Vigenere.py

def find_letter(arg):
    if (arg==0):
        return 'A'
    if (arg==1):
        return 'B'
    if (arg==2):
        return 'C'
    if (arg==3):
        return 'D'
    if (arg==4):
        return 'E'
    if (arg==5):
        return 'F'
    if (arg==6):
        return 'G'
    if (arg==7):
        return 'H'
    if (arg==8):
        return 'I'
    if (arg==9):
        return 'J'
    if (arg==10):
        return 'K'
    if (arg==11):
        return 'L'
    if (arg==12):
        return 'M'
    if (arg==13):
        return 'N'
    if (arg==14):
        return 'O'
    if (arg==15):
        return 'P'
    if (arg==16):
        return 'Q'
    if (arg==17):
        return 'R'
    if (arg==18):
        return 'S'
    if (arg==19):
        return 'T'
    if (arg==20):
        return 'U'
    if (arg==21):
        return 'V'
    if (arg==22):
        return 'W'
    if (arg==23):
        return 'X'
    if (arg==24):
        return 'Y'
    if (arg==25):
        return 'Z'

def find_number(arg):
    if (arg=='A'):
        return 0
    if (arg=='B'):
        return 1
    if (arg=='C'):
        return 2
    if (arg=='D'):
        return 3
    if (arg=='E'):
        return 4
    if (arg=='F'):
        return 5
    if (arg=='G'):
        return 6
    if (arg=='H'):
        return 7
    if (arg=='I'):
        return 8
    if (arg=='J'):
        return 9
    if (arg=='K'):
        return 10
    if (arg=='L'):
        return 11
    if (arg=='M'):
        return 12
    if (arg=='N'):
        return 13
    if (arg=='O'):
        return 14
    if (arg=='P'):
        return 15
    if (arg=='Q'):
        return 16
    if (arg=='R'):
        return 17
    if (arg=='S'):
        return 18
    if (arg=='T'):
        return 19
    if (arg=='U'):
        return 20
    if (arg=='V'):
        return 21
    if (arg=='W'):
        return 22
    if (arg=='X'):
        return 23
    if (arg=='Y'):
        return 24
    if (arg=='Z'):
        return 25
    
def given_frequency(arg):
    if (arg==0):
        return 0.082
    if (arg==1):
        return 0.015
    if (arg==2):
        return 0.028
    if (arg==3):
        return 0.043
    if (arg==4):
        return 0.13
    if (arg==5):
        return 0.022
    if (arg==6):
        return 0.02
    if (arg==7):
        return 0.061
    if (arg==8):
        return 0.07
    if (arg==9):
        return 0.0015
    if (arg==10):
        return 0.0077
    if (arg==11):
        return 0.04
    if (arg==12):
        return 0.024
    if (arg==13):
        return 0.067
    if (arg==14):
        return 0.075
    if (arg==15):
        return 0.019
    if (arg==16):
        return 0.0095
    if (arg==17):
        return 0.06
    if (arg==18):
        return 0.063
    if (arg==19):
        return 0.091
    if (arg==20):
        return 0.028
    if (arg==21):
        return 0.0098
    if (arg==22):
        return 0.024
    if (arg==23):
        return 0.0015
    if (arg==24):
        return 0.02
    if (arg==25):
        return 0.0074

    

encrypted='MYHSIFPFGIMUCEXIPRKHFFQPRVAGIDDVKVRXECSKAPFGHMESJWUSSEHNEZIXFFLPQDVTCEUGTEEMFRQXWYCLPPAMBSKSTTPGSMIDNSESZJBDWJWSPQYINUVRFXPVPCEOZQRBNLUIINSRPXLEEHKSTTPGCEIMCSKVVVTJQRBSIUCKJOIIXXOVHYEFLINOEXFDPZJVFKTETVFXTTVJVRTBXRVGJRAIFPSRGTDXYSIWYXWVFPAQSSEHNEZIXFVRXQPRURVWBXWVCEIMCSKVVVUCXYWJAAGPUHYIDTMJFFSYUSISMIDNSESRRPILVUFSPTEIHYMEGMTVRRPREEDISHXHVTFVQKIIMFRQILVKRCAUPZTVGMCFVTIIQPRUPVEGIMWICFGIAVVRZQASJHKLQLEPUIIQSLRGGSUHSESUQQCWJCLPEWEJPRVDXGRRVHFWINCIPPLMKVYEFTLRGXSAHIJHVTBTHLGZRFDQZGVVKPRUPCSASWYSUAQWEMSUIHTPFDVHEEIVRSYITLRJVWTJXFIIWQAZVGZRYPGYWEIDNXYOKKUKIJOSYZSEEQVLMHPVTKYEXRNOEXAJVBBFAXTHXSYEEBEUSLWONRZQRPAJVTZVZQGRVGJLMGHRBUYZZMERNIFWMEYKSABYTVRRPUIVZKSAAMKHCIYDVVHYEZBETVZRQGCNSEIQSLLARRUICDCIIFWEEQCIHTVESJWITRVSUOUCHESJWMCHXSEXXTRVGJAUILFIKXTTWVELEXXXZSJPUUINWCPNTZZCCIZIEERRPXLMCZSIXDWKHYIMTVFDCEZTEERKLQGEUWFLMKISFFYSWXLGTPAHIIHFKQILVFKLQKIIMEEFJVVCWXTTWVWEZQCXZCEWOGMVGFYFUSIHYISDSUBVWEXRDSEGDXIJCLXRDVLBZZQGWRZSVAILVFYSASJFFKLQJRZHPSRJWRZCIHTRECNQKKSZQVMEGIRQYMZVQZZCMACWKVISGVLFIKXTTAFFCHYXPCWFREDJUSJTMXVZBXQQCAFAVRMCHCWKXXTGYWCHDTRMWTXUBWFTRWKHXVAKLMIQRYVWYTRKCIXGGIRBUMYEVZGFRUCRFQVRFEIFDCIFDXYCJIIWSTOELQPVDSZWMNHFBFXPTWGOZVFWIDWJIDNXYOKMECSNIGSZJWZGSYFILVDRWEXRXCWKDTIUHYINXXKSIRQHWFTDIZLLFTVEDILVKRCAULLARRBGSXFVWEILVVRXQDJDSEAUAPGOJWMCHUWTXMISIGUMQPRUHYIBDAVFKLQNXFCBJDDQKVVTQDTCSNMXAVVHLVZISKVVTQDTCSRRPHSCCEKMHQVBUMQAMSSIXKLMCZEIHTVGSIMEWWFZUMQGWUCEXSXZVMFYDHICJVWFDFIIKIEBIEKYSPTWGWJIKDYVBJPMKIPCLATDVVUZQQCXPCLVXXZVGKIXACFINLMIXFRFATPXKCKLUCORBUATPXKCWIQAAYCUVUAPPCLHUTXPCLXDTEKMFYXXOVQRXFAILGVCAJEJQRRZDRWCUHQGHFBKKUKIPCLVETPMSJXAILVGVYZCEKIIEXBIEARGTXRVAVRIXXYARGTXRVAZRPHEERDEOWMESYIMGXJMFYMGIECKQMRLZBVWKDYRFVRAIGRHKPQNSLOIIYTRPCLLMKIKVVPAKIFTYYYPRZHPMZNSLFYIMGXJMFYPDRKVRXQDRCMKLQJRCCMIPWEKSKLQJRCCMIPPRUHYIGCRRHLVMAWFZUMQGWUCEXRXKYHWSDHPRJVVKUMXVKJAGPZPVVFNMEHYIETZVBKLOWEGHVVAUWKZLOQXXZGNVUIXVBKLQZMEUUSYDJXCUMELMKVZRYPRECKSZTQRBESDPKICLTAUQVBSYFXRRZCQQCMEMFYKDYKVVTQDTCSYEHTXYSGSITVKVVTALIIHFGDTEKSDEOWMESJXTTTFKVVFDGISRXQWEGDZRQHWPCLXTTTVCGPQWEMSKLQESNSIXABEBSKLUHPZTVJDTIRBUFQPYKWWYXISDOBIFWMJZZJQPAFBUIDUYCOUZQCXLFVXTTRZBKLQCEDSFJPTQFQIEONPVHLWGHIKVRXBDAVFCIFJWRZCYZXXVZVXGHJZUYXRDVRBVAIDVCRRHQRIEHNSDAHKVRXIXPCUZZQBIEOTLMCGVHFAAGOKVRXIXPCUZZQNSLHYERJXLFVEZSSCRRKQPWVQLVUICSMKLQEVFAZWQDJKVVWQILZBXWNGYKSJLMKIIWJIZISGCNIDQYKHYIKAMVHYIKSSECKJGAJZZKLMITICDMETXYSPRQKIIKZPXSMTHRXAGWWFVIFWIDGVPHTWSIKXTTCVBJPMKIKVVTQDTCSESIAIKIJJUVLKHFJGAJZZKLMITICDMETPVHLWRXKYHKSRGIVHYIIDVCRKSPDENOPAUILEOKMACECPRVDXIIGKSPDENOPAUILXFVIPLMKVYEFTEERZRFDPVFRROTPVHLWRXKYHWSDPAFFCHAUVVOJSZPAFFCHIWIISJGUTRTSRRPEVFUIIEHAZZCPQPHKCRPXBIEGYEBEMESJWEDPUWVVEXRKVVRMBIFTUIYDGIOTCXTXLGRPXJRZHV'
cipher=[] 
cipher[:0]=encrypted

cipher_copy=cipher
coincidences=[0 for i in range(len(cipher))]

#STEP 1: FIND THE KEY LENGTH

# how many times we'll count coincidences
for i in range(1,len(cipher)):  
    for j in range(len(cipher)):
        if (j+i<len(cipher)):
             if (cipher[j+i]==cipher_copy[j]):
                 coincidences[i]+=1
#print(coincidences) #coincidences[0] is always 0
#as we see we numbers,we can clearly tell that the "big" number are 150<
#how often big numbers occur
last_seen=-1
possible_key_length=[]
for i in range(1,len(coincidences)):
    if coincidences[i]>150:     #150 cause we see that this is the smallest "big" number                                     ----CHANGE
        if (last_seen>0):
            possible_key_length.append(i-last_seen)
        last_seen=i
#print(possible_key_length)   #we see that most times we see the number 7. So we will move on considering 7 is the key length
#we print how often we see the"big" numbers. They occur every seven spots. So 7 is the potention key length.                   
# _ _ _ _ _ _ _
 

frequencies=[0.0 for i in range(26)] #26 letters
key=[0 for i in range(7)] # 7 is the potential key length


for m in range(7): # 7 times so we can find the 7 numbers of the key                                                     ----CHANGE
    max_=-1
    count=0;
    for i in range(m,len(cipher),7):  #how many times we see each letter every 6 letter 
        frequencies[find_number(cipher[i])]+=1 # starting from the letter at the jth place
        count+=1
    for i in range(26):
        frequencies[i]=frequencies[i]/count
    
    for l in range(26): 
        j=0
        mult=0
        for o in range(26):
            if (o+l>=26): 
                mult+=frequencies[j]*given_frequency(o)
                j+=1
            else:
                mult+=frequencies[o+l]*given_frequency(o)

        if (mult>max_):
             max_=mult
             key[m]=l         #finding how many shifts we need



cipher_numbers=[find_number(i) for i in encrypted]
keystream=[]

str1=[]
#making the keystream
j=0
for i in range(len(cipher_numbers)):
    if j==len(key):
        j=0
    keystream.append(key[j])
    j+=1
#print(keystream)

decrypted=[]
for i in range(len(cipher_numbers)):
    decrypted.append(cipher_numbers[i]-keystream[i])
    
    if (decrypted[i]<0 ):
        decrypted[i]=26+(decrypted[i])
    elif (decrypted[i]>25):
        decrypted[i]=(cipher_numbers[i]-keystream[i])-26
    str1.append(decrypted[i]) 



decrypted_=[find_letter(i) for i in decrypted]
print(decrypted_)