CS338 Computer Security
Wednesday, 24 September 2025

+ base64
- a mechanism for representing arbitrary sequences of bytes
    using printable ASCII characters
- it makes the data longer
- why?
    some software messes with binary data
    some situations (including TLS/HTTPS) require
        byte copying to be exact

+ Basic Auth assignment

+ Words
- cryptography - disguising your message
    "encryption"
- cryptanalysis - figuring out the disguised message
    "decryption"
- cryptology - the combo of both cryptography and cryptanalysis
- most people (including JO) say "cryptography" when they mean "cryptology"

+ Symmetric cryptography recap
- Caesar cipher - "dog" + 3 = "grj"; "grj" - 3 = "dog"
    "dog" = plaintext
    "grj" = ciphertext
    3 = key
- RC4
- Vigenere cipher
- ...

- Idea of a key
    - the algorithm is publicly known
    - the sender and receiver both know the same secret number K (key)
    - without the key, an eavesdropper can't decrypt the encrypted message
    - Big Problem: how did the sender and receiver end up with K?
        "key distribution problem", "key sharing", etc.

- Block cipher
    - break plaintext into fixed-sized chunks ("blocks") and encrypt each block

- Stream cipher (e.g., RC4)
    - encrypt one byte at a time

- Block cipher mode of operation
    - ECB
    - CBC

+ Lab