Cryptography Algorithms to encrypt and decrypt

Create an Algorithm directory under Cryptography and create
scripts for Cryptography Algorithms like RSA, Diffie Hellman,
Caesar Cipher and Substitution Cipher to perform key exchange,
encryption and decryption of plain text.

Author: hima-v

Cryptography/Algorithms/Caesar-cipher.py

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+# For caeser cipher the key size is fixed as: key = 3
+
+import string
+
+key = 3
+# store all the alphabets in upper and lowercase in a String variable
+all_letters = string.ascii_letters
+
+# taking inputs
+plain_txt = input("Enter the text you want to Encrypt: \n")
+cipher_txt = []
+
+# create a dictionary which maps the alphabets with respective substitution as per key size
+dict1 = {}
+for i in range(len(all_letters)):
+    dict1[all_letters[i]] = all_letters[(i + key) % len(all_letters)]
+
+# iterate through the plaintext characters and do necessary substittion
+for char in plain_txt:
+    if char in all_letters:
+        temp = dict1[char]
+        cipher_txt.append(temp)
+    else:
+        temp = char
+        cipher_txt.append(temp)
+
+# concatenate all the list elements
+cipher_txt = "".join(cipher_txt)
+
+# create a dictionary for reverse mapping
+dict2 = {}
+for i in range(len(all_letters)):
+    dict2[all_letters[i]] = all_letters[(i - key) % (len(all_letters))]
+decrypt_txt = []
+
+for char in cipher_txt:
+    if char in all_letters:
+        temp = dict2[char]
+        decrypt_txt.append(temp)
+    else:
+        temp = char
+        decrypt_txt.append(temp)
+decrypt_txt = "".join(decrypt_txt)
+
+# display the Cipher Text and Recovered Plain Text
+print("\nCipher Text is: ", cipher_txt)
+print("Recovered plain text: ", decrypt_txt)

Cryptography/Algorithms/Diffie-Hellman.py

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+n = int(input("prime no: \n"))
+g = int(input("prime no: \n"))
+x = 3
+y = 6
+
+
+def al():
+    A = (g**x) % n
+    return A
+
+
+def bob():
+    B = (g**y) % n
+    return B
+
+
+def akey():
+    Bval = bob()
+    k1 = (Bval**x) % n
+    return k1
+
+
+def bkey():
+    Aval = al()
+    k2 = (Aval**y) % n
+    return k2
+
+
+if akey() == bkey():
+    print("shared symmetric key")
+    print(akey(), bkey())
+else:
+    print("error")

Cryptography/Algorithms/README.md

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+# Cryptography Algorithms
+
+# Description
+Its a collection of different cryptography algorithms used for encryption and key exchange protocols.
+Following are the list of algorithms:
+1. Ceaser Cipher
+2. Diffie Hellman Key Exchange Protocol
+3. RSA Encryption and Decryption
+4. Substitution Cipher
+
+# Getting Started
+1. Run the programs locally
+2. Input the parameters such as key size and plain text when prompted

Cryptography/Algorithms/RSA.py

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+import math
+import random
+
+
+# Arguments provide range to generate two prime numbers
+def primenumbers(srt, n):
+    primenos = []
+    prime = [True for i in range(n + 2 - srt)]
+    prime[0] = False
+    prime[1] = False
+    for p in range(srt, int(math.sqrt(n)) + 1):
+        if prime[p] == True:
+            for i in range(p * p, n + 1, p):
+                prime[i] = False
+    for p in range(srt, n + 1):
+        if prime[p]:
+            primenos.append(p)
+    P = random.choice(primenos)
+    primenos.remove(P)
+    Q = random.choice(primenos)
+    return (P, Q)
+
+
+# RSA Algorithm Implementation:
+print("RSA Implementation starts for the connection")
+for c in range(5):
+    print("------")
+
+p, q = primenumbers(1, 30)
+N = p * q
+f = (p - 1) * (q - 1)
+
+
+# Generate Public Key for Encryption
+def publickey():
+    for i in range(2, f):
+        if math.gcd(f, i) == 1:
+            E = i
+            break
+    return E
+
+
+E = publickey()
+
+
+def privatekey():
+    for j in range(1, N):
+        if math.fmod((E * j), (f)) == 1:
+            # if (E * j) % (f) == 1:
+            D = j
+            break
+    return D
+
+
+D = privatekey()
+print("Generated Public Key for Encryption E: ", E)
+print("Generated Private Key for Decryption D: ", D)
+Plain_text = float(input("Enter Plain text in numerical data type: \n"))
+
+x = math.pow(Plain_text, E)
+Cipher_text = math.fmod(x, N)
+print("Generated Cipher Text using Public Key: ", Cipher_text)
+
+D_user = int(input("Enter your private key for Decryption \n"))
+if D_user == D:
+    y = math.pow(Cipher_text, D)
+    Decrypted_text = math.fmod(y, N)
+    print("The Cipher Text has been decrypted: ", Plain_text)
+else:
+    print("ENTERED WRONG PRIVATE KEY")

Cryptography/Algorithms/Substitution-cipher.py

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+import string
+
+# store all the alphabets in upper and lowercase in a String variable
+all_letters = string.ascii_letters
+
+# taking inputs
+key = int(input("Enter the key value in integer: \n"))
+plain_txt = input("Enter the text you want to Encrypt: \n")
+cipher_txt = []
+
+# create a dictionary which maps the alphabets with respective substitution as per key size
+dict1 = {}
+for i in range(len(all_letters)):
+    dict1[all_letters[i]] = all_letters[(i + key) % len(all_letters)]
+
+# iterate through the plaintext characters and do necessary substittion
+for char in plain_txt:
+    if char in all_letters:
+        temp = dict1[char]
+        cipher_txt.append(temp)
+    else:
+        temp = char
+        cipher_txt.append(temp)
+
+# concatenate all the list elements
+cipher_txt = "".join(cipher_txt)
+
+# create a dictionary for reverse mapping
+dict2 = {}
+for i in range(len(all_letters)):
+    dict2[all_letters[i]] = all_letters[(i - key) % (len(all_letters))]
+decrypt_txt = []
+
+for char in cipher_txt:
+    if char in all_letters:
+        temp = dict2[char]
+        decrypt_txt.append(temp)
+    else:
+        temp = char
+        decrypt_txt.append(temp)
+decrypt_txt = "".join(decrypt_txt)
+
+# display the Cipher Text and Recovered Plain Text
+print("\nCipher Text is: ", cipher_txt)
+print("Recovered plain text: ", decrypt_txt)