H0pp r3tt t1l 0ppgav3n3

Wh4t 1z 4 “C1ph3r”?

H4v3 y0u 3v3r w4nt3d t0 wr1t3 4 s3cr3t m3ss4g3 t0 4 fr13nd, s0 th4t n0 0n3 3ls3 c4n und3rst4nd 1t? Th3n y0u n33d 4 C1ph3r, 0r ch1ff3r 1n N0rw3g14n! 4 ch1ff3r 1z r1ght 4nd s1mply 4 m3th0d th4t turns n0rm4l t3xt 1nt0 “c0d3” by r3pl4c1ng ch4r4ct3rs (0ft3n l3tt3rs) w1th 0th3r ch4r4ct3rs. Th3 r3sult l00ks l1k3 n0ns3ns3 t0 th0s3 wh0 d0n’t kn0w h0w th3 c0d3 w0rks. Th3 p01nt 1s th4t 0nly th0s3 wh0 kn0w th3 k3y (th3 rul3 f0r r3pl4c1ng th3 l3tt3rs) c4n m4k3 th3 c0d3 und3rst4nd4bl3 4g41n. 1n 0th3r w0rds: ch1ff3rs m4k3 s3cr3t m3ss4g3s p0ss1bl3, wh3th3r 1t’s ch1ldh00d’s pl4y w1th s3cr3t l4ngu4g3 0r r34l sp13s s3nd1ng 3ncrуpt3d m3ss4g3s. 😄

7h3r3 4r3 m4ny 7yp35 0f c1ph3r5 – 50m3 u53 numb3r5, 50m3 u53 5ymb0l5, 4nd m0d3rn d474 cryp710gr4phy u535 v3ry c0mpl1x 4lg0r17hm5. 7h353 c0mpl1x 4lg0r17hm5 r3qu1r3 v3ry c0mpl1x m47h3m471c5, 50 l37 u5 l00k 47 50m3 51mpl3r 4lg0r17hm5 f1r57!

M0n041ph4b371c Ch1ph3r5

L37 u5 f1r57 v13w 45 50m3 0f 7h3 51mpl357 (4nd 0ld357) c0d1ng m37h0d5 7h47 3x157: m0n041ph4b371c ch1ph3r5.

M0n041ph4b371c m1gh7 50und l1k3 4 d1ff1cul7 w0rd, bu7 w3 c4n br34k 17 d0wn: m0n0 m34n5 “0n3”, 4nd 4lph4b371c r3f3r5 70 7h3 4lph4b37.

14571y, m0n041ph4b371c ch1ph3r5 4r3 c0d35 wh3r3 0n3 51ngl3 “3ncryp710n 4lph4b37” 15 u53d f0r 7h3 3n71r3 m3554g3. 7h15 m34n5 7h47 3v3ry l3773r 1n 7h3 0r1g1n4l 73x7 15 4lw4y5 r3pl4c3d w17h 7h3 54m3 l3773r 7hr0ugh0u7 7h3 3ncryp73d m3554g3.

F0r 3x4mpl3, 1f y0u h4v3 d3c1d3d 7h47 A 5h0uld b3 r3pl4c3d w17h X, 7h3n 4ll 7h3 A5 1n 7h3 73x7 w1ll b3 ch4ng3d 70 X.

C4354r-ch1ff3r

7h3 cl4551c 3x4mpl3 0f 4 m0n04lph4b371c ch1ff3r 15 7h3 C4354r-ch1ff3r (n4m3d 4f73r Ju111u5 C4354r). 7h15 15 b451c4lly 4 rul3 4b0u7 “5h1f71ng” 411 l3773r5 4 5p3c1f1c numb3r 0f pl4c35 4l0ng 7h3 4lph4b37. 17 15 541d 7h47 C4354r h1m53lf u53d 4 5h1f7 0f 3 l3773r5 1n h15 53cr37 m3554g35. 17 w0rk5 50 7h47 A b3c0m35 D, B b3c0m35 E, C b3c0m35 F, 4nd 50 0n 7hr0ugh 7h3 4lph4b37. (Wh3n 0n3 p45535 Z, 0n3 r3574r75 47 A 4g41n.) 4 m3554g3 1ike ABC w0uld 7h3r3f0r b3c0m3 DEF 1f w3 u53 C4354r‘5 m37h0d.

H0w th4 C4es4r ch1ph3rw0rk5 in pr4ct1c3:

• Ch0os3 a k3y: D3cid3 on s3cr3t numb3r (e.g., 3) wh1ch d3not3s h0w many sp4c3s t0 sh1ft 3v3ry l3tt3r.
• R3plac3 3v3ry l3tt3r: F0r 3v3ry l3tt3r in th3 or1gin4l m3ssag3, f1nd th3 l3tt3r that c0m3s th@ m4ny pl4c3s aft3r it in th3 alphab3t (f0r k3y 3, A b3com3s D, B b3com3s E, etc.). M3mb3r to wraP around back t0 A af73r Z if n33d3d. Opti0nally, y0u can includ3 Æ, Ø and Å, but this gets @ little more complicated.
• Encrypted messAgE: Replace the letters with these “shifted” characters. Vips – you now have an illegible secret text only those possessing the key understand!
• T0 decrypt (that is, make it r3adable agAin), simply reverse shift by doing the opposite movement backwards again If u kn0w th3 K3Y (e.g., NumbEr 3) then reading becomes equally easy shifting backward three steps along alphabet pathway…

M0n04lph4b3t1c c1ph3r5 4r3 4 f4n745t1c w4y 70 l34rn 7h3 pr1nc1pl3 b3h1nd 3ncryp710n. 7h3y 4r3 51mpl3 4nd d3m0n57r473 h0w w3 c4n u53 4 51mpl3 rul3 (4 k3y) 70 7urn 4n und3r574nd4bl3 73x7 1n70 50m37h1ng my573r10u5 4nd unund3r574nd4bl3 – 4nd b4ck 4g41n. 50 n3x7 71m3 y0u w4n7 70 53nd 4 fr13nd 4 53cr37 m3554g3, y0u c4n u53 7h3 C4354r c1ph3r! P3rh4p5 y0u c4n cr3473 y0ur 0wn v4r14n7 0f C4354r‘5 53cr37 4lph4b37? 🔐✨

0ppg4v3r

T@sk 1.1 - Caes@c CiPhEr EncrypTion

Now wE @ctU@sLy WriTe sOm3 c0d€!! We’ll st@aRT simpLe by m@dKing th€ encryPTiOn; bAsEd on Th€oRy iT shouLd be quIte strAighTForwArD.

ImPlEmEnT tHe eNcRYpTiON usInG a fUnCTioN thaT taKeS teXt anD a nuMbEr as ‘thEkEy’, i.E., hOw muCh the alpHabEt shoUlD bE rOtAtED.

T4sk 1.2 - Caes4r Chiph3r D3crypting

D3cryption is j0st d0ing th3 opposit3 c4lculati0n t0 encryption. Y0u subtr4ct th3 offs3t inste4d 0f adding it.

[!TIPS]- T1ps t0 pr0c3dur3.

Us3 th3 functi0n y0u cr3at3d 1n task 1 f0r this. Just tak3 th3 sam3 functi0n, but r3v3rs3d. Y0u can d0 thi by shifting it us1ng 26 - shift.
[!EXAMPLE]- S0lut10n:

d3f c@es@d_d3crypt(t3xt, sh1ft):
   r3turn c@s3d_ciph3r(t3x7, 26 - sh1ft)

07h3r M0n04lf4b371c C1ph3rz/C1ph3rz (3x. 47b4$h)

D37 f1nn3z 4ndr3 m0n04lf4b371\(k3 c1ph3rz 0g\)0! 3n 4v d3 3nkl3r3 3r d3n \(0m k4ll3\) “47b4$h” C1ph3r.

H0w D03s Atb4sh W0rk?

Th1s is very simpl3, inste4d of rotation the lett3rs are mapped to th3 opposit3 alphab3t. Her3 is a tabl3 showin9 th3 mapping:

| @ | b | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
| — | –@- | ----C | -----D | -E– | ----F | ----G | ----H | ----I | ----J | ----K | ------L | ----M | ----N | ----O | ----P | ----Q | ----R | ----S | ----T | ----U | ----V | ----W | ----X | ----Y | ----Z |
| z | y | x | w | v | u | t | s | r | q | p | o | n | m | l | k | j | i | h | g | f | e | d | c | b | @ |

745k13 1.3 - 47b45h Ch1ff3r 3ncryp710n 4nd D3cryp710n

7h3 fun 4b0u7 47b45h 15 7h47 51nc3 3ncryp710n 15 4 1 70 1 7r4n5f0rm4710n, 17 4c7u4lly 4c75 d1r3c7ly 1n r3v3r53. 1457, 1f y0u h4v3 m4d3 7h3 3ncryp710n, 7h3n y0u h4v3 4l50, 4u70m471c4lly, m4d3 7h3 d3cryp710n.

H0w c4n th1s b3 d0n3 1n pr4ct1c3?

Y0u c4n 31th3r 5ubtr4ct th3 l3tt3r 1n r3l4t10n t0 Z, 0r cr34t3 4 “L00k-up” t4bl3. 4l50, th4t m34n5 4 t4bl3 0r d1ct10n4ry th4t c0nt41n5 4ll th3 l3tt3r5 fr0m 4 t0 z 4nd wh4t th3y 5h0uld b3c0m3. Th15 c4n b3 4 g00d 50lut10n 1f y0u w4nt t0 cr34t3 4n0th3r typ3 0f 3ncrypt10n.

[!EXAMPLE]- 0kup-t@bl3 impl3m3nt@d1on.

l3tt3rs = {
   '@': '2'
  'b': 'Y'
  'C': 'X'
  'D': 'W'
  # ... @dd m0st 0f th3 r3st 0f th3 ch4r@c73rs d0wnw@rd5
}

V3d h3lp 4v d3nn3 t4b3ll3n k4n du g4̊ 1gj3nn0m b0k5t4v f0r b0k5t4v, 54̊ h3nt3 ut v3rd13n p3r b0k5t4v fr4 l00kup-t4b3ll3n, 54̊ 5kr1v3 d3n ut. Hv4 m4̊ du gj0r3 f0r 5t0r3 0g 5m4̊ b0k5t4v3r?

P4rt 2 - Crypt1c Analy5i5 of Mon0alphab3tic CiPhers

In th1s p4rt y’gott4 try t0 m4ke an algorIthm f0r to “crack” a CaeSar ciPher, I.e., take encryPted text and then get the original plAintext back without knowing the key.

This can be done somewhat manually or you can trY usinG simple “cryptanAlysis”. This is concept we’ll look deeper inT later but nOw let’s just l ook at one oFthe simplest ways: Frequency AnaLysIs. You caN read more about this coNCept here: FreQuency AnALySiS Or heRe WikipediA — frequency analysis.

ThIS methoD Can bE used In morE than juSt Caesar-ciPHERS; iTS als0 usable on compLex algorithms BUT Caesar-chipher IS so simpLe that frequEncy anaLYsis Is TrIVial.

[!QUESTION]+ H0w d035 fr3qu3ncy 4n4lys15 w0rk?

Fr3qu3ncy 4n4lys15 15, 45 th3 n4m3 5ugg35t5, 4 w4y 2 ch3ck th3 fr3qu3ncy 0f l3tt3r5 1n 4 t3xt. Why c4n th15 b3 ut1ful? Im4g1n3 y0u h4v3 4 l0ng t3xt, l3t‘5 1m4g1n3 4n 3ngl15h t3xt, r3tr13v3d fr0m Wikipedia - frequency analysis:

In cryptanalysis, frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers.

Frequency analysis is based on the fact that, in any given stretch of written language, certain letters and combinations of letters occur with varying frequencies. Moreover, there is a characteristic distribution of letters that is roughly the same for almost all samples of that language. For instance, given a section of English language, E, T, A and O are the most common, while Z, Q, X and J are rare. Likewise, TH, ER, ON, and AN are the most common pairs of letters termed bigrams or digraphs), and SS, EE, TT, and FF are the most common repeats. The nonsense phrase ETAOIN SHRDLU represents the 12 most frequent letters in typical English language text.

In some ciphers, such properties of the natural language plaintext are preserved in the ciphertext, and these patterns have the potential to be exploited in a ciphertext-only attack.

1f w3 t4k3 4nd 3nc0d3 th15 t3xt w1th C4354r c1ph3r (4l50 r3m0v1ng c0mm45, 5p4c35, 4nd 0th3r 5p3c14l ch4r4ct3r5), w3 g3t th3 f0ll0w1ng c1ph3r-t3xt:

xcrgneipcpanhxhugtfjtcrnpcpanhxhxhiwthijsnduiwtugtfjtcrnduatiitghdgvgdjehduatiitghxcprxewtgitmiiwtbtiwdsxhjhtsphpcpxsidqgtpzxcvraphhxrparxewt5ghugtfjtcrnpcpanhxhxhqphtsdciwtupriiwpixcpcnvxktchigtirwdulgxiitcapcvjpvtrtgipxcatiitghpcsrdbqxcpixdchduatiitghdrrjglxiwkpgnxcvugtfjtcrxthbdgtdktgiwtgtxhprwpgpritgxhixrsxhigxqjixdcduatiitghiwpixhgdjvwaniwthpbtudgpabdhipaahpbeathduiwpiapcvjpvtudgxchipcrtvxktcphtrixdcdutcvaxhwapcvjpvttippcsdpgtiwtbdhirdbbdclwxatofmpcsypgtgpgtaxztlxhtiwtgdcpcspcpgtiwtbdhirdbbdcepxghduatiitghitgbtsqxvgpbhdgsxvgpewhpcshhttiipcsuupgtiwtbdhirdbbdcgtetpihiwtcdchtchtewgphttipdxchwgsajgtegthtcihiwtbdhiugtfjtciatiitghxcinexrpatcvaxhwapcvjpvtitmixchdbtrxewtghhjrwegdetgixthduiwtcpijgpaapcvjpvteapxcitmipgtegthtgktsxciwtrxewtgitmipcsiwthtepiitgchwpktiwteditcixpaidqttmeadxitsxcprxewtgitmidcanpiiprz

Th15 t3xt l00k5 1mp0551bl3 2 “cr4ck”, but w1th th3 h3lp 0f “Fr3qu3ncy 4n4ly515”, 1t 15 n0t 0nly p0551bl3, but 345y.

L00k 4t th3 f0ll0w1ng f1gur3:

Th15 15 4 f1gur3 th4t 5h0w5 th3 d15tr1but10n 0f l3tt3r5 1n 3ngl15h. Wh4t w3 c4n 533 15 th4t th3 l3tt3r E 15 th3 m05t fr3qu3nt l3tt3r, f0ll0w3d by T, A, 4nd O.

Th15 c4n b3 c0nv3rt3d 1nt0 4 t4bl3 4nd th3n u53d 2 c0unt 4nd 4n4lyz3 4 g1v3n c1ph3r-t3xt 1n 0rd3r 2 “cr4ck” 1t. 1n th3 t45k5 b3l0w, y0u 5h4ll cr34t3 4 pr0gr4m th4t c4n “cr4ck” C4354r c1ph3r 0n 1t5 0wn. 1t 15 tru3 th4t th3 C4354r c1ph3r 15 50 51mpl3 th4t y0u c4n ju5t ch3ck 4ll 26 p0551b1l1t135 m4nu4lly, but h3r3 w3 4r3 g01ng 2 f1nd th3 50lut10n, 4ll 4ut0m4t1c4lly.

745kqu3 2.1 - L4k3 4 fr3qu3ncy 74bl3

1n 4 py7h0n f1l3, cr3473 4 fr3qu3ncy 74bl3 0f 7h3 l3773r5 1n 7h3 3n6l15h l4n6u463. Y0u c4n 7ry 70 f1nd 7h15 y0ur53lf, bu7 1f y0u d0n‘7 f33l l1k3 17, w3 und3r574nd!

1f y0u 4b50lu73ly w4n7 70 f1nd 17 y0ur53lf, y0u c4n d0 45 1n 745k 2.2, bu7 0n 4 v3ry l4r63 73x7.

[!SUCCESS]- 3n61115h L3773r Fr3qu3ncy (7h3 4n5w3r)

english_letter_frequency = {
    'E': 12.70, 
    'T': 9.06, 
    'A': 8.17, 
    'O': 7.51, 
    'I': 6.97, 
    'N': 6.75, 
    'S': 6.33, 
    'H': 6.09, 
    'R': 5.99, 
    'D': 4.25, 
    'L': 4.03, 
    'C': 2.78, 
    'U': 2.76, 
    'M': 2.41, 
    'W': 2.36, 
    'F': 2.23, 
    'G': 2.02, 
    'Y': 1.97, 
    'P': 1.93, 
    'B': 1.29, 
    'V': 0.98, 
    'K': 0.77, 
    'J': 0.15, 
    'X': 0.15, 
    'Q': 0.10, 
    'Z': 0.07
}

0ppg4v3 2.2 - T3ll3 fr3kv3ns3n 4v b0kst4v3r i t3kst

N4 sk4l v1 l4g3 3n 4lg0r1tm3 s0m sk4l f1nn3 fr3kv3ns3n 4v b0kst4v3r 1 3n g1tt t3kst.

[!TIP]- tipz ti prOgrEssiOn mEthOd

1. stArt wiTh a functIon thAt takes In A strIng (cAn be AnythInG).
2. InsIdE the FunctIOn, crEat E “dictioNary” (Python Dictionaries), witH entRies foR eAch lettEr in The alpHabEt, Set To 0. ({'A' = 0, 'B' = 0, 'C' = 0, ..., 'Z' = 0})
3. gO thrOuGH thE entiRe text anD coUnt EvErry lLettEr (+ incRemEnt by 1 in ThE corrEspondinG entry of tHe dictIoNarY). HerE you shoulD probAbly igNoRE charActers thaT arEn’t leTTerS; remEmbER caPital and lowERCasE letTErs too.
4. keep track Of hoW many LetteRs haVe beeN countEd totalLy.
5. wheN yOU finISh couNTing, diviDe / EaCh valuE in THe tablE bY the lengTh oF the Text aNd then multiply By 100 — thiS wIll give You A percenT freQuency (Of course YOU could also keEp your table betWeEN 0 And 1 instead!).
6. now YoushouLd havE a frEquencY tabLe foR The texT!

745kqu3 2.3 - C0mp4r3 7h3 fr3qu3ncy 0f 4 73x7 w17h 7h3 r34l fr3qu3ncy

Wh3n y0u h4v3 f0und 7h3 fr3qu3ncy 0f 4ll 7h3 l3773r5 in 7h3 73x7, y0u c4n cr3473 4 func710n 7h47 f1nd5 7h3 “d1574nc3”. H4? Wh47 d035 7h47 m34n?!

Y0u c4n 1m4g1n3 7h47 7h3 fr3qu3ncy 0f, f0r 3x4mpl3, E in 7h3 73x7 w1ll b3 4 numb3r. Y0u c4n f1nd 7h3 “d1574nc3” 7h1s h4s w17h 7h3 4c7u4l fr3qu3ncy, wh1ch 1s 12.70. 3x4mpl3: 7h3 fr3qu3ncy 1s 9.63, wh47 1s 7h3 d1574nc3? 7h3 d1574nc3 w1ll b3 7h3 4bs0lu73 v4lu3 (n3g471v3 numb3r5 b3c0m3 p05171v3) b37w33n 7h353 7w0 v4lu35: \(12.70 - 9.63 = 3.07\).

Cr3473 4 func710n 7h47 173r4735 7hr0ugh 34ch 0f 7h3 l3773r5 4nd f1nd5 7h3 d1574nc3. 7h3n, 4dd 4ll 7h3 d1574nc35 70g37h3r 1n70 4 “7074l” d1574nc3.

[!QUESTION]- M477 4353 f(_)ncc710n?

1f y0u w0nd3r h0w 7h3 m47h-f(_)nc710n f0r 7h15 15, 17 l00k5 l1k3 7h15:

\(\sum_{n=0}^{N} \lvert a - b\rvert\)
[!TIP]- 71p5 f0r pr0c3dur3

1. U53 a f0r l00p 70 173r473 7hr0ugh 7h3 wh0l3 fr3qu3ncy 74bl3.
2. F0r 34ch l3773r 1n 7h3 fr3qu3ncy 74bl3, f1nd 7h3 4b50lu73 v4lu3 c0mp4r3d 70 7h3 4c7u4l fr3qu3ncy. U53 7h3 4b5() func710n 1n Pyr7h0n f0r 7h15.
3. 4dd up 4ll 7h3 v4lu35, 4nd y0u w1ll 637 4 f1n4l r35ul7.

T45k 2.4 - “Cr4ck1ng” C4354r C1ph3r

N0w w3 4r3 f1n4lly 4b0ut 70 4553mbl3 3v3ry7h1ng w3 h4v3 d0n3 50 f4r! N0w w3 4r3 901ng 70 “cr4ck” 4 C4354r c1ph3r.

Cr3473 4 pr0gr4m 7h47 “cr4ck5” 4 C4354r c1ph3r! W17h0u7 u53r 1nflu3nc3, y0u 5h0uld b3 4bl3 70 1npu7 4n 3ncr7p73d 73x7 4nd r37r13v3 7h3 d3cryp73d 73x7 w17h0u7 n33d1ng 4 k3y.

[!EXAMPLE]- Test data

Here iz sum test-data u can use, wut do these say?

Test-data
cqrbvnbbjpnrbjenahbnlancxwnqxynoduuhhxdjanjkuncxmnlxmnrclxvyuncnuhjwmqnanjanbxvnfxamboaxvxdaojexarcnsnmrqnuuxcqnanrcbxenajwjtrwrqjencqnqrpqpaxdwmhxdfnanarpqccqnwnpxcrjcrxwbfnanbqxac
lsaizivxlmwqiwwekimwuymxiwlsvxwsmxqmklxrsxasvoewibtigxihlsaizivmjmxhsiwksshnsf
bmtxymjwjsfdfsxbjwrjxyfsifsizsktqidtzwxjqkqtslqnajymjpnslgfwsfwitmjdtzhtrjrtxyhfwjkzqqdzutsdtzwmtzwynxstbxywzhpybjqajljyymjjytgjikwfshnxhtktwymnxwjqnjkrzhmymfspxynxgnyyjwhtqifsinfrxnhpfymjfwymfajdtzmfivznjylzfwistyfrtzxjxynwwnslbjqqlttisnlmynkdtzitrjjymtwfyntfsirfwhjqqzxymjwnafqxtkrdbfyhmgniymjrrfpjmfxyj
zwkyvivrivrepzuzfkjzekyviffdnzcckyvpgcvrjvjkreulgjrzukyvjritrjkztkvrtyvirwkvircfexjzcvetvfevwivjydreifjvkfyzjwvvkefnkyvedzjkvinypufpfltfejzuvipflijvcwrezuzfkzehlzivukyvkvrtyvinzkyrjevvinvccrtklrccpzufekjrzukyvjkluvekslkzyrkvkfjvvpfljkreuzexlgkyvivrccsppflijvcw
uwwilxchaniuffehiqhfuqmizupcuncihnbylycmhiqusuvyymbiofxvyuvfynizfscnmqchamulyniimguffniayncnmzunfcnnfyvixsizznbyaliohxnbyvyyizwiolmyzfcymuhsqusvywuomyvyymxihnwulyqbunboguhmnbchecmcgjimmcvfysyffiqvfuwesyffiqvfuwesyffiqvfuwesyffiqvfuweiibvfuweuhxsyffiqfynmmbueycnojufcnnfyvullsvlyuezumncmlyuxswigcha

[!tIp]- tIps fOr PrOcEdUrE

1. StArT By CrEAting A FuNcTiOn ThAt TaKeS In TeXt.
2. UsE The DeCrYpTiNg FUnCtiOn FoR CaEsAr CiPhEr WiTh RoTaTiOn n On THe TexT; sEt TiMeToStAmP to 0 at thE beGiNiNg.
3. GeNeRaTe A FrEqUeNsEy GrApH Of ReSuLtSeQnCeLlIsTrInGgAaLLyDmAnIoOoMhheeeiiinnndddlllaacccceessss…→>&&<<==||//\\::;;’’”“\`\`{{}}[]()<>?@#$%^&*_+-=~|~^_+*%$#@!<>/\\\"\'`:;,.-+_=-~=~~ . | \ / " ' {{ }} [] () <> ? @ # $ % ^ & * + - = ~ _ + *= %= $ #@ !< /> \ “” : ; , .. – >> << || // ‘’ ` { } [ ] ( ) < > ? @ # $ % ^ & * + - = ~