",OUTFILE$ 
 if OUTFILE$ <> "" then #main.destination, OUTFILE$ 
 wait 
 
 [selectDecrypt] 'Set mode to decryption 
 MODE = 2 
 wait 
 
 [selectEncrypt] 'Set mode to encryption 
 MODE = 1 
 wait 
 
 [reset] 'Perform action for the radiobutton named 'selectDecrypt' 
 wait 
 
 [go] 'Do it! 
 #main.key, "!contents? KEY$" 
 #main.source, "!contents? INFILE$" 
 #main.destination, "!contents? OUTFILE$" 
 
 cursor hourglass 
 RESULT = ENCRYPTION(MODE,INFILE$,OUTFILE$,KEY$) 
 cursor normal 
 if RESULT = 1 then notice "DES Crypto Result"+CR$+"Action completed successfully" 
 if RESULT = 2 then notice "Encrypt/decrypt mode not properly selected" 
 if RESULT = 3 then notice "Source file does not exist" 
 if RESULT = 4 then notice "Destination file already exists" 
 if RESULT = 5 then notice "No de/encryption key specified" 
 wait 
 
 [quit] 'End the program 
 close #main 
 end 
 
 function FUNCTION ENCRYPTION(MODE,INFILE$,OUTFILE$,KEY$) 
 
 IF MODE=1 THEN 
 
 OPEN INFILE$ FOR INPUT AS #1 
 IF OUTFILE$="" THEN OUTFILE$="ENCTEMP";TIME$("ms");".TXT" 
 OPEN OUTFILE$ FOR OUTPUT AS #2 
 #main.tesrc,"" 
 #main.tesrc,DATE$();" ";TIME$() 
 #main.tesrc,"----------------------------------------------" 
 #main.tedes,"" 
 #main.tedes,DATE$();" ";TIME$() 
 #main.tedes,"----------------------------------------------" 
 
 
 WHILE EOF(#1)=0 
 LINE INPUT #1,TEXT$ 
 TEXT$=TEXT$+CR$ 
 #main.tesrc,TEXT$; 
 DO 
 '''''''''''''''''''''''PAD ''''''''''''''''PAD WITH NUL WHEN BLOCK LESS THAN 8 BYTES'''''''''''''''''''' BYTES'''''''''''''''' 
 DX$=LEFT$(TEXT$,8) 
 PDX$=chr$(0)+chr$(0)+chr$(0)+chr$(0)+chr$(0)+chr$(0)+chr$(0)+chr$(0) 
 PDX$=DX$+LEFT$(PDX$,8-LEN(DX$)) 
 ENC$=DESencrypt$(PDX$,KEY$) 
 HX$=SpecHex$(ENC$) 
 #main.tedes,HX$ 
 PRINT #2,HX$ 
 TEXT$=MID$(TEXT$,9) 
 SCAN 
 LOOP WHILE LEN(TEXT$)>0 
 WEND 
 
 CLOSE #1 
 CLOSE #2 
 #main.tesrc,"----------------------------------------------" 
 #main.tedes,"----------------------------------------------" 
 
 ENCRYPTION = 1 
 
 ELSE 
 
 OPEN INFILE$ FOR INPUT AS #1 
 IF OUTFILE$="" THEN OUTFILE$="DECTEMP";TIME$("ms");".TXT" 
 OPEN OUTFILE$ FOR OUTPUT AS #2 
 #main.tesrc,"" 
 #main.tesrc,DATE$();" ";TIME$() 
 #main.tesrc,"----------------------------------------------" 
 #main.tedes,"" 
 #main.tedes,DATE$();" ";TIME$() 
 #main.tedes,"----------------------------------------------" 
 
 WHILE EOF(#1)=0 
 LINE INPUT #1,TEXT$ 
 #main.tesrc,TEXT$ 
 DX$=PackHex$(LEFT$(TEXT$,16)) 
 DEC$=DESdecrypt$(DX$,KEY$) 
 
 '''''''''''''''''REMOVE PADDING ADDED DURING ENCRYPTION'''''''''''''''''''''' ENCRYPTION''''''''''''''''' 
 TDEC$="" 
 FOR I=1 TO 8 
 CH$=MID$(DEC$,I,1) 
 IF CH$>CHR$(0) THEN TDEC$=TDEC$+CH$ 
 NEXT 
 #main.tedes,TDEC$; 
 PRINT #2,TDEC$; 
 
 WEND 
 
 CLOSE #1 
 CLOSE #2 
 #main.tesrc,"----------------------------------------------" 
 #main.tedes,"----------------------------------------------" 
 
 ENCRYPTION = 1 
 
 END IF 
 end function END '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' FUNCTION 
 
 END 
 
 ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' 
 ' COPY GLOBALS AND ALL BELOW THIS LINE UNDER YOUR SOFTWARE AND YOU WILL 
 ' DESencrypt AND ' DESdecrypt FUNCTIONS IN YOUR SOFTWARE. ' DES WORKS WITH 
 ' 8 BYTE KEYS ON 8 BYTE DATA. IT ENCRYPTS TO 8 BYTE BLOCK AND WILL DECRYPT 
 ' DECRYPT FROM 8 BYTE BLOCK TO PLAIN TEXT. IF YOU HAVE LONGER DATA THEN YOU MUST 
 ' DIVIDE IT INTO 8 BYTE BLOCKS. SHORTER THAN 8 BYTE BLOCKS MUST BE PADDED 
 ' AND UNPADDED ' BY YOUR SOFTWARE LOGIC. ABOVE, DEMO SHELL SHOWS THIS NICELY. 
 ' HOWEVER, DOWN BELOW ' THE GUI SHELL, IT CALLS THE FUNCTIONS BELOW. DES NEEDS 
 ' ALL THOSE COMPLEX FUNCTIONS ' AND NUMBERS. YOU SHOULD NOT TOUCH OR WORRY 
 ' ABOUT THEM. JUST USE DESencrypt( DATA$,KEY$) ' DATA$,KEY$ ) OR DESdecrypt( ENCDATA$,KEY$ ). 
 ' YOU CAN DECRYPT WITH THE EXACT SAME KEY YOU USED FOR ' ENCRYPTION. 
 '''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' Function ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''' 
 
 FUNCTION PackHex$( y$ ) 
 z$="" 
 for j=1 to len(y$) step 2 
 n=HEXDEC(mid$(y$,j,2)) 
 z$=z$+chr$(n) 
 next j 
 PackHex$=z$ 
 end function END FUNCTION 
 
 FUNCTION SpecHex$( y$ ) 
 z$="" 
 FOR j=1 TO LEN(y$) 
 n=ASC(MID$(y$,j,1)) 
 IF n<16 THEN z$=z$+"0" 
 z$=z$+DECHEX$(n) 
 NEXT j 
 SpecHex$=z$ 
 END FUNCTION 
 
 FUNCTION RANDOMKEY$() 
 X$="" 
 FOR I=1 TO 8 
 X$=X$+CHR$(RND(1)*255) 
 NEXT I 
 RANDOMKEY$=X$ 
 END FUNCTION 
 
 SUB Initialize 
 CR$=CHR$(13):LF$=CHR$(10) 
 
 INITIALTR$="" 
 DATA 58,50,42,34,26,18,10,2,60,52,44,36,28,20,12,4,62,54,46,38,30,22,14,6,64,56,48,40,32,24,16,8,57,49,41,33,25,17,9,1,59,51,43,35,27,19,11,3,61,53,45,37,29,21,13,5,63,55,47,39,31,23,15,7 58,50,42,34,26,18,10,2,60,52,44,36,28,20,12,4,62,54,46,38,_ 
 30,22,14,6, 64,56,48,40,32,24,16,8,57,49,41,33,25,17,9,1,59,51,43,_ 
 35,27,19,11,3,61,53,45,37,29,21,13,5,63,55,47,39,31,23,15,7 
 FOR I=1 TO 64:READ X:INITIALTR$=INITIALTR$+CHR$(X): NEXT I 
 
 FINALTR$="" 
 DATA 40,8,48,16,56,24,64,32,39,7,47,15,55,23,63,31,38,6,46,14,54,22,62,30,37,5,45,13,53,21,61,29,36,4,44,12,52,20,60,28,35,3,43,11,51,19,59,27,34,2,42,10,50,18,58,26,33,1,41,9,49,17,57,25 40,8,48,16,56,24,64,32,39,7,47,15,55,23,63,31,38,6,46,14,54,_ 
 22,62,30,37,5,45,13,53,21,61,29,36,4,44,12,52,20,60,28,35,3,43,11,_ 
 51,19,59,27,34,2,42,10,50,18,58,26,33,1,41,9,49,17,57,25 
 FOR I=1 TO 64:READ X:FINALTR$=FINALTR$+CHR$(X): NEXT I 
 
 SWAP$="" 
 DATA 33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,01,02,03,04,05,06,07,08,09,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32 33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,_ 
 53,54,55,56,57,58,59,60,61,62,63,64,01,02,03,04,05,06,07,08,09,10,_ 
 11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32 
 FOR I=1 TO 64:READ X:SWAP$=SWAP$+CHR$(X): NEXT I 
 
 
 KEYTR1$="" 
 DATA 57,49,41,33,25,17,9,1,58,50,42,34,26,18,10,2,59,51,43,35,27,19,11,3,60,52,44,36,63,55,47,39,31,23,15,7,62,54,46,38,30,22,14,6,61,53,45,37,29,21,13,5,28,20,12,4,0,0,0,0,0,0,0,0 57,49,41,33,25,17,9,1,58,50,42,34,26,18,10,2,59,51,43,35,_ 
 27,19,11,3,60,52,44,36,63,55,47,39,31,23,15,7,62,54,46,38,30,22,_ 
 14,6,61,53,45,37,29,21,13,5,28,20,12,4,0,0,0,0,0,0,0,0 
 FOR I=1 TO 64:READ X:KEYTR1$=KEYTR1$+CHR$(X): NEXT I 
 
 KEYTR2$="" 
 DATA 14,17,11,24,1,5,3,28,15,6,21,10,23,19,12,4,26,8,16,7,27,20,13,2,41,52,31,37,47,55,30,40,51,45,33,48,44,49,39,56,34,53,46,42,50,36,29,32,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 14,17,11,24,1,5,3,28,15,6,21,10,23,19,12,4,26,8,16,7,27,20,_ 
 13,2,41,52,31,37,47,55,30,40,51,45,33,48,44,49,39,56,34,53,46,42,_ 
 50,36,29,32,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 
 FOR I=1 TO 64:READ X:KEYTR2$=KEYTR2$+CHR$(X): NEXT I 
 
 ETR$="" 
 DATA 32,1,2,3,4,5,4,5,6,7,8,9, 8,9,10,11,12,13,12,13,14,15,16,17,16,17,18,19,20,21,20,21,22,23,24,25,24,25,26,27,28,29,28,29,30,31,32,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 32,1,2,3,4,5,4,5,6,7,8,9,8,9,10,11,12,13,12,13,14,15,_ 
 16,17,16,17,18,19,20,21,20,21,22,23,24,25,24,25,26,27,28,29,_ 
 28,29,30,31,32,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 
 FOR I=1 TO 64:READ X:ETR$=ETR$+CHR$(X): NEXT I 
 
 
 PTR$="" 
 DATA 16,7,20,21,29,12,28,17,1,15,23,26,5,18,31,10,2,8,24,14,32,27,3,9,19,13,30,6,22,11,4,25,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 16,7,20,21,29,12,28,17,1,15,23,26,5,18,31,10,2,8,24,14,_ 
 32,27,3,9,19,13,30,6,22,11,4,25,0,0,0,0,0,0,0,0,0,0,0,0,0,0,_ 
 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 
 FOR I=1 TO 64:READ X:PTR$=PTR$+CHR$(X): NEXT I 
 
 
 DIM ROTS(16) 
 DATA 1,1,2,2,2,2,2,2,1,2,2,2,2,2,2,1 
 FOR I=1 TO 16:READ X:ROTS(I)=X: NEXT I 
 
 DIM S(8,64) 
 DATA 14, 4, 13, 1, 2, 15, 11, 8, 3, 10, 6, 12, 5, 9, 0, 7,_ 
 0, 15, 7, 4, 14, 2, 13, 1, 10, 6, 12, 11, 9, 5, 3, 8,_ 
 4, 1, 14, 8, 13, 6, 2, 11, 15, 12, 9, 7, 3, 10, 5, 0,_ 
 15, 12, 8, 2, 4, 9, 1, 7, 5, 11, 3, 14, 10, 0, 6, 13 
 
 DATA 15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10,_ 
 3, 13, 4, 7, 15, 2, 8, 14, 12, 0, 1, 10, 6, 9, 11, 5,_ 
 0, 14, 7, 11, 10, 4, 13, 1, 5, 8, 12, 6, 9, 3, 2, 15,_ 
 13, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14, 9 
 
 DATA 10, 0, 9, 14, 6, 3, 15, 5, 1, 13, 12, 7, 11, 4, 2, 8,_ 
 13, 7, 0, 9, 3, 4, 6, 10, 2, 8, 5, 14, 12, 11, 15, 1,_ 
 13, 6, 4, 9, 8, 15, 3, 0, 11, 1, 2, 12, 5, 10, 14, 7,_ 
 1, 10, 13, 0, 6, 9, 8, 7, 4, 15, 14, 3, 11, 5, 2, 12 
 
 DATA 7, 13, 14, 3, 0, 6, 9, 10, 1, 2, 8, 5, 11, 12, 4, 15,_ 
 13, 8, 11, 5, 6, 15, 0, 3, 4, 7, 2, 12, 1, 10, 14, 9,_ 
 10, 6, 9, 0, 12, 11, 7, 13, 15, 1, 3, 14, 5, 2, 8, 4,_ 
 3, 15, 0, 6, 10, 1, 13, 8, 9, 4, 5, 11, 12, 7, 2, 14 
 
 DATA 2, 12, 4, 1, 7, 10, 11, 6, 8, 5, 3, 15, 13, 0, 14, 9,_ 
 14, 11, 2, 12, 4, 7, 13, 1, 5, 0, 15, 10, 3, 9, 8, 6,_ 
 4, 2, 1, 11, 10, 13, 7, 8, 15, 9, 12, 5, 6, 3, 0, 14,_ 
 11, 8, 12, 7, 1, 14, 2, 13, 6, 15, 0, 9, 10, 4, 5, 3 
 
 DATA 12, 1, 10, 15, 9, 2, 6, 8, 0, 13, 3, 4, 14, 7, 5, 11,_ 
 10, 15, 4, 2, 7, 12, 9, 5, 6, 1, 13, 14, 0, 11, 3, 8,_ 
 9, 14, 15, 5, 2, 8, 12, 3, 7, 0, 4, 10, 1, 13, 11, 6,_ 
 4, 3, 2, 12, 9, 5, 15, 10, 11, 14, 1, 7, 6, 0, 8, 13 
 
 DATA 4, 11, 2, 14, 15, 0, 8, 13, 3, 12, 9, 7, 5, 10, 6, 1,_ 
 13, 0, 11, 7, 4, 9, 1, 10, 14, 3, 5, 12, 2, 15, 8, 6,_ 
 1, 4, 11, 13, 12, 3, 7, 14, 10, 15, 6, 8, 0, 5, 9, 2,_ 
 6, 11, 13, 8, 1, 4, 10, 7, 9, 5, 0, 15, 14, 2, 3, 12 
 
 DATA 13, 2, 8, 4, 6, 15, 11, 1, 10, 9, 3, 14, 5, 0, 12, 7,_ 
 1, 15, 13, 8, 10, 3, 7, 4, 12, 5, 6, 11, 0, 14, 9, 2,_ 
 7, 11, 4, 1, 9, 12, 14, 2, 0, 6, 10, 13, 15, 3, 5, 8,_ 
 2, 1, 14, 7, 4, 10, 8, 13, 15, 12, 9, 0, 3, 5, 6, 11 
 
 FOR I=1 TO 8 
 FOR J=1 TO 64 
 READ X 
 S(I,J)=X 
 NEXT J 
 NEXT I 
 
 END SUB 
 
 SUB TRANSPOSE BYREF DAT$, T$, N 
 DIM iDAT(64),iT(64),iX(64) 
 
 FOR I=1 TO 64 
 IF MID$(DAT$,I,1)=CHR$(1) THEN iDAT(I)=1 ELSE iDAT(I)=0 
 iX(I)=iDAT(I) 
 iT(I)=ASC(MID$(T$,I,1)) 
 NEXT 
 
 FOR I=1 TO N 
 iDAT(I)=iX( iT(I) ) 
 NEXT 
 ZAT$="" 
 FOR I=1 TO 64: ZAT$=ZAT$+CHR$(iDAT(I)): NEXT I 
 DAT$=LEFT$(ZAT$ ,64) 
 END SUB 
 
 SUB ROTATE BYREF KEY$ 
 X$=LEFT$(KEY$,56) 
 X$=MID$(X$,2,55) 
 X$=LEFT$(X$,27)+LEFT$(KEY$,1)+MID$(X$,29) 
 X$=LEFT$(X$,55)+MID$(KEY$,29,1 ) 
 KEY$=LEFT$(X$,56) 
 END SUB 
 
 SUB UNROTATE BYREF KEY$ 
 X$=LEFT$(KEY$,56) 
 X$=MID$(KEY$,28,1)+LEFT$(X$,55) 
 X$=LEFT$(X$,28)+MID$(KEY$,56,1)+MID$(X$,30) 
 KEY$=LEFT$(X$,56) 
 END SUB 
 
 SUB F I, BYREF KEY$, BYREF A$, BYREF X$ 
 DIM Z(64),Y(64) 
 E$=LEFT$(A$,56) 
 CALL TRANSPOSE E$,ETR$, 48 
 FOR J=1 TO ROTS(I) 
 CALL ROTATE KEY$ 
 NEXT 
 IKEY$=LEFT$(KEY$,56) 
 CALL TRANSPOSE IKEY$,KEYTR2$, 48 
 FOR J=1 TO 48 
 IF ASC(MID$(E$,J,1))+ASC(MID$(IKEY$,J,1))=1 THEN Y(J)=1 ELSE Y(J)=0 
 NEXT 
 FOR K=1 TO 64: Z(K)=ASC(MID$(X$,K,1)): NEXT 
 FOR K=1 TO 8 
 R=32*Y(6*K-5)+16*Y(6*K)+8*Y(6*K-4)+4*Y(6*K-3)+2*Y(6*K-2)+Y(6*K-1)+1 
 IF ODD(S(K,R) / 8) THEN Z(4*K-3)=1 ELSE Z(4*K-3)=0 
 IF ODD(S(K,R) / 4) THEN Z(4*K-2)=1 ELSE Z(4*K-2)=0 
 IF ODD(S(K,R) / 2) THEN Z(4*K-1)=1 ELSE Z(4*K-1)=0 
 IF ODD(S(K,R)) THEN Z(4*K) =1 ELSE Z(4*K) =0 
 NEXT 
 X$="" 
 FOR K=1 TO 64: X$=X$+CHR$(Z(K)): NEXT 
 CALL TRANSPOSE X$,PTR$,32 
 END SUB 
 
 SUB F2 I, BYREF KEY$, BYREF A$, BYREF X$ 
 DIM Z(64),Y(64) 
 E$=LEFT$(A$,64) 
 CALL TRANSPOSE E$, ETR$, 48 
 IKEY$=LEFT$(KEY$,64) 
 CALL TRANSPOSE IKEY$, KEYTR2$ , 48 
 FOR J=1 TO 48 
 IF ASC(MID$(E$,J,1))+ASC(MID$(IKEY$,J,1))=1 THEN Y(J)=1 ELSE Y(J)=0 
 NEXT J 
 FOR J=1 TO ROTS(17-I) 
 CALL UNROTATE KEY$ 
 NEXT J 
 FOR K=1 TO 64: Z(K)=ASC(MID$(X$,K,1)): NEXT 
 FOR K=1 TO 8 
 R=32*Y(6*K-5)+16*Y(6*K)+8*Y(6*K-4)+4*Y(6*K-3)+2*Y(6*K-2)+Y(6*K-1)+1 
 IF ODD(S(K,R) / 8) THEN Z(4*K-3)=1 ELSE Z(4*K-3)=0 
 IF ODD(S(K,R) / 4) THEN Z(4*K-2)=1 ELSE Z(4*K-2)=0 
 IF ODD(S(K,R) / 2) THEN Z(4*K-1)=1 ELSE Z(4*K-1)=0 
 IF ODD (S(K,R)) THEN Z(4*K)=1 ELSE Z(4*K)=0 
 NEXT 
 X$="" 
 FOR K=1 TO 64: X$=X$+CHR$(Z(K)): NEXT 
 CALL TRANSPOSE X$,PTR$,32 
 END SUB 
 
 FUNCTION ODD( N ) 
 IF INT(N) MOD 2 = 0 THEN ODD=0 ELSE ODD=1 
 END FUNCTION 
 
 FUNCTION DESencrypt$( PTEXT$, KY$ ) 
 PLAINTEXT$=BINARY$(PTEXT$) 
 KEY$=BINARY$(KY$) 
 A$=LEFT$( PLAINTEXT$,64 ) 
 CALL TRANSPOSE A$, INITIALTR$ ,64 
 CALL TRANSPOSE KEY$, KEYTR1$ ,56 
 FOR I=1 TO 16 
 B$=LEFT$( A$,64 ) 
 A$=MID$(B$,33,32) 
 CALL F I,KEY$,A$,X$ 
 FOR J=1 TO 32 
 IF ASC(MID$(B$,J,1))+ASC(MID$(X$,J,1))=1 THEN A$=A$+CHR$(1) ELSE A$=A$+CHR$(0) 
 NEXT 
 NEXT 
 CALL TRANSPOSE A$,SWAP$,64 
 CALL TRANSPOSE A$,FINALTR$,64 
 DESencrypt$=ASCII$(A$) 
 END FUNCTION 
 
 FUNCTION DESdecrypt$( CTEXT$, KY$ ) 
 CRYPTEXT$=BINARY$(CTEXT$) 
 KEY$=BINARY$(KY$) 
 A$=LEFT$( CRYPTEXT$,64 ) 
 CALL TRANSPOSE A$, INITIALTR$ ,64 
 CALL TRANSPOSE KEY$, KEYTR1$ ,56 
 FOR I=1 TO 16 
 B$=LEFT$( A$,64 ) 
 A$=MID$(B$,33,32) 
 CALL F2 I,KEY$,A$,X$ 
 FOR J=1 TO 32 
 IF ASC(MID$(B$,J,1))+ASC(MID$(X$,J,1))=1 THEN A$=A$+CHR$(1) ELSE A$=A$+CHR$(0) 
 NEXT 
 NEXT 
 CALL TRANSPOSE A$,SWAP$,64 
 CALL TRANSPOSE A$,FINALTR$,64 
 DESdecrypt$=ASCII$(A$) 
 END FUNCTION 
 
 FUNCTION BINARY$( BMP$ ) 
 BITMAP$="" 
 FOR j=1 TO 8 
 L1=ASC(MID$(BMP$,j,1)) 
 FOR i=7 TO 0 STEP -1 
 IF ( 2^i AND L1 ) THEN BITMAP$=BITMAP$+CHR$(1) ELSE BITMAP$=BITMAP$+CHR$(0) 
 NEXT i 
 NEXT j 
 BINARY$=BITMAP$ 
 END FUNCTION 
 
 FUNCTION ASCII$( BMP$ ) 
 BITMAP$="" 
 C=0 
 N=7 
 FOR j=1 TO 64 
 C=C+ASC(MID$(BMP$,j,1))*2^N 
 N=N-1 
 IF ( j MOD 8 )=0 THEN 
 BITMAP$=BITMAP$+CHR$(C) 
 N=7 
 C=0 
 END IF 
 NEXT j 
 ASCII$=BITMAP$ 
 END FUNCTION 

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56 BITS DES KEY LENGTH

You may wonder why 56 bits used instead of 64 bits. This is because every byte has 7 data bits and 1 parity bit, so bit. Thus, 8 x 7 = 56 bits is the effective key size. Highest parity bit is generally ignored but if it is checked checked, it is then expected that DES keys must have odd parity.

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3DES: TRIPLE DES

The 56 bits DES algorithm, also known as single DES , has been cracked for few some years now. This can be done with specially designed parallel computers with high speed hardware DES crypto processors. processors .

Another method used is a network effort made with tens of thousands of Pentium PCs connected over the Internet. Using a A special screensaver which uses the idle time of these PCs to search a key space assigned to each PC. This way, every each PC is given a different key space from a master computer. After several days or months months, eventually one of the PCs finds the key and reports this to the master computer via Internet. This attack is possible if a known plaintext attack ((plaintext attack}} is possible. Otherwise this sort of brute force attack is difficult. However, most attractive target financial encrypted data such as enciphered PIN numbers have a known plaintext pattern and thus such an attack is possible. Due to this all all, major credit card systems are now have since changed to 3DES:TRIPLE DES.

TRIPLE DES: DES uses two 56 bit DES keys in what is known as EDE mode: mode, or Encrypt Decrypt Encrypt mode. So, effectively it is 112 bit encryption but encryption. Even so, EDE remains generally termed as 128 bit encryption. So, this is the so called strong 128 bit encryption, single encryption . Single DES run three times with two keys. Ofcourse, keys . Of course, you can also use 3 keys in EEE mode with three keys in EDE mode, but the classic 3DES is is 112 bit 3DES which encrypts with EDE and decrypts with DED using two 56 bit keys. keys . 3DES is very strong and we can say it is unbreakable until quantum computers can be built feasibly. become a reality. Otherwise, even with the fastest of fastest 'fastest of the fastest' computers in the world the world, the 112 bits key space can not feasibly be feasibly searched with using a brute force trial and error method until finding to find all the right bits. There are 2 to the power 112 different possible keys. keys . This is a big number.

How big ? big? Let's say if every bit can be represented by one electron electron, which is not possible but still we say if this is possible possible, and we were to store every possible than possibility, then you will have

• 5 192 296 858 534 827 628 530 496 329 220 096 electrons x 112 bits. bits
• 1 electron is 1/1837 weight of an AMU, atomic mass unit. unit
• 1 gram is equivalent to 1,675,000,000,000,000,000,000,000 AMU. AMU

According to this this, the weight of our data only in on our hard disk will be 188,996 kg ( 416,292 lb) or almost 200 metric tons. metrictons. Which is to say you will have pretty heavy metal on your desktop. Not a good idea to put all this data in your laptop. In fact according to current hard disk technology your hard disk harddisk will instantly turn into a black hole and suck up everything around if you try to store all 3DES keys possible.

So, don't mess around with 3DES without adult supervision.

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MAC : MESSAGE AUTHENTICATION CODE

One of the useful applications of DES or and 3DES is is MAC: Message Authentication Code which . MAC is used for to ensure INTEGRITY while DES/3DES is used for ensures CONFIDENTIALITY. This means, using MAC, MAC , you can create a digital signature to assure that no part of the message, not even one bit have been hasbeen changed in transit.

For example, if you are making a wire transfer from account number 1029121 to to 2771099 for $125.00 $125.00, you will not want someone messing around with data to divert the funds to account no number 2499911 instead of 2771099 or change instead. Neither, do you want the amount of transfer to $12500.00. changed from $125.00 to $12,500.00. Hence, between the two banks they exchange the MAC'ing keys to be used for transfers and transfers. Each transferred message is then they sign each transfer by dividing the transfer message divided into 8 byte blocks. These 8 byte blocks and are then successively encrypt each block with encrypted using either DES or 3DES, 3DES , XOR ing the next block to the encrypted value of previous block until the end of the message. Finally, you get a message is reached.

This procedure results in an 8 byte hexadecimal value. Usually you take the left half 4 bytes and call this the the MAC of the message. message . You append this MAC to the end of each message. Finally the receipient recipient gets the message and MAC after which the . The receiver then recomputes the MAC again using the agreed upon key and compares the transmitted MAC. MAC . If the two MACs MAC 's don't match this will mean somebody messed with match, then the data has been violated in transit. Ofcourse the

Of course this same technique can be applied to files stored on the disk, you disk. You can put a MAC at the end of each line of a critical contract and one MAC for the entire document. If the document MAC doesn't match to the expected MAC , then you can check every line with the Line MACs MAC s to see which line of the document was documentwas altered. Likewise, you can put MACs MAC s to every accounting record to detect an any internal fraud of by an employee messing with company accounts.

MAC is the backbone of is the backboneof many financial transactions like credit card systems. For example the latest EMV chip cards from Visa and Mastercard or Contactless cards like Mastercard PayPass or Visa Wave uses variants of MAC like CVV, ARQC, ARPC, CVV , ARQC , ARPC , etc to provide end to end message integrity of credit and debit card transactions. From the date, time, amount, currency code, card number and random elements ARQC Authorisation (Authorisation Request Cryptogram Cryptogram) is generated by the chip card in a POS or ATM during transaction which . During the transaction, the MAC is checked by the issuer issuing bank online and to determine if it is good this is a corresponding valid transaction. If found to be 'good', the ARPC Authorisation (Authorisation Response Cryptogram Cryptogram) is sent back to the terminal which submits this code to the card. The card and finally the card decide then decides whether this response is bona fide and accepts or rejects the completion succesfully or declination of the transaction. the transaction accordingly.

All of this happens these global events happen in just a few seconds around the globe and using a smartcard with a chip smaller than 1 millimeter square and lately square. More recently, these transactions are being made with contacless contactless cards wirelessly over the air. So, these things are Encryption is not science fiction but real life events happening behind the scenes things of everyday of such everday events like as checking out from the supermarket with your credit card or using your digital mobile phone.

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CONCLUSION

So you now have a DES algorithm written in Liberty Basic and a simple GUI application to play around. for experimentation. It is not a fast implementation which implementation. Such speed would require a DLL written in C. Maybe, because of Perhaps this it could be more interesting for some to see brings to mind a MATRIX style, Hollywood movie scene like those HEX numbers floating on the window, window; a cliche of our times when our hero or heroine sits in front of a display to watch these speeding numbers and bingo , bingo, gets into the deepest secret secrets of the next ICBM launch site, etc. launch.

Enjoy. It will get more interesting with public key cryptography on the next article.

CryptoMan

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Copyright (c) 2006, Verisoft
www.verisoft.com
onur@verisoft.com [[Cryptography with Liberty Basic 102: Classical Cryptography: DES by CryptoMan]]

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