Introduction
Cryptography is the science of protecting information by transforming it into an unreadable form so that only intended parties can read it. The word comes from the Greek kryptos (hidden) + graphein (to write) — literally "hidden writing." In cyber security, cryptography is the mathematical engine behind almost every protection you rely on: HTTPS in your browser, WhatsApp messages, mobile-banking PINs, digital signatures on government documents, password storage, and blockchain.
For BCS, Bank IT and IT-officer exams, this is one of the highest-yield topics. Examiners love it because it produces clean, factual MCQs: which algorithm is symmetric, how many keys are used, what a hash guarantees, key sizes of DES/AES/RSA, and one-liner definitions of confidentiality, integrity and authentication. If you master the three pillars — symmetric encryption, asymmetric (public-key) encryption, and hashing — you can answer the overwhelming majority of cryptography questions with confidence.
Learning objectives
By the end of this lesson you should be able to:
- Define cryptography and explain the goals of the CIA triad plus authentication and non-repudiation.
- Distinguish plaintext, ciphertext, key, encryption and decryption precisely.
- Explain how symmetric encryption works, its speed advantage, and its key-distribution problem.
- Explain how asymmetric (public-key) encryption works, and how the same key pair gives both confidentiality and digital signatures.
- Explain what a cryptographic hash function is, its required properties, and why hashing is not encryption.
- Name and compare the standard algorithms — DES, 3DES, AES, RSA, ECC, Diffie-Hellman, MD5, SHA-1, SHA-256 — with their key/output sizes.
- Solve exam-style problems: number of keys needed in a network, modular RSA-style calculations, and identifying which technique provides which security service.