Cryptography using python

CRYPTOGRAPHY USING
PYTHON
BY: KAUSHIK RAMABHOTLA(DR3ADL0CKS)

$WHOAMI
• I am Kaushik Ramabhotla
• I am an undergrad(sophomore) at Amrita University,Amritapuri,Kerala.
• Electronics and Communications guy.
• I was a part of India’s best CTF – TEAM BIOS.(July ‘18 to April ‘19).

What are u gonna gain through this session??
• Python Overview and Installation
• Reverse cipher
• Caesar cipher
• Rot13 algorithm
• Base64 encryption-decryption
• Simple Substitution Cipher
• Python modules of cryptography
• Substitution cipher
• One Time Pad Cipher (theory)
• Symmetric and asymmetric crypto
• The RSA algorithm

What the heck is cryptography?
Cryptography is the science developed soulfully for providing security while exchanging
data or a message between two users
I WOULD SAY IT AS AN ART OF COMMUNICATION VIA CODED
MESSAGES!

BASIC TERMINOLOGY
PLAINTEXT
CIPHER TEXT
ENCRYPTION
DECRYPTI
ON
CRYPTOGRA
PHY

Why use python?
• Python is an open source high level language. It is designed as such that it can be
easily understood by most of the readers. It uses English key words frequently
• To conclude, Python is neither better or worse than any other programming
language as long as you use a good crypto library. However, if you're thinking about
implementing something like RSA yourself or some other crypto algorithm you found
in a book/paper, then most probably the code will be vulnerable unless you're an
expert and you really know what you're doing. This applies to any language.

Cryptography Packagescryptography,
Have a look at them
 Python includes a package called cryptography which provides cryptographic
primitives.
package is achieved through following command:
“pip install cryptography ”
 There are various packages with both high level recipes and low level interfaces
to common cryptographic algorithms such as symmetric ciphers, message
digests and key derivation functions.
 Throughout this tutorial, we will be using various packages of Python for
implementation of cryptographic algorithms.

Reverse cipher
As the name suggests , It is easily understood that it is
an encryption that is just the reverse of the plain text or
for an instance , it will be a water image of our original
message
message = 'kaushik'
translated = ''
i = len(message) - 1
while i >= 0:
translated = translated + message[i]
i = i - 1
print(translated)

DISADVANTAGES?
ANYONE CAN EASILY BREAK THE
CIPHER TEXT TO GET THE ORIGINAL
MESSAGE. HENCE, REVERSE CIPHER
IS NOT CONSIDERED AS GOOD
OPTION TO MAINTAIN SECURE
COMMUNICATION CHANNEL.
This cipher is a child’s play!

CAESAR CIPHER
The Caesar cipher is named after Julius Caesar, who, according to
Suetonius, used it with a shift of three to protect messages of military
significance. It is the most basic encryption known to the world.

IMPLEMENTATION
plain = input('enter the string : ')
shift = int(input('enter the shift : '))
result = ''
for i in range(len(plain)):
char = plain[i]
if char.isupper():
result += chr((ord(char) + shift - 65) % 26 + 65)
else:
result += chr((ord(char) + shift - 97) % 26 + 97)
print(result)

BRUTE FORCE TECHNIQUE(REV-
ENGINEERING)
message = input('enter the cipher text')
letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
for key in range(len(letters)):
translated = ''
for x in message:
if x in letters:
num = letters.find(x)
num = num - key
if num < 0:
num = num + len(letters)
translated = translated + letters[num]
else:
translated = translated + x
print('key %s: %s' % (key, translated))

ROT -13 ALGORITHM
• It is simply the abbreviated form of “Rotate by 13 pieces”.
• It is a special case of Caesar cipher where the shift is fixed to 13.
• Every letter is shifted by 13 places to encrypt or decrypt the message.

DRAWBACK
• The rot13 cipher uses a shift of 13 and also is a special case of
Caesar cipher
• Therefore, the algorithm is very easy to break in by brute-force
method or,
frequency analysis method. Therefore, it does not include any
practical use.

BASE64 ENCRYPTION-DECRYPTION
• BASE64 encryption converts binary data to text format
• When you encode text in ASCII, you start with a text string and
convert it to a sequence of bytes.
• When you encode data in Base64, you start with a sequence of bytes
and convert it to a text string.

CODE PART
• There is a module for base64 in python(in-built)
• For decoding…
import base64
x = base64.b64encode(bytes(input(''), 'utf-8'))
print(x)
import base64
x = base64.b64decode(bytes(input(''), 'utf-8'))
print(x)

DRAWBACKS…
• BASE64 algorithm is basically used to store passwords in databases
• Any online decoder tool can be used to decipher the information you
want
• Or you can just decipher them by yourself!

ONE TIME PAD CIPHER(O.T.P)
• It is similar to vienere cipher.
• It is an unbreakable cipher.
• As the name suggests, key is used one time only and never used
again for any other message to be encrypted
• The key is as long as the given message
• The key is auto generated

ENCRYPTION/DECRYPTION WITH ONE
TIME PAD
• To encrypt a letter, a user needs to write a key underneath the
plaintext. The plaintext letter is placed on the top and the key letter on
the left. The cross section achieved between two letters in the plain
text.

DECRYPTION
• To decrypt a letter, user takes the key letter on the left and finds cipher
text letter in that row. The plain text letter is placed at the top of the
column where the user can find the cipher text letter.
• Python has a handy module named-’onetimepad’ which has a
command line encryption tool that uses encryption mechanism similar
to the algorithm.

FACTS:
• So, if you want to encrypt the message "If you want to survive out
here, you've got to know where your towel is.”, its length is 55
excluding the spaces and punctuation. To encrypt with this O.T.P, we
need a key of length =55(same as plaintext)
• Let’s use the key
“kcqyzhepxautiqekxejmoretzhztrwwqdylbttvejmedbsanybpxqik”.
• Now imagine if a cryptanalyst got hold of the ciphertext, brute force
would not work, because the no. of possibilities are (26^55)=i.e.
666,091,878,431,395,624,153,823,182,526,730,590, 376,250,379,52
8,249,805,353,030,484,209,594,192,101,376 possible keys.

VIGENERE CIPHER :
• This cipher is almost as of Caesar cipher but with a little twist.
• Caesar cipher is one-character shift, whereas vigenere cipher involves
multiple alphabets shift
• For encryption the mathematical equation is as follows:
• For decryption the mathematical equation is as follows:

FACTS :
• In this cipher, the key length is directly proportional to security of the
plaintext

CODE PART
plaintext = input()
key = input('enter')
key_length = len(key)
key_as_int = [ord(i) for i in key]
plaintext_int = [ord(i) for i in plaintext]
ciphertext = ''
for i in range(len(plaintext_int)):
value = (plaintext_int[i] + key_as_int[i % key_length]) % 26
ciphertext += chr(value + 65)
print(ciphertext)
ciphertext = input('enter')
key = input()
key_length = len(key)
key_as_int = [ord(i) for i in key]
ciphertext_int = [ord(i) for i in ciphertext]
plaintext = ''
for i in range(len(ciphertext_int)):
value = (ciphertext_int[i] - key_as_int[i % key_length]) % 26
plaintext += chr(value + 65)
print(plaintext)
Encryption part
Decryption
part

MERITS AND DE-MERITS
• The strength of vignere cipher is that it is not susceptible to frequency
analysis due to the fact that the shifts are more. A codebreaker using
frequency analysis method/graph cannot help us in decrypting the
original message. Though it was strong cipher after its invention in
16th century. In 1863, ”Fredrich kakiski” was the first to publish a
successful general attack on the Vigenère cipher
• The primary weakness of the Vigenère cipher is the repeating nature
of its key. If a cryptanalyst correctly guesses the length of the key,
then the ciphertext can be treated as interwoven Caesar ciphers,
which, individually, can be easily broken

CRYPTOGRAPHY WITH PYTHON –
SYMMETRIC AND ASYMMETRIC
CRYPTOGRAPHY
• First we will look into Symmetric part
• Symmetric encryption is a type of encryption where the key is same
for encryption and decryption

TYPES:
• There are two types of symmetric encryption, they are:
1. Block ciphers
2. Stream ciphers
• Block ciphers : In a block cipher, the plaintext is broken into blocks of
a set length and the bits in each block are encrypted together.
• A few of the most popular block ciphers are DES/3DES, AES,
Blowfish, and Twofish.

DATA ENCRYPTION CIPHER(DES)
• DES is a block cipher with a block size of 64bit and a 56bit key since 8
of the 64 bits of the key are not used by the encryption algorithm
(function as check bits only)

BLOCK CIPHER MODES OF OPERATION
• One of the main issue with block ciphers is that it allows only to
encrypt message the same size as their block length. If you use an
algorithm in which a block has a size of 64bit to encrypt a message of
65bit, you need a way to define how the second block should be
encrypted. To issue this problem, different modes of operations were
discovered. We shall discuss the most used operations they are: 1.
ECB 2. ECB modes respectively

ECB MODE(ELECTRONIC CODEBOOK
MODE)
ECB is the simplest mode of operation. In this mode, each part of
plaintext is encrypted separately. The main disadvantage to this mode is
that if there are identical plaintext parts, when encrypted with a given
key, produce same ciphertexts, which allows an attacker to learn some
info about the plaintext.

CIPHER BLOCK CHAINING (CBC) MODE
• In the cipher block chaining, an initialisation vector(I.V) is exclusively
added to the encryption process with the key. For the first round of
encryption, it is a random public value. For the next subsequent
rounds, the I.V becomes the ciphertext of the previous round’s
encryption

IMPLEMENTATION:
import pyDes
data = "DES Algorithm Implementation"
k = pyDes.des("DESCRYPT", pyDes.CBC, "00000000", pad=None, padmode=pyDes.PAD_PKCS5)
d = k.encrypt(data)
print("Encrypted: %r" % d)
print("Decrypted: %r" % k.decrypt(d))

THINGS THAT ARE NEW IN THE PREVIOUS
CODE ARE::
• 1. initialisation vector:
• 2.padding:

ASYMMETRIC CRYPTOGRAPHY
• As discussed earlier, it is quite opposite to symmetric cryptography and
it requires two keys(one for encryption and one for decryption)
• The public key is used for encryption
• The private key is used for decryption

HOW TO GET THE KEYS?
The cryptographic strength is primarily linked to the length of the RSA modulus n. In
2017, a sufficient length is deemed to be 2048 bits.

UNDERSTANDING RSA ALGORITHM
• RSA is considered as the most secure way of encryption
• Invented by Rivest, Shamir, Adleman in 1978. and hence named RSA.
• It includes working with prime numbers
• The integers used by this method are sufficiently large to make it
difficult to solve.
• Here also, there are two sets of keys:private and public.
How it works?

STEPS IN RSA ALGORITHM
Step1:generating the RSA modulus
Step 2: generating derived
number(e),public key, private key
Step 3: finally encrypting or decrypting

LET’S DEFINE VARIABLES THAT WILL BE
USED:
• P,q two very large prime numbers.
• N = p*q
• E = public key exponent
• D = private key exponent
• M = unpadded message
• C = cipher text

KEY GENERATION:
• Calculation of phi(Euler's totient function) = (p-1)*(q-1)
• Choose an integer e such that (1<e<phi) and GCD(e,phi)=1
• Normally, e=22i+1(i>0). Small values of e can make RSA vulnerable to
some attacks under certain conditions
• Compute d as d = (1/e) * (mod(phi))
• The pair(e,n) is known as public key
• The pair (d,n) is known as private key

ENCRYPTION AND DECRYPTION
• Computes ciphertext c = me mod n
• Transmits the ciphertext c using a reliable channel
• While decrypting one can compute m = cd mod n (Since c = me mod n,
cd mod n = me*d mod n = m).

PADDING AND PADDING SCHEMES
• RSA without padding is referred as “Textbook RSA”. There are a
number of attacks possible if the message is not padded before RSA-
encryption.
• The padding bytes follow a specific standard, for
example, PKCS,OAEP,etc. padding ensures that
the plaintext does not become vulnerable to
these attacks.
Direct square
root
Hastad’s
broadcast
attack
Chosen
ciphertext
attacks

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