References

• Wikipedia: AES
• Wikipedia: Block Cipher
• Wikipedia: Block Cipher Mode of Operation
• NIST FIPS Publication 197 – The specification with implementation details and test vectors

What Symmetric Algorithms are Currently Used?

• Signal messaging app uses AES-CBC
• The Transport Layer Security (TLS) Protocol Version 1.3 uses either AES-GCM, AES-CCM, or ChaCha20
• SSH uses AES for symmetric encryption, AES-GCM and ChaCha20-Poly1305 for AEAD encryption.

What is AES?

From Wikipedia: AES

AES Characteristics

• AES is an open standard. The details of its operation has been public for over 20 years.

• " is the first (and only) publicly accessible cipher approved by the U.S. National Security Agency (NSA) for top secret information when used in an NSA approved cryptographic module."

• AES is very efficient (fast, easy to implement in hardware and software)

AES is a Block Cipher 1

From Wikipedia: Block Cipher

• Encryption: \(E_K(P) := E(K,P): \{0,1\}^k \times \{0,1\}^n \rightarrow \{0,1\}^n\)

• Where \(K\) is the key of length \(k\) bits, \(P\) is the plaintext of length \(n\) bits

• Decryption: \(E_K^{-1}(C) := D_K(C) = D(K,C): \{0,1\}^k \times \{0,1\}^n \rightarrow \{0,1\}^n\)

AES is a Block Cipher 2

• AES is a block cipher it works on blocks of 128 bits (16 bytes) and uses key sizes of 128 bits (16 bytes), 192 bits (24 bytes), or 256 bits (32 bytes)

• From Wikipedia: AES: “AES is based on a design principle known as a substitution–permutation network, and is efficient in both software and hardware.”

Substitution-Permutation

From Wikipedia

AES is a Block Cipher 3

• Block Ciphers are used via certain block cipher modes of operation which must be carefully used to get the full protection of the algorithm.

• The length of the key is vital to the security of the algorithm, you should never directly use “text” as a key to a block cipher.

Cryptography in Python

• We will use the pyca/cryptography
• This is a very popular Python cryptography package and is available for many platforms
• We will be using the Hazardous Materials module for educational purposes
• Symmetric Encryption

Testing AES Encryption in Python

AES is well known with published examples

from cryptography.hazmat.primitives.ciphers import (Cipher, algorithms, modes)

plaintext = bytes.fromhex("00112233445566778899aabbccddeeff")
key = bytes.fromhex("000102030405060708090a0b0c0d0e0f")
encryptor = Cipher(algorithms.AES(key), modes.ECB()).encryptor()
ciphertext = encryptor.update(plaintext)
print(ciphertext.hex())
# Expected output: 69c4e0d86a7b0430d8cdb78070b4c55a

Testing AES Decryption in Python

decryptor = Cipher(algorithms.AES(key), modes.ECB()).decryptor()
dtext = decryptor.update(ciphertext)
print(dtext.hex())

Test a larger Key (256 bits)

plaintext = bytes.fromhex("00112233445566778899aabbccddeeff")
key = bytes.fromhex("000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f")
encryptor = Cipher(algorithms.AES(key), modes.ECB()).encryptor()
ciphertext = encryptor.update(plaintext)
print(ciphertext.hex())
# Expected output: 8ea2b7ca516745bfeafc49904b496089

Encrypting an Image

• We saw with our homemade substitution cipher that we could see patterns in the ciphertext
• AES works with block of 16 bytes so we probably would not see pattern in “text” based ciphertext
• What about an image?

Reading Images in Python

Will use matplotlib and numpy to get an image

import matplotlib.pyplot as plt
import matplotlib.image as mpimg
import numpy as np

img = mpimg.imread('test.jpg')
plt.figure()
plt.imshow(img)

My Test Image

Screen shot of the Image tutorial page

Image as an Array of Bytes 1

print(f"The image size in bytes: {img.size}")
print(f"The number of dimensions in the image array: {img.ndim}")
print(f"The size of each image dimension: {img.shape}")
print(f"The type of data in the image arra: {img.dtype}")

Image as an Array of Bytes 2

• The image size is 3,117,102 bytes Why so big?
• The image array has 3 dimensions(!) with sizes: (739, 1406, 3) Why?
• The type of data in array is an unsigned 8 bit integer (a raw byte)
• Interpretation: height of 739 pixels, width of 1406 pixels. Each pixel’s color is specified by a combination of three bytes RGB

Encrypting the Image Array

• As a block cipher AES works on 128 bits (16 bytes) at a time
• we will need to group together 16 bytes at a time.
• We’ll group 4x4 blocks of pixels and encrypt each color separately to get a block of 128 bits.

Python Image Array Encryption

Not to be used for security!!!

def jpgEncrypt(npimg, encrypt=True):
    shape = npimg.shape
    # Chop off rows and columns to be a multiple of 4
    nshape = ((shape[0]//4)*4, (shape[1]//4)*4, shape[2])
    cimg = np.ndarray(nshape, np.uint8)
    # Encrypt by block
    for i in range(0, nshape[0]//4):
        for j in range(0, nshape[1]//4):
            for k in range(0, 3):
                mybytes = npimg[4*i:4*(i + 1), 4*j:4*(j+1), k].tobytes()
                if encrypt:
                    cbytes = encryptor.update(mybytes)
                else:
                    cbytes = decryptor.update(mybytes)
                deserialized_bytes = np.frombuffer(cbytes, dtype=np.uint8)
                cimg[4*i:4*(i + 1), 4*j:4*(j+1), k] = np.reshape(deserialized_bytes, newshape=(4, 4))
    return cimg

Image Encryption Result

This is suitable for “top secret” information?

What Went Wrong?

The image (uncompressed) has lots of 128 bit blocks with the same information, i.e., groups of identical 4x4 pixels, the block encryption algorithm processes these into exactly the same new blocks of information!

I guess we can’t use AES?

The Use of Block Ciphers

From Block cipher mode of operation

BCM References

• Wikipedia: Block cipher mode of operation
• NIST: Block Cipher Modes
• NIST: BCM Current Modes

Confidentiality Only Modes I

Just a sample, there are more…

• Electronic Code Book (ECB)
• Cipher Block Chaining (CBC) “CBC has been the most commonly used mode of operation…”
• Cipher Feed Back (CFB)
• Output Feed Back (OFB) “allows many error-correcting codes to function normally even when applied before encryption”
• Counter Mode (CTR)

Confidentiality Only Modes II

Electronic Code Book (ECB) I

From Wikipedia: Block cipher mode of operation

Electronic Code Book (ECB) II

From Wikipedia: Block cipher mode of operation

Cipher Block Chaining (CBC)

From Wikipedia: Block cipher mode of operation

Initialization Vectors

From Wikipedia: Block cipher mode of operation

AES-CBC in Python

# Set up encryptor and decryptor
key = bytes.fromhex("000102030405060708090a0b0c0d0e0f")
# Need to set the initialization vetor
iv = bytes.fromhex("e0e1e2f3f4a5a6a7b8b90a0b0c0d0e0f")
encryptor = Cipher(algorithms.AES(key), modes.CBC(iv)).encryptor()
decryptor = Cipher(algorithms.AES(key), modes.CBC(iv)).decryptor()

Result of AES-CBC Image Encryption

When I use AES with CBC to encrypt my screen shot I get:

Counter Mode (CTR)

From Wikipedia: Block cipher mode of operation

The nonce is like an initialization vector

Other Important Modes

• Galois Counter Mode (GCM): Provides confidentiality and authenticity (integrity) and is fast (newest), option in WPA3, TLS, SSH
• CCM: Provides confidentiality and authenticity used in WPA2, TLS
• XTS: “…designed to protect the confidentiality of data on block-oriented storage devices without providing authentication, in order to avoid expansion of the data;” See Wikipedia: Disk Encryption for all the software/products using this mode.