Table of Contents for

Cracking Codes with Python

INDEX

Symbols

+ (addition and concatenation operator), 12, 23–24, 89–90

+= (augmented addition assignment operator), 91–92

/= (augmented division assignment operator), 92

*= (augmented multiplication assignment operator), 92

-= (augmented subtraction assignment operator), 92

\ (backslash), 28–29

: (colon), 26

/ (division operator), 12

" (double quote), 29–30

__ (double underscore), 95

== (equal to comparison operator), 43–45

** (exponent operator), 201

> (greater than comparison operator), 43–44

>= (greater than or equal to comparison operator), 43–44

# (hash mark), 34

// (integer division operator), 181

< (less than comparison operator), 43–44

<= (less than or equal to comparison operator), 43–44

% (modulo operator), 173

* (multiplication and replication operator), 12, 24, 89–90

\n (newline escape character), 28–29

!= (not equal to comparison operator), 43–45

() (parentheses), 14, 28, 35

' (single quote), 22, 28–30

[] (square brackets), 26

%s (string formatting), 75

- (subtraction operator), 12

\t (tab escape character), 28–29

''' or """ (triple quotes), 164–165

A

absolute path, 131

addition operator (+), 12

Adleman, Leonard, 337

affine cipher, 177–181

decrypting with, 179–180, 194–195

encrypting with, 179, 193–194

hacking, 201–204

affineCipher.py, 186–187

affineHacker.py, 198–199

affineKeyTest.py, 192

al_sweigart_privkey.txt, 342, 343

al_sweigart_pubkey.txt, 342, 343, 346, 363

and operator, 106, 108

append() list method, 121, 155–156, 253–254

arguments, 28

default, 157

defining functions with parameters, 83–84

for file permissions, 131, 132

keyword

end, for print(), 306

key, for sort(), 273

repeat, for itertools.product(), 307

reverse, for sort(), 273, 275

lists as, 121–122, 301

optional, 131

passing references as, 121, 234

assignment statements, 16

augmented assignment operators, 91–92

authentication, 337–338

B

Babbage, Charles, 247, 282

backslash (\), 28–29

Bellaso, Giovan Battista, 247

Bernstein, Daniel J., xxi–xxii

blocks

of code, 45–46

in cryptography, 350

converting block to string, 354–355

converting string to block, 350–352

Boolean operators, 106–108

Boolean values, 43

breakpoints, for debugging, 379–380

break statement, 311–312

brute-force attack, 69

built-in functions, 56

C

Caesar cipher, 4

decrypting with, 7

encrypting with, 7

hacking, 69

caesarCipher.py, 54–55

caesarHacker.py, 70–71

candidates (possible cipherwords), 223

case (of letters)

converting with string methods, 134, 216–217

sensitivity, in Python, 18

ceil() function (math module), 103–104

chiffre indéchiffrable, 247

cipher disk, 4

cipherletters, 222

cipherletter mappings, 224, 232–241

adding letters to, 233–234

intersecting, 234–235

ciphers, vs. codes, 3–4

ciphertext, 4

cipher wheel, 4

decrypting with, 6–7

encrypting with, 5–6

cipherwords, 222

clock arithmetic, 172–173

close() file object method, 132

code breaker, 2

codes, vs. ciphers, 3–4

colon (:), 26

comments, 34

comparison operators, 43–45

composite numbers, 323, 326–327

concatenation operator (+), 23–24, 89–90

condition, 42–43

confidentiality, 338

constants, 57, 313

continue statement, 202–204

copy.deepcopy() function, 122

copy() function (pyperclip module), 56, 65

cryptanalyst, 2

cryptographers, 2

Adleman, Leonard, 337

Bellaso, Giovan Battista, 247

Kerckhoffs, Auguste, 70

Rivest, Ron, 337

Schneier, Bruce, 337

Shamir, Adi, 337

Turing, Alan, 188, 268, 352

Vigenère, Blaise de, 247

cryptography, 2

cryptomath.py, 182

Cuisenaire rods, 174–175

D

data types, 13

Boolean, 43

dictionary, 146–150

floating-point, 13

integer, 13

list, 86–90

string, 22

tuple, 190

debugging

in IDLE, 375–379

setting breakpoints, 379–380

decoding, 3

decrypting, 2

with the affine cipher, 179–180, 194–195

with the Caesar cipher, 7

with a cipher wheel, 6–7

with the public key cipher, 368

with the reverse cipher, 39

with the simple substitution cipher, 216

with the transposition cipher, 100–101

with the Vigenère cipher, 248–249

deepcopy() function (copy module), 122

def statement, 82–84

detectEnglish.py, 143–144

dictionaries, 146–150

adding or changing items in, 147

in operator and, 148

len() function and, 148

lists, differences from, 147

dictionary attacks, 250–251, 280

dictionary.txt, 280

diff tool, checking source code with, 31–32

digital signatures, 338

division operator (/), 12

integer (//), 181

divisors. See factors

double encryption, 8

double quote ("), 29–30

dunder (__, double underscore), 95

dunder name dunder (__name__) variable, 95–96

duplicating lists, 122

E

elif statement, 61

else statement, 60

encoding, 3

encrypting

with the affine cipher, 179, 193–194

with the Caesar cipher, 7

with a cipher wheel, 5–6

with the public key cipher, 367–368

with the reverse cipher, 39

with the simple substitution cipher, 215

with the tranposition cipher, 78–80

with the Vigenère cipher, 248–249

encryption key, 4

end keyword argument, for print(), 306

endswith() string method, 135

English, detecting programmatically, 141–143, 156–159

equal to operator (==), 43–45

Eratosthenes, sieve of, 328–331

errors, 15. See also debugging

finding with diff tool, 31–32

ImportError, 55

IndexError, 25

ModuleNotFoundError, 226

NameError, 33

SyntaxError, 15, 28, 29

escape characters, 28–30

ETAOIN, 260

Euclid’s algorithm, 176–177

extended, 181

execution, program

defined, 34

terminating with sys.exit(), 124

exists() function (os.path module), 133

exit() function (sys module), 124

exiting programs, 35–36, 124

exponent operator (**), 201

expressions, 12–15

defined, 13

evaluating, 14–15

extend() list method, 301

F

factors

of composite numbers, 323

greatest common divisors, 173–175, 176–177

of prime numbers, 322, 357

of spacings, in Kasiski examination, 283–284, 297

False (Boolean value), 43

file editor (IDLE), 30

file objects

close() file object method, 132

open() function, 131

read() file object method, 132

write() file object method, 131

files, plain text, 128, 130–134

find() string method, 62–63

float, 13

float() function, 154

floating-point numbers, 13

floor() function (math module), 103–104

for statement, 58–59

freqAnalysis.py, 265–266

frequency

defined, 260

of English letters, 261

frequency analysis, 259, 285–287

frequency match score, 262–264, 286

functions, 28

calling from modules, 65

passing as values, 272–273

G

Gadsby (Wright), 266

gcd() function, 176–177

GCD (greatest common divisor), 173–175, 176–177. See also factors

generating keys

affine cipher, 195–196

public key cipher, 340

simple substitution cipher, 218

global scope, 84

googol, 322

greater than operator (>), 43–44

greater than or equal to (>=) operator, 43–44

greatest common divisor (GCD), 173–175, 176–177. See also factors

H

hacker, 2

hacking

affine cipher, 201–204

Caesar cipher, 69

public key cipher, why we can’t, 355–357

simple substitution cipher, 222–225, 241–245

transposition cipher, 166–168

Vigenère cipher, 280–282

hash mark (#), 34

hello.py, 31

hybrid cryptosystems, 347

I

IDLE, xxvii, 12, 30

debugging with, 375–379

opening programs, 34

running programs, 33

saving programs, 32

if statement, 59–60

ImportError, 55

import statement, 55, 56

IndexError, 25

indexing, 24–27

infinite loop, 195

in operator, 61–62

in place modification, of lists, 122

input() function, 35, 50

insert() list method, 367

installing Python, xxv–xxvi

integer division operator (//), 181

integer (int) data type, 13

interactive shell, xxvii, 12

intersected mapping, 225, 234–235

int() function, 154

islower() string method, 216–217

isupper() string method, 216–217

itertools module, 307–308

J

join() string method, 93–94, 253–254

K

Kasiski examination, 282–284

Kasiski, Friedrich, 282

Kerckhoffs, Auguste, 70

Kerckhoffs’s principle, 70

key, encryption, 4

key keyword argument, for sort(), 273

L

len() function, 41–42

using with dictionaries, 148

using with lists, 89

less than operator (<), 43–44

less than or equal to operator (<=), 43–44

list-append-join process, 253–254

list() function, 123, 213, 253–254, 274–275

lists, 86–87

append() method, 121, 155–156, 253–254

as arguments, 121–122, 301

concatenating, 89–90

dictionaries, differences from, 147, 149

duplicating, 122

extend() method, 301

in operator, 89

insert() method, 367

len() method, 89

in place modification, 122

references, 119–121

replicating, 89–90

sort() method, 212–213, 271–275

local scope, 84

loops

for, 58–59

with the range() function, 72–73

while, 42–43

Lovelace, Ada, 163

lower() string method, 134

M

machine-in-the-middle (MITM) attack, 339

'__main__' string value, 95

main() function

in affineCipher.py, 188–189, 196

in affineHacker.py, 200, 204

in makePublicPrivateKeys.py, 343

and __name__ variable, 95

in publicKeyCipher.py, 373

in simpleSubCipher.py, 211, 219

in simpleSubHacker.py, 231, 245

in transpositionDecrypt.py, 102–103, 110

in transpositionEncrypt.py, 85, 95

in transpositionFileCipher.py, 132–133, 138

in transpositionHacker.py, 164–165, 169

in transpositionTest.py, 118, 124

in vigenereCipher.py, 252–253, 257

in vigenereHacker.py, 294, 313

makePublicPrivateKeys.py, 340–341

makeWordPatterns.py, 225

man-in-the-middle (MITM) attack, 339

mappings. See cipherletter mappings

math.ceil() function, 103–104

math.floor() function, 103–104

math operators

+ (addition operator), 12

/ (division operator), 12

** (exponent operator), 201

% (modulo operator), 173

* (multiplication operator), 12

- (subtraction operator), 12

math.sqrt() function, 327, 356

max() function, 364

methods

defined, 62

passing as values, 272–273

min() function, 364

MITM (man-in-the-middle or machine-in-the-middle) attack, 339

modular arithmetic, 172–173

modular inverse, 180–181

in affine cipher, 179–180, 181

in public key cipher, 344

ModuleNotFoundError, 226

modules

calling functions from, 65

importing, 56

modulo operator (%), 173

Morse code, 3

Morse, Samuel, 3

multiline strings, 164–165

multiple assignment, 175–176

multiplication operator (*), 12

multiplicative cipher, 177–181

N

__name__ variable, 95–96

NameError, 33

negative indexes, 25–26

newline

escape character (\n), 28–29

removing with the strip() method, 167

not equal to (!=) operator, 43–45

not in operator, 61–62

not operator, 107

O

one-time pad cipher, 316

open() function, 131

opening programs, 34

operators, 12

assignment, 16

augmented assignment, 91–92

comparison, 43–45

in, 61–62

math, 12

not in, 61–62

order of operations, 14

or operator, 107, 108

os.path.exists() function, 133–134

P

parameters. See also arguments

defining default arguments, 157

defining functions with, 83–84

optional, 131

passing references as, 121, 234

scope of, 84

parentheses (()), 14, 28, 35

passingReference.py, 121

path (of files), 131

pattern object, regular expression, 231

permutation, 78

PKI (public key infrastructure), 338

plaintext, 4

plain text files, 128, 130–134

potential decryption letters, 223

pow() function, 353–354

precedence, 14

primality test, 322

prime factorization, 323

prime numbers, 322–323

finding

with sieve of Eratosthenes, 328–330

with trial division algorithm, 326–327

use in public key cryptography, 340, 356–357

primeNum.py, 324–326

print() function, 27–28, 34–35, 306

product() function (itertools module), 307–308

pseudorandom numbers, 116–117

public key cipher

decrypting with, 368

encrypting with, 367–368

hacking, why we can’t, 355–357

publicKeyCipher.py, 357–360

public key cryptography, 336

example key files, 342

formula to generate keys, 340

public key infrastructure (PKI), 338

pyperclip.copy() function, 56

pyperclip.py, xxvi

Q

quotes

enclosing strings

multiline, 164–165

single line, 29–30

escaping in strings, 28–29

R

Rabin-Miller primality test, 331–333

random numbers, 116–117

random.randint() function, 116–117

random.seed() function, 116–117

random.shuffle() function, 122

range() function, 72–73

read() file object method, 132

references, to lists, 119–121

regex objects, 231

regular expressions

calling sub() method on, 241

finding characters with, 230–231

repeat keyword argument, for itertools.product(), 307

replace() string method, 243

replication, 24, 118

replication operator (*), 24, 89–90

return statement, 94–95

return value, 35, 94–95

reverse cipher, 39

reverseCipher.py, 40

reverse keyword argument, for sort(), 273, 275

Rivest, Ron, 337

round() function, 103–104

RSA cipher, 337

running programs, 33

Russell, Bertrand, 209

S

saving programs, 32

Schneier, Bruce, 337

scope, 84

secrets module, 318

seed() function (random module), 116–117

set() function, 298

Shamir, Adi, 337

Shannon, Claude, 70

Shannon’s Maxim, 70, 190

shuffle() function (random module), 122

sieve of Eratosthenes, 328–331

signatures, digital, 338

silent mode, 306

simpleSubCipher.py, 209–210

simpleSubHacker.py, 226–229

simple substitution cipher, 208

decrypting, 216

encrypting, 215

hacking, 222–225, 241–245

single quote ('), 22, 28–30

slicing, 26–27

sort() list method, 212–213, 271–275

source code, 31

spaces, removing with the strip() method, 167

split() string method, 150–151

sqrt() function (math module), 327, 356

square brackets ([]), 26

square root, 327

startswith() string method, 135

statements, 16

assignment, 16

break, 311–312

continue, 202–204

def, 82–84

elif, 61

else, 60

for, 58–59

if, 59–60

import, 55, 56

return, 94–95

while, 42–43

str() function, 154

strings, 22–27

building with list-append-join process, 253–254

concatenation, 23

endswith() method, 135

escape characters, 28–30

find() method, 62–63

formatting (%s), 75

indexing, 24–27

interpolation, 75

islower() method, 216–217

isupper() method, 216–217

join() method, 93–94, 253–254

lower() method, 134

multiline, 164–165

quotes, 28–30

replace() method, 243

replication, 24, 118

slicing, 26–27

split() method, 150–151

startswith() method, 135

str() function, 154

strip() method, 167

title() method, 134–135

upper() method, 134

stringTest.py, 254

strip() method, 167

sub() regular expression method, 241

subtraction operator (-), 12

symbol set, 57

locating symbols in, 63

wraparound in, 64, 177–178, 193

SyntaxError, 15, 28, 29

sys.exit() function, 124

sys.path.append() function, 226

T

tab

escape character (\t), 28–29

removing with the strip() method, 167

terminating programs, 35–36

testing programs, 113–114

textbook RSA, 337

time.time() function, 136–137

title() string method, 134–135

transposition cipher, 77, 78–81

decrypting with, 100–101

encrypting with, 78–80

hacking, 166–168

transpositionDecrypt.py, 101–102

transpositionEncrypt.py, 81–82

transpositionFileCipher.py, 130

transpositionHacker.py, 162–163

transpositionTest.py, 114–115

trial division algorithm, 326–327, 328

triple quotes (''' or """), 164–165

True (Boolean value), 43

tuples, 190, 268

Turing, Alan, 188, 268, 352

two-time pad cipher, 319–320

U

upper() string method, 134

V

Vail, Alfred, 3

values, 12

variables, 15–18

names, 18

overwriting, 17

Vigenère, Blaise de, 247

Vigenère cipher, 248–251, 316, 319–320

decrypting with, 248–249

encrypting with, 248–249

hacking, 280–282

vigenereCipher.py, 251–252

vigenereDictionaryHacker.py, 280–281

vigenereHacker.py, 251–252

W

while statements, 42–43

whitespace characters, 167

word patterns, 222–223

wordPatterns.py, 225

wraparound

in modular arithmetic, 172–173

of symbol sets, 64, 177–178, 193

wrapper functions, 213–214

Wright, Ernest Vincent, 266

write() file object method, 131