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# Cryptography — Basics

### 1. What is Cryptography?

Cryptography is the study of techniques used to secure communication and information from unauthorized access. It converts readable data (**plaintext**) into an unreadable form (**ciphertext**) using a set of rules called an **algorithm** (or **cipher**), usually controlled by a secret value called a **key**.

The reverse process — converting ciphertext back to plaintext — is called **decryption**.

```
Plaintext  --[Encryption + Key]-->  Ciphertext
Ciphertext --[Decryption + Key]-->  Plaintext
```

### 2. Goals of Cryptography (CIA + 2)

| Goal                | Meaning                                    |
| ------------------- | ------------------------------------------ |
| **Confidentiality** | Only authorized parties can read the data  |
| **Integrity**       | Data has not been altered in transit       |
| **Authentication**  | Confirms the identity of the sender        |
| **Non-repudiation** | Sender cannot deny having sent the message |
| **Availability**    | Data/systems remain accessible when needed |

### 3. Types of Cryptography

#### 3.1 Symmetric Key Cryptography

* Same key is used for both encryption and decryption.
* Fast, but key distribution is a challenge (both parties must securely share the same secret key).
* Examples: Caesar Cipher, DES, AES, Rail Fence Cipher.

```
Key (K) is same on both sides
Plaintext --Encrypt(K)--> Ciphertext --Decrypt(K)--> Plaintext
```

#### 3.2 Asymmetric Key Cryptography (Public Key Cryptography)

* Uses a **pair of keys**: a **public key** (shared with everyone) and a **private key** (kept secret).
* What is encrypted with the public key can only be decrypted with the corresponding private key, and vice versa.
* Slower than symmetric cryptography but solves the key-distribution problem.
* Examples: RSA, ECC, Diffie-Hellman.

### 4. Classical Cipher Techniques

Classical ciphers (used historically, mainly for learning purposes today) fall into two broad categories:

#### 4.1 Substitution Cipher

* Each character in the plaintext is **replaced** by another character according to a fixed rule.
* The *positions* of characters stay the same, but the *identity* of each character changes.
* Example: **Caesar Cipher**.

#### 4.2 Transposition Cipher

* The characters of the plaintext are **rearranged** (their positions are shuffled) according to a fixed system.
* The *identity* of each character stays the same, but the *position* changes.
* Example: **Rail Fence Cipher**.

### 5. Modern Cryptographic Concepts

#### 5.1 Hashing

A **hash function** takes an input of any size and produces a fixed-size output (the **hash** or **digest**). It is a one-way function: you cannot practically reverse a hash back to the original input.

* Used to check data integrity.
* Examples: SHA-1, SHA-256, MD5 (MD5 and SHA-1 are now considered weak/broken for security-critical use).

#### 5.2 Digital Signatures

A digital signature proves that a message was created by a specific sender (**authentication**) and was not altered (**integrity**), and the sender cannot later deny sending it (**non-repudiation**).

General process:

1. Sender hashes the message.
2. Sender encrypts (signs) the hash using their **private key** → this is the "signature."
3. Receiver hashes the received message independently, then decrypts the signature using the sender's **public key** to recover the original hash.
4. If both hashes match, the signature is valid.

Digital signatures use asymmetric cryptography, but note: they provide authentication and integrity, **not confidentiality** (the message itself isn't hidden unless separately encrypted).

### 6. Key Terms Glossary

| Term               | Definition                                                                                               |
| ------------------ | -------------------------------------------------------------------------------------------------------- |
| Plaintext          | Original, readable message                                                                               |
| Ciphertext         | Encrypted, unreadable message                                                                            |
| Key                | Secret value controlling encryption/decryption                                                           |
| Cipher             | Algorithm used to perform encryption/decryption                                                          |
| Cryptanalysis      | The study of breaking ciphers without the key                                                            |
| Brute Force Attack | Trying every possible key until the correct one is found                                                 |
| Modulo (mod)       | Remainder after division; used heavily in cipher math (e.g. wrapping the alphabet A-Z back to A after Z) |

### 7. Practicals in This Set

1. **Practical 1** — Caesar Cipher (Substitution Technique)
2. **Practical 2** — Rail Fence Cipher (Transposition Technique)
3. **Practical 3** — RSA Digital Signature (Asymmetric Cryptography)


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