# CECS 229 Programming Assignment 3 solution

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## Objectives:

1. Encrypt and decrypt text using an affine transformation.
2. Encrypt and decrypt text using the RSA cryptosystem.

NOTES:
1. Unless otherwise stated in the FIXME comment, you may not change the outline of the
algorithm provided by introducing new loops or conditionals, or by calling any built-in
functions that perform the entire algorithm or replaces a part of the algorithm.

2. You may use the utility functions found in util.py.

3. You may use any functions you implemented in previous programming assignments for this
course. If you choose to use them, please make sure to copy and paste their
implementation into pa3.py file, so that they are uploaded to CodePost when you submit

Problem 1:
Complete the function affine_encrypt(text, a, b) that returns the cipher text encrypted
using key ( a , b ). You must verify that the gcd(a, 26) = 1 before making the encryption. If
gcd(a, 26) != 1, the function must raise a ValueError exception with message “The
given key is invalid.” .
In [ ]:
def affine_encrypt(text, a, b):
“””
encrypts the plaintext ‘text’, using an affine transformation key (a, b)
:param: text – str type; plaintext as a string of letters
:param: a – int type; integer satisfying gcd(a, 26) = 1

Problem 2:
Complete the function affine_decrypt(ciphertext, a,b) that decrypts the text given in
ciphertext which was encrypted using key ( a , b ). You must verify that the gcd(a, 26) = 1. If
gcd(a, 26) != 1, the function must raise a ValueError exception with message “The
given key is invalid.” .
:param: b – int type; shift value
:raise: ValueError if gcd(a, 26) is not 1.

:return: str type; the encrypted message as string of uppercase letters
“””
# FIXME: raise an error if the gcd(a, 26) is not 1
cipher = “”
for letter in text:
if letter.isalpha():
# FIXME: Use util.py to initialize ‘num’ to be
# the integer corresponding to the current letter
num = None
# FIXME: Encrypt the current ‘num’ using the
# affine transformation with key (a, b).
# Store the result in cipher_digits.
cipher_digits = ‘None’
if len(cipher_digits) == 1:
# FIXME: If the cipherdigit is 0 – 9,
# prepend the string with a 0
# to make it a two-digit number
cipher_digits = None

# FIXME: Use util.py to append to the cipher the ENCRYPTED letter
# corresponding to the current cipher digits
cipher += ‘None’
return cipher
In [ ]:
def affine_decrypt(ciphertext, a, b):
“””
decrypts the given cipher, assuming it was encrypted using an affine transformatio
:param: ciphertext – str type; a string of digits
:param: a – int type; integer satisfying gcd(a, 26) = 1.

:param: b – int type; shift value
:return: str type; the decrypted message as a string of uppercase letters
“””
a_inv = 0 # FIXME: complete this line so that a_inv holds the inverse of a under m
text = “”
for letter in ciphertext:
if letter.isalpha():
letter = letter.upper()
# FIXME: Use util.py to find the integer `num` that corresponds
# to the given letter

Problem 3:
Complete the function encryptRSA(text, n, e) which uses RSA to encrypt the string
text using key (n, e) .
EXAMPLE
>> encryptRSA(“REPEAT”, 2537, 13)
‘194319342299’
num = None

# FIXME: Decrypt the integer that corresponds to the current
# encrypted letter using the decryption function for an affine
# transformation with key (a, b) so that letter_digits holds
# the decrypted number as a string of two digits
letter_digits = ‘None’
if len(letter_digits) == 1:
# FIXME: If the letter number is between 0 – 9, inclusive,
# prepend the string with a 0
letter_digits = None
# FIXME: Use util.py to append to the text the decrypted
# letter corresponding to the current letter digits
text += ‘None’
return text
In [ ]:
def encryptRSA(plaintext, n, e):
“””
encrypts plaintext using RSA and the key (n, e)
:param: text – str type; plaintext as a string of letters
:param: n – int type; positive integer that is the modulo in the RSA key
:param: e – int type; positive integer that is the exponent in the RSA key
:return: str type; the encrypted message as a string of digits
“””
text = plaintext.replace(‘ ‘, ”) # removing whitespace

# FIXME: Use util.py to initialize ‘digits’ as a string of
# the two-digit integers that correspond to the letters of ‘text’
digits = ‘None’
# FIXME: Use util.py to initialize ‘l’ with the length of each RSA block
l = 0
# FIXME: Use a loop to pad ‘digits’ with enough 23’s (i.e. X’s)
# so that it can be broken up into blocks of length l
# creating a list of RSA blocks
blocks = [digits[i:i + l] for i in range(0, len(digits), l)]
cipher = “”
for b in blocks:
# FIXME: Initialize ‘encrypted_block’ so that it contains

Problem 4:
Complete the implementation of the function decryptRSA(cipher, p, q, e) which
decrypts cipher , assuming it was encrypted using RSA and key .
EXAMPLE:
>> decryptRSA(‘03412005’, 43, 59, 23)
STOP
# the encryption of block ‘b’ as a string
encrypted_block = ‘None’
if len(encrypted_block) < l:
# FIXME: If the encrypted block contains less digits
# than the block size l, prepend the block with enough
# 0’s so that the numeric value of the block
# remains the same, but the new block size is l,
# e.g. if l = 4 and encrypted block is ‘451’ then prepend

# one 0 to obtain ‘0451’
encrypted_block = None
# FIXME: Append the encrypted block to the cipher
cipher += ‘None’
return cipher
(n = p ⋅ q, e)
In [ ]:
def decryptRSA(cipher, p, q, e):
“””
decrypts the cipher, which was encrypted using RSA and the key (p * q, e)
:param: cipher – ciphertext as a string of digits
:param: p, q – prime numbers used as part of the key n = p * q to encrypt the ciph
:param: e – integer satisfying gcd((p-1)*(q-1), e) = 1
:return: str type; the decrypted message as a string of letters
“””

n = p * q
ciphertext = cipher.replace(‘ ‘, ”)
# FIXME: Use util.py to initialize `l` with the size of
# each RSA block
l = 0
# FIXME: Use a Python list comprehension to break the ciphertext
# into blocks of equal length ‘l’. Initialize ‘blocks’ so that it
# contains these blocks as elements
blocks = []

text = “” # initializing the variable that will hold the decrypted text
# FIXME: Compute the inverse of e
e_inv = None
for b in blocks:
# FIXME: Use the RSA decryption function to decrypt
# the current block

decrypted_block = ‘None’
if len(decrypted_block) < l:
# FIXME: If the decrypted block contains less digits
# than the block size l, prepend the block with
# enough 0’s so that the numeric value of the block
# remains the same, but the new block size is l,
# e.g. if l = 4 and decrypted block is ’19’ then prepend
# two 0’s to obtain ‘0019’
decrypted_block = None
# FIXME: Use util.py to append to text the decrypted block
# transformed into letters
text += ‘None’
return text