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Theory:-
- The term GSM stands for Global System for Mobile Communications.
- Mobile communications have to communicate using the global positioning for providing the user with better and efficient communication service by protecting against unauthorized access a privacy of mobile subscribers against any kind of eavesdropping.
- The system of GSM provides a way of ciphering the text which is an crucial way to protect user from evesdropping . and for the whole ciphering process we require the TSMI i.e.,Temporary Subscriber Identity. there's is a 4 to 8 digit PIN asssociated to validate the identity of the subscribers. The PIN here stands for Password Identification Number.
- The way this whole process is carried out is shown as follows accoring to the steps:
- At the time of service provisioning the IMSI,
- individual subscriber authentication key (Ki),
- authentication algorithm (A3) performed
- cipher key generation algorithm (A8) performed
- encryption algorithm (A5) performed
Source code in Python for A3 Algorithm of GSM:-
import random
k=random.getrandbits(128)
m=random.getrandbits(128)
kb=bin(k)[6:]
mb=bin(m)[4:]
kbl=kb[0:64]
kbr=kb[64:]
mbl=mb[0:64]
mbr=mb[64:]
a1=int(kbl,2)
int(mbr,2)
a2=int(kbr,2)
int(mbl,2)
a3=a1^a2
a4=bin(a3)[2:].zfill(64)
a5=a4[0:32]
a6=a4[32:]
a7=int(a5,2)
int(a6,2)
print("128 Bit Key = ",kb)
print("128 Random Bits Generated = ",mb)
print("RES/SRES =",bin(a7)[2:].zfill(len(a5)))
Source code in Python for A8 Algorithm of GSM:-
import re import
copy import sys
reg_x_length = 19
reg_y_length = 22
reg_z_length = 23
key_one = ""
reg_x = []
reg_y = []
reg_z = []
def loading_registers(key): i = 0
while(i < reg_x_length):
reg_x.insert(i, int(key[i]))
i = i + 1
j = 0
p = reg_x_length while(j <
reg_y_length):
reg_y.insert(j,int(key[p])) p = p + 1
j = j + 1
k = reg_y_length + reg_x_length r = 0
while(r < reg_z_length):
reg_z.insert(r,int(key[k])) k =
k + 1
r = r + 1
def set_key(key):
if(len(key) == 64 and re.match("^([01])+", key)): key_one=key
loading_registers(key) return True
return False
def get_key():
return key_one
def to_binary(plain): s = ""
i = 0
for i in plain:
binary = str(' '.join(format(ord(x), 'b') for x in i)) j = len(binary)
while(j < 8):
binary = "0" + binary s =
s + binary
j = j + 1
binary_values = []
k = 0
while(k < len(s)):
binary_values.insert(k, int(s[k]))
k = k + 1 return
binary_values
def get_majority(x,y,z):
if(x + y + z > 1):
return 1
else:
return 0
def get_keystream(length):
reg_x_temp = copy.deepcopy(reg_x)
reg_y_temp = copy.deepcopy(reg_y)
reg_z_temp = copy.deepcopy(reg_z)
keystream = []
i = 0
while i < length:
majority = get_majority(reg_x_temp[8], reg_y_temp[10],
reg_z_temp[10])
if reg_x_temp[8] == majority:
new = reg_x_temp[13] ^ reg_x_temp[16] ^ reg_x_temp[17]
^ reg_x_temp[18]
reg_x_temp_two = copy.deepcopy(reg_x_temp) j = 1
while(j < len(reg_x_temp)):
reg_x_temp[j] = reg_x_temp_two[j-1] j = j +
1
reg_x_temp[0] = new
if reg_y_temp[10] == majority:
new_one = reg_y_temp[20] ^ reg_y_temp[21]
reg_y_temp_two = copy.deepcopy(reg_y_temp) k = 1
while(k < len(reg_y_temp)):
reg_y_temp[k] = reg_y_temp_two[k-1] k = k
+ 1
reg_y_temp[0] = new_one
if reg_z_temp[10] == majority:
new_two = reg_z_temp[7] ^ reg_z_temp[20] ^ reg_z_temp[21] ^
reg_z_temp[22]
reg_z_temp_two = copy.deepcopy(reg_z_temp) m =
1
while(m < len(reg_z_temp)):
reg_z_temp[m] = reg_z_temp_two[m-1] m =
m + 1
reg_z_temp[0] = new_two
keystream.insert(i, reg_x_temp[18] ^ reg_y_temp[21] ^
reg_z_temp[22])
i = i + 1
return keystream
def convert_binary_to_str(binary): s = ""
length = len(binary) - 8 i = 0
while(i <= length):
s = s + chr(int(binary[i:i+8], 2)) i = i + 8
return str(s)
def encrypt(plain):
s = ""
binary = to_binary(plain)
keystream = get_keystream(len(binary)) i = 0
while(i < len(binary)):
s = s + str(binary[i] ^ keystream[i]) i = i + 1
return s
def decrypt(cipher):
s = ""
binary = []
keystream = get_keystream(len(cipher)) i = 0
while(i < len(cipher)):
binary.insert(i,int(cipher[i]))
s = s + str(binary[i] ^ keystream[i]) i = i + 1
return convert_binary_to_str(str(s))
def user_input_key():
tha_key = str(input('Enter a 64-bit key: '))
if (len(tha_key) == 64 and re.match("^([01])+", tha_key)): return
tha_key
else:
while(len(tha_key) != 64 and not re.match("^([01])+", if
tha_key)):
(len(tha_key) == 64 and re.match("^([01])+",
tha_key)):
return tha_key
tha_key = str(input('Enter a 64-bit key: '))
return tha_key
def user_input_choice():
someIn = str(input('[0]: Quit\n[1]: Encrypt\n[2]: Decrypt\nPress 0, 1, or 2: ')) if (someIn == '0' or someIn == '1' or someIn == '2'): return someIn
else:
while(someIn != '0' or someIn != '1' or someIn != '2'):
if (someIn == '0' or someIn == '1' or someIn ==
'2'): return someIn
someIn = str(input('[0]: Quit\n[1]: Encrypt\n[2]:
Decrypt\nPress 0, 1, or 2: ')) return
someIn
def user_input_plaintext(): try:
someIn = str(input('Enter the plaintext: '))
except:
someIn = str(input('Try again: '))
return someIn
def user_input_ciphertext():
ciphertext = str(input('Enter a ciphertext: ')) if
(re.match("^([01])+", ciphertext)):
return ciphertext
else:
while(not re.match("^([01])+", ciphertext)):
if (re.match("^([01])+", ciphertext)):
return ciphertext
ciphertext = str(input('Enter a ciphertext: '))
return ciphertext
def main_fun():
key = str(user_input_key())
set_key(key)
first_choice = user_input_choice() if(first_choice ==
'0'):
print('Have an awesome day!!!') sys.exit(0)
elif(first_choice == '1'):
plaintext = str(user_input_plaintext())
print(plaintext) print(encrypt(plaintext))
elif(first_choice == '2'):
ciphertext = str(user_input_ciphertext()) print(decrypt(ciphertext))
main_fun()
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