Encryption Fundamentals

After this lesson, you will be able to: Understand symmetric/asymmetric encryption and encrypt files hands-on with Python.

Encryption converts readable plaintext into ciphertext using a key. Done right, it makes leaked data useless to the attacker. This lesson covers the math at a conceptual level and gets you encrypting files with Python's `cryptography` library.

Prerequisites:Data Classification

Symmetric vs asymmetric

Symmetric (AES), one key encrypts and decrypts. Fast, ideal for bulk data. Problem: how do you share the key? Asymmetric (RSA, ECC), keypair: public encrypts, private decrypts. Slow but solves key sharing. Used to wrap symmetric keys (TLS, S/MIME).

Encrypt a file in Python

Using the `cryptography` library, install with `pip install cryptography`.

python

from cryptography.fernet import Fernet

# 1. Generate a key (save this securely!)
key = Fernet.generate_key()
f = Fernet(key)

# 2. Encrypt
with open('secret.txt', 'rb') as src:
    plaintext = src.read()
ciphertext = f.encrypt(plaintext)
with open('secret.txt.enc', 'wb') as dst:
    dst.write(ciphertext)

# 3. Decrypt
with open('secret.txt.enc', 'rb') as src:
    decrypted = f.decrypt(src.read())
print(decrypted)

Hashing is not encryption

SHA-256, bcrypt, argon2 are one-way functions, used for passwords, integrity. You cannot 'decrypt' a hash. Confusing the two is a classic interview trap.

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