Fundamentals of Encryption Technologies · Topic 1 of 2
Encryption Development and Fundamentals
Learn how encryption developed from classical ciphers to public key cryptography, plus plaintext, keys, and symmetric versus asymmetric encryption.
- 5 min
- Three stages: classical cryptography, modern cryptography from Shannon in 1949, and public key cryptography from Diffie and Hellman in 1976.
- Substitution replaces characters, transposition rearranges them; the Caesar cipher is a substitution cipher.
- Symmetric encryption uses one shared key, is fast, and has a key distribution problem.
What it is
Encryption turns readable data (plaintext) into unreadable data (ciphertext) using an algorithm and a key. Decryption reverses the process with the right key. Without the key, an attacker who captures the ciphertext learns nothing useful.
Encryption mainly protects confidentiality. Related cryptographic tools add integrity (hash), authentication and non-repudiation (digital signatures). These four security goals come back in every later chapter.
How it works
How encryption developed
The course splits the history into three stages.
| Stage | Period | Key idea | Examples |
|---|---|---|---|
| Classical cryptography | Ancient times to the early 20th century | The secret is the method itself; substitution and transposition done by hand or by machine | Scytale, Caesar cipher, Enigma machine |
| Modern cryptography | From 1949 | Shannon's paper "Communication Theory of Secrecy Systems" makes cryptography a science; security rests on the key, not on hiding the algorithm | DES published as a US standard in 1977 |
| Public key cryptography | From 1976 | Diffie and Hellman's paper "New Directions in Cryptography" shows two parties can agree on a key over an open channel | DH key exchange, RSA (1977) |
A substitution cipher replaces each letter with another, as the Caesar cipher does by shifting letters a fixed number of places. A transposition cipher keeps the letters but changes their order, as the Scytale does by winding a strip round a rod.
Later milestones are worth knowing too. AES replaced DES as the US standard in 2001, and China published its own commercial algorithms, such as SM2, SM3 and SM4.
Basic terms
| Term | Meaning |
|---|---|
| Plaintext | The original readable data |
| Ciphertext | The encrypted, unreadable data |
| Algorithm | The mathematical rules that encrypt and decrypt |
| Key | The secret value that controls the algorithm's output |
| Encryption | Plaintext to ciphertext |
| Decryption | Ciphertext to plaintext |
Modern cryptography follows Kerckhoffs's principle: a system should stay secure even when everything except the key is public. That is why DES, AES and RSA are published openly, and why the key must be long and kept secret.
Symmetric encryption
In symmetric encryption, the sender and receiver use the same key to encrypt and decrypt. It is fast and suits bulk data, such as the payload of an IPsec or SSL connection.
Its weakness is key distribution. Both sides must already share the key, and sending it in clear text exposes it. With many users, every pair needs its own key, so the number of keys grows quickly.
Asymmetric encryption
In asymmetric encryption, each party has a key pair: a public key that anyone may have and a private key that only the owner keeps. Data encrypted with one key of the pair can be decrypted only with the other key.
- To send a confidential message, encrypt with the receiver's public key. Only the receiver's private key can decrypt it.
- To prove who you are, sign with your own private key. Anyone can verify it with your public key.
Asymmetric encryption solves key distribution, because public keys can be shared openly. It is much slower than symmetric encryption, so it is normally used for small data such as keys and signatures, not bulk traffic.
Symmetric and asymmetric compared
| Feature | Symmetric | Asymmetric |
|---|---|---|
| Keys | One shared key | Public and private key pair |
| Speed | Fast | Slow |
| Typical use | Bulk data encryption | Key exchange, digital signatures, small data |
| Key distribution | Hard, key must be shared secretly | Easy, public key can be published |
| Examples | DES, 3DES, AES, SM1, SM4, RC4 | RSA, DH, DSA, ECC, SM2 |
Real systems combine both. Asymmetric cryptography protects or agrees on a symmetric session key, and the session key then encrypts the data. This is the idea behind the digital envelope, IKE and the SSL handshake.
A worked example
Alice wants to send a large file to Bob securely. Encrypting the whole file with Bob's RSA public key would be very slow.
Instead, Alice creates a random AES key and encrypts the file with it. She then encrypts only the AES key with Bob's public key and sends both. Bob decrypts the AES key with his private key and then decrypts the file quickly with AES.
How the exam asks about this
- "Which event marks the start of public key cryptography?" → The 1976 Diffie and Hellman paper "New Directions in Cryptography".
- "Which key decrypts data that was encrypted with Bob's public key?" → Bob's private key.
- "Which is the main drawback of symmetric encryption?" → Key distribution and management, because both sides need the same secret key.
What to remember for the exam
- Three stages: classical cryptography, modern cryptography from Shannon in 1949, and public key cryptography from Diffie and Hellman in 1976.
- Substitution replaces characters, transposition rearranges them; the Caesar cipher is a substitution cipher.
- Symmetric encryption uses one shared key, is fast, and has a key distribution problem.
- Asymmetric encryption uses a public and private key pair, is slow, and is used for key exchange and signatures.
- Encrypt for confidentiality with the receiver's public key; sign with the sender's private key.
Security comes from keeping the key secret, not the algorithm. An answer that says an algorithm is secure because it is hidden is wrong for modern cryptography.
Common mistakes
Mixing up which key encrypts. For confidentiality, the sender uses the receiver's public key. Using the sender's own private key gives a signature, which anyone can read with the sender's public key.
Thinking asymmetric replaces symmetric. Asymmetric algorithms are too slow for bulk data. They protect the session key, and a symmetric algorithm still encrypts the traffic.
Calling the Caesar cipher transposition. The Caesar cipher shifts each letter to another letter, which is substitution. Transposition keeps the same letters and changes their positions.
Check yourself
Which security goal does encryption mainly provide?
Confidentiality. Integrity, authentication and non-repudiation come from hashes and digital signatures.
Why are symmetric and asymmetric encryption usually combined?
Asymmetric cryptography safely delivers or agrees on a session key, and the faster symmetric algorithm then encrypts the bulk data.
Who published the 1949 paper that started modern cryptography?
Claude Shannon, with "Communication Theory of Secrecy Systems".
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