Practice Exercises

Practice Exercises 1A

Using the cipher wheel or St. Cyr Slide, encrypt the following sentences with key 5.

1. Eat food.
2. Mostly plants.
3. Not too much.

Using the cipher wheel or St. Cyr Slide, decrypt the following sentences with key 17.

4. Z YRMV DP WRKYVI'J VPVJ.
5. REU DP DFKYVI’J EFJV.
6. ZE R ARI LEUVI DP SVU.

Answers:

1. Jfy ktti.
2. Rtxyqd uqfsyx.
3. Sty ytt rzhm.
4. I have my father’s eyes.
5. And my mother’s nose.
6. In a jar under my bed.

Practice Exercises 1B

Encrypt the following sentences in the Caesar cipher with the given keys:

1. "AMBIDEXTROUS: Able to pick with equal skill a right-hand pocket or a left." with the key 4.
2. "GUILLOTINE: A machine which makes a Frenchman shrug his shoulders with good reason." with key 17.
3. "IMPIETY: Your irreverence toward my deity." with key 21.

Decrypt the following ciphertexts with the given keys:

4. "ZXAI: P RDHIJBT HDBTIXBTH LDGC QN HRDIRWBTC XC PBTGXRP PCS PBTGXRPCH XC HRDIAPCS." with key 15.
5. "MQTSWXSV: E VMZEP EWTMVERX XS TYFPMG LSRSVW." with key 4.

Encrypt the following sentence with the key 0:

6. "This is a silly example."

Here are some words and next to them the same word encrypted. What key was used?

7. ROSEBUD – LIMYVOX
8. YAMAMOTO – PRDRDFKF
9. ASTRONOMY – HZAYVUVTF

The following sentence was encrypted with the key 8. What does it decrypt to when you use the key 9 instead?

10. "UMMSVMAA: Cvkwuuwv xibqmvkm qv xtivvqvo i zmdmvom bpib qa ewzbp epqtm."

Answers:

1. EQFMHIBXVSYW: Efpi xs tmgo amxl iuyep wompp e vmklx-lerh tsgoix sv e pijx.
2. XLZCCFKZEV: R drtyzev nyzty drbvj r Wivetydre jyilx yzj jyflcuvij nzky xffu ivrjfe.
3. DHKDZOT: Tjpm dmmzqzmzixz ojrvmy ht yzdot.
4. KILT: A costume sometimes worn by Scotchmen in America and Americans in Scotland.
5. IMPOSTOR: A rival aspirant to public honors.
6. This is a silly example.
7. 20
8. 17
9. 7
10. LDDJMDRR: Tmbnllnm ozshdmbd hm okzmmhmf z qdudmfd sgzs hr vnqsg vghkd.

Practice Exercises 3A

1. What are the operators for addition, subtraction, and multiplication?
2. Which is the operator for division: / or \ ?
3. Which of these is an integer value, and which is a floating point value?
   42
3.141592
4. Which of these lines are not expressions?
   4 x 10 + 2
3 * 7 + 1
2 +
42
   

Answers:

1. + - *
2. /
3. 42 is an integer value, 3.141592 is a floating point value.
4. 2 + is not a valid expression because there needs to be a value on the right side of the + operator.

Practice Exercises 3B

1. If you enter this code into the interactive shell, what does it print out?

   >>> spam = 20
   >>> spam

2. If you enter this code into the interactive shell, what does it print out?

   >>> spam = 20
   >>> spam = 30
   >>> spam

3. If you enter this code into the interactive shell, what does it print out?

   >>> spam = 20
   >>> spam = spam + 20
   >>> spam

4. If you enter this code into the interactive shell, what does it print out?

   >>> spam = 20
   >>> spam + 20
   40
   >>> spam

5. If you enter this code into the interactive shell, what does it print out?

   >>> spam = 20
   >>> SPAM = 30
   >>> spam

6. If you enter this code into the interactive shell, what does it print out?

   >>> spam = 

7. If you enter this code into the interactive shell, what does it print out?

   >>> monkeyplumber

8. Which of these lines is an expression, and which of these is a statement?

   >>> 2 + 2
   4
   >>> spam = 42

9. Is this line an expression or a statement?

   >>> 5
   5

Answers:

1. 20
2. 30
3. 40
4. 20
5. 20
6. This produces an error because the value to store in spam is missing.
7. This produces an error because no variable named monkeyplumber has been created yet.
8. 2 + 2 is an expression. spam = 42 is a statement (specifically, an assignment statement)
9. An expression. A value by itself is an expression (which simply evaluates to itself).

Practice Exercises 4A

1. If you enter this code into the interactive shell, what does it print out?

>>> spam = 'Cats'
>>> spam

2. If you enter this code into the interactive shell, what does it print out?

>>> spam = 'Cats'
>>> spam + spam + spam

3. If you enter this code into the interactive shell, what does it print out?

>>> spam = 'Cats'
>>> spam * 3

4. If you enter this code into the interactive shell, what does it print out?

>>> print("Dear Alice,\nHow are you?\nSincerely,\nBob")

5. If you enter this code into the interactive shell, what does it print out?

>>> print("Hello" + 'Hello')

Answers:

1. Cats
2. CatsCatsCats
3. CatsCatsCats
4. Dear Alice,
How are you?
Sincerely,
Bob
5. HelloHello

Practice Exercises 4B

If you enter the following pieces of code into the interactive shell, what does it print out?

1. >>> spam = 'Four score and seven years is eighty seven years.'
   >>> print(spam[0])

2. >>> spam = 'Four score and seven years is eighty seven years.'
   >>> print(spam[5])

3. >>> spam = 'Four score and seven years is eighty seven years.'
   >>> print(spam[-1])

4. >>> spam = 'Four score and seven years is eighty seven years.'
   >>> print(spam[-3])

5. >>> spam = 'Four score and seven years is eighty seven years.'
   >>> print(spam[0:4] + spam[5])

6. >>> spam = 'Four score and seven years is eighty seven years.'
   >>> print(spam[-3:-1])

7. >>> spam = 'Four score and seven years is eighty seven years.'
   >>> print(spam[:10])

8. >>> spam = 'Four score and seven years is eighty seven years.'
   >>> print(spam[-5:])

9. >>> spam = 'Four score and seven years is eighty seven years.'
   >>> print(spam[:])

Answers:

1. F
2. s
3. .
4. r
5. Fours
6. rs
7. Four score
8. ears.
9. Four score and seven years is eighty seven years.

Practice Exercises 4C

1. Which window has the >>> prompt? The interactive shell or the file editor?
2. Which window runs one Python instruction at a time, immediately? The interactive shell or the file editor?
3. What does the following Python instruction print out?

#print('Hello world!')

4. What function displays text on the screen?
5. What function lets the user type text into the program?
6. What does it mean if your program calls input() and you type in blah, and the program displays this error:

NameError: name 'Albert' is not defined

Answers:

1. The interactive shell has the >>> prompt.
2. The interactive shell.
3. It prints out nothing. This is a comment and is ignored.
4. print()
5. input()
6. You are running Python 2 instead of Python 3.

Practice Exercises 5A

What do the following pieces of code display on the screen?

1. print(len('Hello') + len('Hello'))

2. i = 0
   while i < 3:
       print('Hello')
       i = i + 1

3. i = 0
   spam = 'Hello'
   while i < 10:
       spam = spam + spam[i]
       i = i + 1
   print(spam)

4. i = 0
   while i < 4:
       while i < 6:
           i = i + 2
           print(i)

Answers:

1. 10

2. Hello
   Hello
   Hello

3. HelloHelloHello

4. 2
   4
   6

Practice Exercises 6A

Using the cipher program, encrypt the following sentences with the given keys:

1. “'You can show black is white by argument,' said Filby, 'but you will never convince me.'” with the key 8.
2. “1234567890” with key 21.

Using the cipher program, decrypt the following ciphertexts with the given keys:

3. “'Kv uqwpfu rncwukdng gpqwij.'” with key 2.
4. “Xqp whh ahoa kb pda sknhz swo ejreoexha.” with key 22.

Using the cipher program, encrypt the following sentence with the key 0:

5. “This is still a silly example.”

Answers:

1. 'GWC KIV APWE JTIKS QA EPQBM JG IZOCUMVB,' AIQL NQTJG, 'JCB GWC EQTT VMDMZ KWVDQVKM UM.'
2. 1234567890
3. 'IT SOUNDS PLAUSIBLE ENOUGH.'
4. BUT ALL ELSE OF THE WORLD WAS INVISIBLE.
5. THIS IS STILL A SILLY EXAMPLE.

Practice Exercises 6B

1. What Python instruction would import a module named watermelon.py?
2. The variable SPAM is a constant. Will this code cause an error?: SPAM = 'hello'

What do the following pieces of code display on the screen?

3. for i in 'hello':
       print(i)

4. print('Hello'.lower())

5. print('Hello'.upper())

6. print('Hello'.upper().lower().upper().lower())

BONUS: What does this program display on the screen? (Guess, and then type it in and run it to find out.)

spam = 'foo'
for i in spam:
    spam += i
print(spam)

Answers:

1. import watermelon (Note that there is no .py)
2. No, it will not cause an error. Constants are just regular variables and can have their values changed. But it is a generally accepted convention to not change the values in constants.

3. h
   e
   l
   l
   o

4. hello
5. HELLO
6. hello

BONUS: foofoo Even though the spam variable is having strings added to it, only the value of spam when the for loop first started is iterated over.

Practice Exercises 6C

What do the following pieces of code display on the screen?

1. if 10 < 5:
       print('Hello')

2. if 10 < 5:
       print('Hello')
   else:
       print('Goodbye')

3. if 10 < 5:
       print('Hello')
   elif 5 == 5:
       print('Alice')
   else:
       print('Goodbye')

4. if 10 < 5:
       print('Hello')
   elif False:
       print('Alice')
   else:
       print('Goodbye')

5. if 10 < 5:
       print('Hello')
   elif False:
       print('Alice')
   elif 5 != 5:
       print('Bob')
   else:
       print('Goodbye')

6. if 10 < 5:
       print('Hello')
   elif 5 == 5:
       print('Alice')
   else:
       print('Goodbye')

7. if 10 < 5:
       print('Hello')
   else:
       print('Goodbye')
   elif 5 == 5:
       print('Alice')

8. print('f' in 'foo')

9. print('f' not in 'foo')

10. print('foo' in 'f')

11. print('hello'.find('o'))

12. print('hello'.find('oo'))

Answers:

1. Nothing.
2. Goodbye
3. Alice
4. Goodbye
5. Goodbye
6. Alice
7. This program crashes with an error because the else statement always comes at the end.
8. True
9. False
10. False
11. 4
12. -1 (The integer -1 is returned if the string cannot be found.)

Practice Exercises 7A

Break the following ciphertexts:

1. R UXEN VH TRCCH,
2. FR DBMMR EHOXL FX,
3. CXPNCQNA FN'AN BX QJYYH,
4. OBR OZKOMG QOFSTFSS.
5. PDKQCD IU DAWZ DWO OQOLEYEKJO,
6. FTMF U WQQB GZPQD YK TMF,
7. AR ITMF YUSTF TMBBQZ,
8. DA D NCMVIF OJ OCZ NDUZ JA V MVO.
9. ZFBI. J'N QSFUUZ TVSF NZ DBU XPVME FBU NF.

Answers:

1. I LOVE MY KITTY,
2. MY KITTY LOVES ME,
3. TOGETHER WE'RE SO HAPPY,
4. AND ALWAYS CAREFREE.
5. THOUGH MY HEAD HAS SUSPICIONS,
6. THAT I KEEP UNDER MY HAT,
7. OF WHAT MIGHT HAPPEN,
8. IF I SHRANK TO THE SIZE OF A RAT.
9. YEAH. I’M PRETTY SURE MY CAT WOULD EAT ME.

Practice Exercises 8A

With paper and pencil, encrypt the following messages with the key 9 using the transposition cipher. The (s) parts mark a single space. The number of characters has been provided for your convenience.

1. Underneath(s)a(s)huge(s)oak(s)tree(s)there(s)was(s)of(s)swine(s)a(s)huge(s)company, (61 characters)
2. That(s)grunted(s)as(s)they(s)crunched(s)the(s)mast:(s)(s)For(s)that(s)was(s)ripe,(s)and(s)fell(s)full(s)fast. (79 characters)
3. Then(s)they(s)trotted(s)away,(s)for(s)the(s)wind(s)grew(s)high:(s)(s)One(s)acorn(s)they(s)left,(s)and(s)no(s)more(s)might(s)you(s)spy. (96 characters)

Answers:

The (s) parts mark a single space:

1. Uhot(s)(s)on(s)ahoamdakef(s)pe(s)(s)r(s)harhtesunnur(s)wgyegewie,aeean(s)t(s)(s)sec
2. Tteestr(s)lheydthif(s)ad(s)(s):apeft(s)ct(s)tela(s)arh(s)(s),lsgsueFw(s)(s)tr(s)n(s)oaaf.utcmrsnunhha(s)(s)dl
3. T(s)a(s)(s)hc(s)d(s)uhtwtg:ol(s)m(s)erahr(s)renisnoyee(s)nfogp(s)t,(s)wO(s)t(s)hytt(s)w(s)nt,mt.hefiheh(s)o(s)edoni(s)earyy(s)rdgayneo

Practice Exercises 8B

In the following programs, is spam a global or local variable? Or are there both a global and local variables named spam?

1. spam = 42

2. def foo():
       spam = 42

3. spam = 42
   def foo():
       spam = 42

4. spam = 42
   def foo():
       print(spam)

5. spam = 42
   def foo():
       global spam
       print(spam)

6. spam = 42
   def foo():
       global spam
       spam = 99
       print(spam)

7. spam = 42
   def foo():
       spam = 99
       print(spam)

Answers:

1. Global
2. Local
3. Two variables, one global and one local.
4. Global
5. Global
6. Global
7. Two variables, one global and one local.

Practice Exercises 8C

What value do each of the following expressions evaluate to?

1. [0, 1, 2, 3, 4][2]
2. [1, 2, 3, 4][2]
3. [[1, 2], [3, 4]][0]
4. [[1, 2], [3, 4]][1]
5. [[1, 2], [3, 4]][0][1]
6. [[1, 2], [3, 4]][1][1]
7. ['hello'][0]
8. ['hello'][0][1]
9. [2, 4, 6, 8, 10][1:3]
10. list('Hello world!')
11. list(range(10))
12. list(range(10))[2]

Answers:

1. 2
2. 3
3. [1, 2]
4. [3, 4]
5. 2
6. 4
7. 'hello'
8. 'e'
9. [4, 6]
10. ['H', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '!']
11. [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
12. 2

Practice Exercises 8D

What value do each of the following expressions evaluate to?

1. len([2, 4])
2. len([])
3. len(['', '', ''])
4. [4, 5, 6] + [1, 2, 3]
5. 3 * [1, 2, 3] + [9]
6. 42 in [1, 2, 3]
7. 42 in [41, 42, 42, 42]

Answers:

1. 2
2. 0
3. 3
4. [4, 5, 6, 1, 2, 3]
5. [1, 2, 3, 1, 2, 3 ,1, 2, 3, 9]
6. False
7. True

Practice Exercises 8E

1. What are the four augmented assignment operators?

What do the following pieces of code display on the screen?

2. spam = 'hello'
   spam += 'world'
   print(spam)

3. spam = 3
   spam += 3
   spam += 3
   print(spam)

4. ''.join(['hello', 'world'])

5. ' '.join(['my', 'very', 'special', 'mudder'])

Answers:

1. +=, -=, *=, /=
2. helloworld
3. 9
4. helloworld
5. my very special mudder

Practice Exercises 9A

With paper and pencil, decrypt the following messages with the key 9. The (s) parts mark a single space. The total number of characters is already counted for you.

1. H(s)cb(s)(s)irhdeuousBdi(s)(s)(s)prrtyevdgp(s)nir(s)(s)eerit(s)eatoreechadihf(s)paken(s)ge(s)b(s)te(s)dih(s)aoa.da(s)tts(s)tn (89 characters)
2. A(s)b(s)(s)drottthawa(s)nwar(s)eci(s)t(s)nlel(s)ktShw(s)leec,hheat(s).na(s)(s)e(s)soogmah(s)a(s)(s)ateniAcgakh(s)dmnor(s)(s) (86 characters)
3. Bmmsrl(s)dpnaua!toeboo'ktn(s)uknrwos.(s)yaregonr(s)w(s)nd,tu(s)(s)oiady(s)hgtRwt(s)(s)(s)A(s)hhanhhasthtev(s)(s)e(s)t(s)e(s)(s)eo (93 characters)

Answers:

1. He picked up the acorn and buried it straight By the side of a river both deep and great.
2. At length he came back, and with him a She And the acorn was grown to a tall oak tree.
3. But with many a hem! and a sturdy stroke, At length he brought down the poor Raven's own oak.

Practice Exercises 9B

If you enter the following pieces of code into the interactive shell, what does it print out?

1. >>> math.ceil(3.1)
2. >>> math.ceil(3.0)
3. >>> math.floor(3.1)
4. >>> math.floor(3.0)
5. >>> round(3.1)
6. >>> round(3.0)
7. >>> round(3.5)
8. >>> True and True
9. >>> True and False
10. >>> False and True
11. >>> False and False
12. >>> True or True
13. >>> True or False
14. >>> False or True
15. >>> False or False
16. >>> not True
17. >>> not not True

Answers:

1. 4
2. 3
3. 3
4. 3
5. 3
6. 3
7. 4
8. True
9. False
10. False
11. False
12. True
13. True
14. True
15. False
16. False
17. True

Practice Exercises 9C

1. Draw out the complete truth tables for the and, or, and not operators.
2. Which is correct?
   • if __name__ == '__main__':
   • if __main__ == '__name__':
   • if _name_ == '_main_':
   • if _main_ == '_name_':

Answers:

1. True	and	True	is	True
   True	and	False	is	False
   False	and	True	is	False
   False	and	False	is	False

   
   
   

   True	or	True	is	True
   True	or	False	is	True
   False	or	True	is	True
   False	or	False	is	False

   
   
   

   not	True	is	False
   not	False	is	True

2. if __name__ == '__main__':

Practice Exercises 10A

1. If you ran this program and it printed the number 8, what would it print the next time you run it?
   

   import random
   random.seed(9)
   print(random.randint(1, 10))

2. What does this program print out?
   

   spam = [1, 2, 3]
   eggs = spam
   ham = eggs
   ham[0] = 99
   print(ham == spam)

3. What module contains the deepcopy() function?
4. What does this program print out?
   

   import copy
   spam = [1, 2, 3]
   eggs = copy.deepcopy(spam)
   ham = copy.deepcopy(eggs)
   ham[0] = 99
   print(ham == spam)

Answers:

1. 7
2. True
3. The copy module.
4. False

Practice Exercises 11A

1. Which is correct: os.exists() or os.path.exists()
2. When is the UNIX epoch?

What do the following expressions evaluate to?

3. 'Foobar'.startswith('Foo')
4. 'Foo'.startswith('Foobar')
5. 'Foobar'.startswith('foo')
6. 'bar'.endswith('Foobar')
7. 'Foobar'.endswith('bar')
8. 'The quick brown fox jumped over the yellow lazy dog.'.title()

Answers:

1. os.path.exists()
2. Midnight on January 1st, 1970
3. True
4. False
5. False
6. False
7. True
8. 'The Quick Brown Fox Jumped Over The Yellow Lazy Dog.'

Practice Exercises 12A

1. What is a dictionary file?
2. Why is being able to detect English useful for cryptanalysis?
3. Would a dictionary file that only had a couple hundred words work for detecting English?

Answers:

1. A text file with a large collection of English words.
2. Detecting English allows your program to figure out if it has found the correct decryption key.
3. It would not work very well. The more words the dictionary file has, the more accurately it can detect English.

Practice Exercises 12B

1. What is a Python dictionary?
2. Python lists use square brackets [ and ] to type them out. What do dictionaries use?
3. What does the following code print?
   >>> spam = {'name': 'Al'}
>>> spam['name']
>>> print(spam['name'])

Answers:

1. A data type of values that can store multiple values like a list, except they can use any value for their index.
2. Curly braces { and }
3. Al

Practice Exercises 12C

1. What does this code print?
   >>> spam = {'eggs': 'bacon'}
>>> print(len(spam))
2. What does this code print?
   >>> spam = {'eggs': 'bacon'}
>>> print('eggs' in spam)
3. What does this code print?
   >>> spam = {'eggs': 'bacon'}
>>> print('bacon' in spam)
4. What for loop code would print out the values in the following spam dictionary?
   >>> spam = {'name': 'Zophie', 'species':'cat', 'age':8}

Answers:

1. 1
2. True
3. False
4. for k in spam:
    print(spam[k])

Practice Exercises 12D

1. What are three differences between dictionaries and lists?
2. What does the following print?
   >>> print('Hello, world!'.split())
3. What is the None value?
4. How many different values does the NoneType data type have?
5. What happens if your code attempts to divide by zero?

Answers:

1. Dictionary key-value pairs are not in any order.
   Dictionaries cannot be concatenated with the + operator.
   Lists have integer indexes, but dictionary keys can be of any data type.
2. ['Hello,', 'world!']
3. A Python value that represents the lack of a value.
4. Only one: ,code>None
5. It will crash with a DivideByZero error message.

Practice Exercises 12E

1. Where does the append() method add values to a list?
2. What is a default argument?
3. What will the following print?
   def spam(eggs=42):     print(eggs)
spam()
spam('Hello')
4. What percentage of words in this sentence are valid English words?
   "Whether it's flobulllar in the mind to quarfalog the slings and arrows out outrageous guuuuuuuuur."

Answers:

1. To the end of the list.
2. A default value used for a parameter if the function call does not pass an argument for that parameter.
3. 42
Hello
4. 80%

Practice Exercises 13A

1. What does this expression evaluate to: ' Hello world'.strip()
2. Which characters are whitespace characters?
3. Why does 'Hello world'.strip('o') evaluate to a string that still has o's in it?
4. Why does 'xxxHelloxxx'.strip('X') evaluate to a string that still has x's in it?

Answers:

1. 'Hello world'
2. The space, newline, and tab characters.
3. The o's are not at the beginning or end of the string.
4. This expression removes uppercase X's, not lowercase x's.

Practice Exercises 14A

1. What do the following expressions evaluate to?
   11 % 5
21 % 7
17 % 1000
5 % 5
   

Answers:

1. 1
0
17
0
   

Practice Exercises 14B

1. Is 4 a factor of 8?
2. Is 4 a factor of 10?
3. Is 4 a factor of 12?
4. What is the greatest common factor of 10 and 15?
5. What is the greatest common factor of 6 and 8?

Answers:

1. Yes
2. No
3. Yes
4. 5
5. 2

Practice Exercises 14C

1. What does spam contain after executing spam, eggs = 'hello', 'world'?
2. The greatest common factor of 17 and 31 is 1. Are 17 and 31 relatively prime?
3. Why aren't 6 and 8 relatively prime?
4. If two numbers are realtively prime, are they also coprime?
5. What's the difference between a divisor and a factor?
6. Write out the code for the Python function that calculates greatest common divisor.
7. Write it out again, just to help yourself memorize it.

Answers:

1. 'hello
2. Yes
3. Because the GCD of 6 and 8 is 2, not 1.
4. Yes. These terms mean the same thing.
5. TODO

6. def gcd(a, b):
       while a != 0:
           a, b = b % a, a
       return b

7. def gcd(a, b):
       while a != 0:
           a, b = b % a, a
       return b

Practice Exercises 14D

1. In the shift cipher, the symbol's number is added to the key's number. What is done in the multiplicative cipher?

Answers:

1. The symbol's number is multiplied by the key's number.

Practice Exercises 14E

1. What is the multiplicative inverse a number A?
2. What is a modular inverse of A mod C?

Answers:

1. 1 / A
2. The number B such that (A * B) % C = 1, for some number A.

Practice Exercises 15A

1. The affine cipher can be thought of as the combination of which two other ciphers?
2. What is a tuple?
3. How is a tuple different from a list?
4. Why does having Key A be 1 make the affine cipher weak?
5. Why does having Key B be 0 make the affine cipher weak?

Answers:

1. The shift and multiplicative ciphers.
2. A data type in Python similar to lists, except immutable.
3. Tuples are immutable, meaning they cannot have their items changed.
4. Because multiplying the symbol's number by 1 does not change it.
5. Because adding 0 to the symbol's number does not change it.

Practice Exercises 16A

1. What does 2 ** 5 evaluate to?
2. What does 6 ** 2 evaluate to?
3. What does the following code print out?

4. for i in range(5):
       if i == 2:
           continue
       print(i)

5. Does the main() function of affineHacker.py get called if another program runs import affineHacker?

Answers:

1. 32
2. 36
3. 0
   1
   3
   4
4. No.

Practice Exercises 17A

1. Why can't brute force be used against a simple substitution cipher, even with a powerful supercomputer?

Answers:

1. There are 403,291,461,126,605,635,584,000,000 possible decryptions, which is too many even for all the world's supercomputers.

Practice Exercises 17B

1. What does the spam variable contain after running this code?
   spam = [4, 6, 2, 8]
spam.sort()
2. What is a wrapper function?
3. What does 'Hello'.isupper() evaluate to?
4. What does 'hello'.islower() evaluate to?
5. What does 'HELLO 123'.isupper() evaluate to?
6. What does '123'.islower() evaluate to?

Answers:

1. spam contains [2, 4, 6, 8]
2. A function that calls another function and returns its return value, often with a slight change to the arguments or return value.
3. False
4. True
5. True
6. False

Practice Exercises 17C

1. How can you modify an encryption algorithm to encrypt spaces and punctuation characters?

Answers:

1. Add the new characters to the LETTERS

Practice Exercises 18A

1. What is the word pattern for the word "hello"?
2. Do "mammoth" and "goggles" have the same word pattern?
3. If the variable spam contains a list of lists of strings, how can you display this is an orderly, pretty way on the screen?
4. Which could be the possible plaintext word for the cipherword PYYACAO: ALLEGED, EFFICIENTLY, or POODLE?

Answers:

1. 0.1.2.2.3
2. Yes
3. Use the pprint.pprint() function.
4. ALLEGED (EFFICIENTLY has too many letters, POODLE has too few)

Practice Exercises 19A

1. The vigenere is similar to which other cipher?
2. How many possible keys are there for a simple substitution key with a key length of 10?
   * Hundreds
   * Thousands
   * Millions
   * Much, much more than a million
   

Answers:

1. The Caesar Shift Cipher
2. Much, much more than a million

Practice Exercises 20A

1. What is frequency analysis?
2. What are the six most common letters in English?
3. What does the spam variable contain after running this code?
   spam = [4, 6, 2, 8]
spam.sort(reverse=True)
   

Answers:

1. Frequency analysis is a cryptanalysis technique of counting the frequency of letters in the ciphertext and comparing it to the frequency in English.
2. ETAOIN
3. [8, 6, 4, 2]

Practice Exercises 21A

1. What is a dictionary attack?
2. What does Kasiski Examination of a ciphertext reveal?
3. What two things does converting a list value to a set value with the set() function do?
4. If the spam variable contains ['cat', 'dog', 'mouse', 'dog']. This list has 4 items in it. How many items does the list returned from list(set(spam)) have?
5. What does the following code print?
   print('Hello', end='')
   print('World')

Answers:

1. A dictionary attack is a brute force cryptanalysis technique where every word in the English dictionary is used as a key.
2. Kasiski Examination reveals the likely length of the simple substitution key.
3. It removes duplicates from the list and removes the order of the items.
4. HelloWorld

Practice Exercises 22A

1. Why is there no "one-time pad" program presented in this chapter?
2. What cipher is the "two-time pad" equivalent to?
3. Would using a key twice as long as the plaintext message make the one-time pad twice as secure?

Answers:

1. Because it is mathematically impossible to hack the one-time pad?
2. The "two-time" pad is equivalent to a simple substitution cipher with a key that is half the size of the message.
3. No. Any key longer than the message is only equally powerful to a key that is the same length as the message.

Practice Exercises 23A

1. How many primes numbers are there?
2. What are integers that are not prime called?
3. What are two algorithms for finding prime numbers?

Answers:

1. There are an infinite number of primes.
2. Composites are numbers that are not prime.
3. The Sieve of Eratosthenes and Rabin-Miller are both algorithms for finding prime numbers.

Practice Exercises 24A

1. What's the difference between a symmetric cipher and an asymmetric cipher?
2. Alice generates a public key and a private key. After a while, she accidentally deletes her private key. Will other people be able to send her encrypted messages? Will she be able to decrypt messages previously sent to her?
3. What is authentication and confidentiality? How are they different?

Answers:

1. Symmetric ciphers use the same key for encryption and decryption. Assymetric ciphers use different keys for encryption and decryption.
2. Without her private key, Alice will not be able to decrypt and read messages sent to her that were encrypted with her public key.
3. Authentication is a person being able to prove they are who they say they are. Confidentiality is keeping a secret message secret, and readable by only the person with the decryption key. If you receive an encrypted message from Bob, but it is actually Alice pretending to be Bob, you have confidentiality but no authentication. If you receive an encrypted message from Bob (and you know it is from Bob), but the encryption can be hacked, you have authentication but not confidentiality.

Practice Exercises 24B

1. What is nonrepudiation?
2. Alice generates a public key and a private key. After a while, she loses her private key. Will she be able to digitally sign documents? Will other people able to verify her previously signed documents?
3. The function call ord('A') returns the integer 65. What does chr(65) return?

Answers:

1. Nonrepudiation is the guarantee that someone cannot later deny they wrote a message.
2. Alice will not be able to digitally sign new documents if she loses her key.
3. chr(65) returns 'A'

Practice Exercises 24C

1. In cryptography, what is a block?
2. What's the data type of the value b'hello'?
3. The encode() string method returns a value of what data type?
4. The decode() bytes method returns a value of what data type?

Answers:

1. In cryptography, a block is a fixed length of symbols that are encrypted or decrypted as a group. Block ciphers don't encrypt individual symbols, but rather blocks of symbols.
2. This is a value of the bytes data type.
3. The encode() string method returns a bytes value.
5. The decode() bytes method returns a string value.

Practice Exercises 24D

1. The variable spam contains the list ['cat', 'dog', 'mouse']. What does it contain after spam.insert(1, 'hello') runs?
2. What does the function call pow(2, 4) evaluate to?
3. What does the function call pow(2, 4, 10) evaluate to?

Answers:

1. ['cat', 'hello', 'dog', 'mouse']
2. 16
3. 6