Description
• Please read sections 2.1–2.3, 2.7–2.9, and 2.11 of your textbook. You might also find
the remaining sections of chapter 2 (especially 2.5 and 2.12) fun and interesting.
• Do Problems 19, 20, and 25 at the end of chapter 2 (section 2.13).
• Do the following additional problems:
1. Alice and Bob agree on the key word “virgo” for Vigenère encryption between
them. Alice sends Bob the encrypted message
ZDVOGZIMKGYZFVDDVXUBPA
Decrypt this message.
(You can do this by hand pretty easily, but this problem
is here mostly so that if you do write some computer code to do Vigenère encryption/decryption, you can test it before moving on to the next few (harder)
problems.)
2. Alice and Bob have agreed upon a 4-letter password for Vigenère encryption. You
(Eve) don’t know their password, but you intercept the following ciphertext sent
from Bob to Alice. Use letter frequencies as outlined in class to figure out their
key, and decipher the message.
FEWCNWQBMSNSTEJYWOTMXDGVXYCVCYYODSGDQEUOFOTNBAUDQEDKLKDYWEQP
JLKPNSROWTFOOEPNRNICXMGDQIPQHOWEBEVRNMCCJPWXLHNSWEKRJVGXNIVR
NRVRNTKWNNQBCHGSWCNSWAVSXNVYNXRVJIPWHSGVOTQKVAPGQOTSBEUKWDUV
NERCDNFOATJOKLCXTEVYOTJOEETIORGOMOODQAVSYRQFRDGKWDVRNNSENSVS
XNUDQEOKWNGBRNYRRCJSYRQFRDGSC
3. You have intercepted the following ciphertext sent from Alice to Bob. All that
you know is that they are using Vigenère encryption. Figure out their key, and
decipher the message.
DOEESFDAWTSRJSXSHRZFHJGBIEAGIEOIGKWYANVWKVPHAAGYKNZLVVJBTUYP
QROWRBREKSQUAMBUOYRELKSYENPZWPDHXOOFXRXOWACAISFGECNDOEHYFSZB
ZOKGIFLRHVIPPHBKVRWDPSGFQNDMDBJHBKPEAALLOAZHXDCBGEWXFBIMRHCV
JTHXJVAWEAYRWSMJOACEESBXXIKVKVPHWMZYCRXQDYQMTYSNJDTTZNYKMGDX
JOMKCJWTLGBFWIWZSFQDWWBYUYHMRBJOMHFBLOLWHBENOWGGENLGIGDAYJWP
WNLWQHNIMASF
For the last two problems, you will most likely want to use a computer. If you do, here
are those two ciphertexts in a format that you should be able to copy and paste directly into
some computer code:
ciphertext2 = “\
FEWCNWQBMSNSTEJYWOTMXDGVXYCVCYYODSGDQEUOFOTNBAUDQEDKLKDYWEQP\
JLKPNSROWTFOOEPNRNICXMGDQIPQHOWEBEVRNMCCJPWXLHNSWEKRJVGXNIVR\
NRVRNTKWNNQBCHGSWCNSWAVSXNVYNXRVJIPWHSGVOTQKVAPGQOTSBEUKWDUV\
NERCDNFOATJOKLCXTEVYOTJOEETIORGOMOODQAVSYRQFRDGKWDVRNNSENSVS\
XNUDQEOKWNGBRNYRRCJSYRQFRDGSC”
ciphertext3 = “\
DOEESFDAWTSRJSXSHRZFHJGBIEAGIEOIGKWYANVWKVPHAAGYKNZLVVJBTUYP\
QROWRBREKSQUAMBUOYRELKSYENPZWPDHXOOFXRXOWACAISFGECNDOEHYFSZB\
ZOKGIFLRHVIPPHBKVRWDPSGFQNDMDBJHBKPEAALLOAZHXDCBGEWXFBIMRHCV\
JTHXJVAWEAYRWSMJOACEESBXXIKVKVPHWMZYCRXQDYQMTYSNJDTTZNYKMGDX\
JOMKCJWTLGBFWIWZSFQDWWBYUYHMRBJOMHFBLOLWHBENOWGGENLGIGDAYJWP\
WNLWQHNIMASF”
Also for your convenience, here are the typical frequencies of letters in English, in various
formats:
As a C or C++ array:
float letter_frequencies[26] = {
0.082, 0.015, 0.028, 0.043, 0.127, 0.022, 0.020, 0.061, 0.070,
0.002, 0.008, 0.040, 0.024, 0.067, 0.075, 0.019, 0.001, 0.060,
0.063, 0.091, 0.028, 0.010, 0.023, 0.001, 0.020, 0.001
}
As a Python list:
letter_frequencies = [
0.082, 0.015, 0.028, 0.043, 0.127, 0.022, 0.020, 0.061, 0.070,
0.002, 0.008, 0.040, 0.024, 0.067, 0.075, 0.019, 0.001, 0.060,
0.063, 0.091, 0.028, 0.010, 0.023, 0.001, 0.020, 0.001,
]
As a Python dictionary:
letter_frequencies = {
“A”: 0.082, “B”: 0.015, “C”: 0.028, “D”: 0.043, “E”: 0.127, “F”: 0.022,
“G”: 0.020, “H”: 0.061, “I”: 0.070, “J”: 0.002, “K”: 0.008, “L”: 0.040,
“M”: 0.024, “N”: 0.067, “O”: 0.075, “P”: 0.019, “Q”: 0.001, “R”: 0.060,
“S”: 0.063, “T”: 0.091, “U”: 0.028, “V”: 0.010, “W”: 0.023, “X”: 0.001,
“Y”: 0.020, “Z”: 0.001,
}
