sdes Algorithm
The Simplified Data Encryption Standard (SDES) algorithm is a symmetric key block cipher that serves as a simplified version of the Data Encryption Standard (DES) algorithm. It was primarily designed for educational purposes to provide learners with a more straightforward understanding of the fundamental concepts of cryptography and encryption techniques. SDES operates on an 8-bit block of plaintext data, using a 10-bit key for both encryption and decryption processes. The algorithm involves several steps, including key generation, initial permutation, round operations, and inverse permutation, which are all aimed at scrambling the input data to generate the encrypted output or ciphertext.
The SDES algorithm consists of two rounds of processing, with each round involving four primary operations: substitution, permutation, key mixing, and expansion. The substitution operation replaces input bits with new bits according to a predefined substitution box (S-box), while the permutation operation rearranges the bits based on a specified permutation rule. Key mixing combines the input data with a round key, which is derived from the original 10-bit key using a key generation process. The expansion operation expands the input data to a larger size, allowing further manipulations in subsequent rounds. After completing the two rounds, the algorithm performs an inverse permutation on the final output to produce the ciphertext. The same process, with slight modifications, is used to decrypt the ciphertext back to the original plaintext.
def apply_table(inp, table):
"""
>>> apply_table("0123456789", list(range(10)))
'9012345678'
>>> apply_table("0123456789", list(range(9, -1, -1)))
'8765432109'
"""
res = ""
for i in table:
res += inp[i - 1]
return res
def left_shift(data):
"""
>>> left_shift("0123456789")
'1234567890'
"""
return data[1:] + data[0]
def XOR(a, b):
"""
>>> XOR("01010101", "00001111")
'01011010'
"""
res = ""
for i in range(len(a)):
if a[i] == b[i]:
res += "0"
else:
res += "1"
return res
def apply_sbox(s, data):
row = int("0b" + data[0] + data[-1], 2)
col = int("0b" + data[1:3], 2)
return bin(s[row][col])[2:]
def function(expansion, s0, s1, key, message):
left = message[:4]
right = message[4:]
temp = apply_table(right, expansion)
temp = XOR(temp, key)
l = apply_sbox(s0, temp[:4]) # noqa: E741
r = apply_sbox(s1, temp[4:])
l = "0" * (2 - len(l)) + l # noqa: E741
r = "0" * (2 - len(r)) + r
temp = apply_table(l + r, p4_table)
temp = XOR(left, temp)
return temp + right
if __name__ == "__main__":
key = input("Enter 10 bit key: ")
message = input("Enter 8 bit message: ")
p8_table = [6, 3, 7, 4, 8, 5, 10, 9]
p10_table = [3, 5, 2, 7, 4, 10, 1, 9, 8, 6]
p4_table = [2, 4, 3, 1]
IP = [2, 6, 3, 1, 4, 8, 5, 7]
IP_inv = [4, 1, 3, 5, 7, 2, 8, 6]
expansion = [4, 1, 2, 3, 2, 3, 4, 1]
s0 = [[1, 0, 3, 2], [3, 2, 1, 0], [0, 2, 1, 3], [3, 1, 3, 2]]
s1 = [[0, 1, 2, 3], [2, 0, 1, 3], [3, 0, 1, 0], [2, 1, 0, 3]]
# key generation
temp = apply_table(key, p10_table)
left = temp[:5]
right = temp[5:]
left = left_shift(left)
right = left_shift(right)
key1 = apply_table(left + right, p8_table)
left = left_shift(left)
right = left_shift(right)
left = left_shift(left)
right = left_shift(right)
key2 = apply_table(left + right, p8_table)
# encryption
temp = apply_table(message, IP)
temp = function(expansion, s0, s1, key1, temp)
temp = temp[4:] + temp[:4]
temp = function(expansion, s0, s1, key2, temp)
CT = apply_table(temp, IP_inv)
print("Cipher text is:", CT)
# decryption
temp = apply_table(CT, IP)
temp = function(expansion, s0, s1, key2, temp)
temp = temp[4:] + temp[:4]
temp = function(expansion, s0, s1, key1, temp)
PT = apply_table(temp, IP_inv)
print("Plain text after decypting is:", PT)
