What does Kerckhoffs' principle say?

The algorithm is public; only the key is secret

Both the algorithm and key are secret

The key is public; the algorithm secret

Which pairs the type with its property?

Symmetric: same key, fast but key distribution is hard; public key: key pair, slower but solves distribution

Symmetric: key pair, slow; public key: one key, fast

Symmetric: hash, fixed length; public key: one key

Both use one key but different algorithms

What is a hash function used for?

One-way fixed-length digest for integrity and passwords

Reversible encryption of big files

Asked 2/13
2071 Magh · 3 marks
2073 Magh · 3.5 marks

Short note: cryptography

Short answer

Cryptography hides information by converting plaintext to ciphertext using keys, providing confidentiality, integrity, authentication, and non-repudiation. Symmetric cryptography uses one shared key (fast but key distribution is hard); asymmetric uses public/private key pairs (solves distribution but slower). Hash functions produce fixed-length digests for integrity checks and password storage.

Write a short note on cryptography.

Cryptography is the science of hiding information by converting it into a form that unauthorised persons cannot read, so that only those who hold the secret key can recover it. It provides confidentiality, integrity, authentication and non-repudiation.

Terms

Plaintext (P): the original readable message.
Ciphertext (C): the scrambled message.
Encryption: C = E(K, P). Decryption: P = D(K, C).
Key: the secret value controlling the cipher. Kerckhoffs' principle: the algorithm is public; only the key is secret.

 Plaintext --[Encrypt, key K1]--> Ciphertext --[Decrypt, key K2]--> Plaintext

Types

Secret-key (symmetric) cryptography: the same key is used for encryption and decryption (DES, 3DES, AES). Fast, but key distribution is a problem.
Public-key (asymmetric) cryptography: a pair of keys: the public key for encryption and the private key for decryption (RSA). Solves key distribution, but is slower.
Hash functions (one-way): produce a fixed-length digest (MD5, SHA-1, SHA-256); used for integrity checks and password storage.

Uses

Secure communication (SSL/TLS, VPN), digital signatures, password storage and encrypted file systems.

Classical ciphers include substitution (Caesar) and transposition ciphers.

✓ Answer checked by Claude Opus 5.5 (high effort)

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