Question

Decrypting the APCO cipher without the key.

Decryption without the key is obviously a much more difficult process. Indeed, the purpose of encryption is to make it as difficult as possible for anyone who does not know the key to read the plain text. We will be using an unsophisticated password cracking technique called a brute force attack. A brute force attack on the APCO cipher works by trying every possible four digit key (from 0000 to 9999) and keeping the one that gives the best result. How do we know what the “best result” is? The simplest way to test the quality of the decryption key is to count the spaces in the resulting plain text. A space character is the most commonly occurring character in normal text. The key that gives the most spaces in the plain text will almost certainly be the correct key.

Begin by copying the cipher text into your program as asciiz in the data section. (

Lx!ydw!vki!dhwu\"rj!vlqfu/$jv#{bu#xig#{ptvx!qi$ukpit.#mu\"zet\"wlf\"dkf\"rj!ylweqp0!kw$xcv$ujh$bih$ph#jpqomtjqitu/$jv#{bu#xig#iqqfl!qi$cgomfh/$jv#{bu#xig#iqqfl!qi$jpfvffxpjv|0!kw$xcv$ujh$tgdwpp#sg\"omhjw0!kw$xcv$ujh$tgdwpp#sg\"gesmqitu/$jv#{bu#xig#wqtlrh\"rj!jrtf.#mu\"zet\"wlf\"zmovhv!qi$egvtbku0!yh$icg$fxhvzvkmoi#ffhrvf\"xw-\"zi!jdh!prxikqk!dhjpth$vu/$xg#{fth$bno$hqlrh\"gmsgfx!vr$igdzfp/$xg#{fth$bno$hqlrh\"gmsgfx!vki!qwlft#{b{1$Jp#wiqux-\"wlf\"siskrh!ydw!ur$gcu$mkni!vki!ruitgqx!rhvjqg0!vkeu\"vsng#sg\"lxt\"qsjulitv#evvksskwmfu#moulwugg$pp#muu#ffkqk!thgfkyie.#jpt#kpqg$pt#jpt#iwko0!kq$ujh$twsisndxjxh$egjvfg#sg\"fsnrdvjurr!qqpz0

)

Next, write a procedure that counts the number of characters in the cipher text. This information will be useful in the other procedures. Verify this procedure works by testing it on a short string of known length.

Then, write a procedure countSpaces that takes two arguments: the number of characters and the address of the plain text and returns the number of space characters in the given plain text. Verify this procedure works by testing it on a short string with a known number of spaces.

Next, write a procedure decrypt that takes four arguments: the number of characters, the key, the address of the cipher text, and the address of the plain text. This procedure decrypts the cipher text using the given key and writes the result to the plain text.

You can test this procedure on the following short cipher text string: ( to check if its working)

b\"eg!fhj!ikmk\"npnpr$qsuwuw#zxz|~

This cipher text should provide a best key of 1234 and a decrypted text of:

a bc def ghij klmno pqrstu vwxyz (answer)

Place calls to the two above procedures in a loop that steps through all of the possible keys and saves the best one. Once the best key has been found, print it. Run the decrypt function one last time using the best key, then print the resulting plain text.

You can assume that there will never be more than 1000 characters in the cipher text. Note: This program runs very slowly on the long cipher text.

Mips programming ?????

See the following pseudo code:

int numChar = countChar(cipherText)

for (key = 0 to 9999)

decrypt(numChar, key, cipherText, plainText)

int numSpaces = countSpaces(numChar, plainText)

if numSpaces > bestSpaces

bestSpaces = numSpaces

bestKey = key

end for

decrypt(numChar, bestKey, cipherText, plainText)

print bestKey

print plainText

end

Note: the ASCII value for a space is 32. Decrypting the APCO cipher. If the user has the key, then decryption is a simple task. You just reverse the procedure above. Instead of adding the key to the plain text, you subtract it from the cipher text Cipher Text L Ascii k t t p V Z y W 76 116 125 39 104 116 107 122 36 121 110 112 119 37 125 118 Key Subtract 6 4 5 7 5 6 7 5 6 7 4 6 7 72 111 119 32 100 111 101 115 32 116 104 105 115 32 119 111 Plain Text d H O e t hi W S W O

i was doing it on Mars mips but i am getting lots  of error and the submission date is close i am stuck on how do i use it to decrypt long test and get the best possible key

1. Correctly decrypt the long cipher text and print the key

2. Properly use procedures.

Answer 1

Hi Student ,

I will be writing the entire code in Python and let me know if you have any concern , Please repost your query separately

# To get the count of Cipher Text , means total number Of Chars in Cipher Text

def countCipherText(cipherText):

NumberOfCharacterInCipher=0

for e in cipherText:

NumberOfCharacterInCipher+=1  

return NumberOfCharacterInCipher

# To get the count Of Spaces In Plain Text

def countSpaceInPlainText(NumOfCharacterInCipher,AddrOfThePlainText):

return AddrOfThePlainText.count(' ')

  

#To decrypt the string based on the Number Of Character , Based on the Input key And the cipher text

def decryptString(NumOfChar,key,addOfCipherText,addOfPlainText):

result = ''

for l in addOfCipherText:

try:

i = (key.index(l) - NumOfChar) % 26

result += key[i]

except ValueError:

result += l

return result

AddressOfThePlainText='Abcdse nmnmsdnmd salklds slklklds'

cipherText='Lx!ydw!vki!dhwu\"rj!vlqfu/$jv#{bu#xig#{ptvx!qi$ukpit.#mu\"zet\"wlf\"dkf\"rj!ylweqp0!kw$xcv$ujh$bih$ph#jpqomtjqitu/$jv#{bu#xig#iqqfl!qi$cgomfh/$jv#{bu#xig#iqqfl!qi$jpfvffxpjv|0!kw$xcv$ujh$tgdwpp#'

key = 'abcdefghijklmnopqrstuvwxyz'

print "Best Key ",key

print "Decrypted String ",decryptString(countCipherText(cipherText), key, cipherText, AddressOfThePlainText)