Encryption and hashing

Encryption

• Encryption turns readable plaintext into unreadable ciphertext using a key.
• Only someone with the right key can decrypt it back.
• There are two families — symmetric and asymmetric — plus one-way hashing.

Symmetric encryption

• Symmetric encryption uses the same key for encrypting and decrypting.
• It is fast and ideal for bulk data (a whole disk, a video stream).
• Its weakness is key distribution — how do you share the secret key safely in the first place?

Asymmetric encryption

• Each user has a key pair: a public key they publish and a private key they keep secret.
• Data encrypted with the public key can be decrypted only with the matching private key.
• To message Alice: encrypt with her public key; only she can decrypt. No prior key exchange needed — but it is much slower, so not used for large data.

Hybrid approach (what real systems use)

• Use asymmetric to exchange a fresh session key, then use that symmetric key for the data:
• the client makes a random session key → encrypts it with the server's public key → the server decrypts it with its private key → both now share it for fast symmetric encryption.
• This is how HTTPS and SSH work.

Hashing (one-way)

• A cryptographic hash turns any input into a fixed-size digest; the same input always gives the same digest, and a tiny change alters it completely.
• It is one-way — you cannot recover the input.
• Used for password checks, integrity checks and digital signatures. (Hashing is not encryption — there's no key to reverse it.)

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