Cryptography Fundamentals

By the end, you can

• List the four goals of cryptography (confidentiality, integrity, authentication, non-repudiation) and explain why plain encryption provides confidentiality but not integrity or authenticity.
• Apply the Caesar cipher formula (shift mod 26) to encrypt and decrypt a letter.
• Explain how frequency analysis breaks substitution ciphers, and how Kasiski's examination extends the attack to polyalphabetic (Vigenère) ciphers by recovering the key length.
• State Kerckhoffs's principle and explain why published algorithms (AES, RSA) are more trustworthy than secret ones — security must rest on the key, not on the obscurity of the algorithm.
• Compute a byte-level XOR and explain why XORing twice with the same key returns the original plaintext (A ⊕ K ⊕ K = A).
• Describe the three conditions for a perfect one-time pad (key truly random, at least as long as the message, never reused) and explain why it is unbreakable yet impractical.
• Distinguish block ciphers from stream ciphers, and name the four AES round steps (SubBytes, ShiftRows, MixColumns, AddRoundKey).
• Explain why ECB mode is insecure (identical plaintext blocks yield identical ciphertext, leaking patterns) and how CBC/CTR modes fix it with a per-message IV/nonce.
• Explain the difference between confidentiality and integrity, and name two mechanisms that provide integrity (MACs and digital signatures).
• Describe the key-distribution problem and explain why public-key cryptography solves it by removing the need to share a secret in advance.