Lesson 4: Cryptography, Hashing, and Digital Signatures

Cryptography protects information by transforming it so that unauthorized users cannot meaningfully read or alter it. In security, cryptography supports confidentiality, integrity, authentication, and non-repudiation.

Important terms:

• Plaintext — original readable data
• Ciphertext — encrypted unreadable output
• Encryption — converting plaintext to ciphertext
• Decryption — restoring ciphertext to plaintext
• Key — secret or controlled value used by an algorithm

Cryptography is not magic. If keys are poorly managed, even strong algorithms can fail in practice.

There are two broad encryption approaches.

Symmetric encryption — the same key is used to encrypt and decrypt.

• Advantages: fast, efficient for large amounts of data.
• Challenge: both parties must protect and share the same secret key securely.

Asymmetric encryption — a pair of keys is used: a public key for sharing and a private key kept secret.

• Advantages: supports secure key exchange, helps with digital signatures.
• Challenge: slower than symmetric methods.

A simple exam comparison:

In real systems, both are often used together. For example, a secure connection may use asymmetric methods to establish trust and exchange secrets, then use symmetric encryption for efficient communication.