Complete Guide to Encryption & Decryption: Understand Cryptanalysis with Notes 🔐

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### Encryption
- **Definition**: Encryption is the process of converting plaintext (readable data) into ciphertext (unreadable data) using an algorithm and a key.
- **Purpose**: To protect the confidentiality of data by making it unreadable to unauthorized users.
- **Types**:
- **Symmetric Encryption**: Uses the same key for both encryption and decryption (e.g., AES, DES).
- **Asymmetric Encryption**: Uses a pair of keys – a public key for encryption and a private key for decryption (e.g., RSA, ECC).
- **Process**:
1. **Plaintext**: Original readable data.
2. **Encryption Algorithm**: The method used to transform the plaintext.
3. **Key**: A value used by the algorithm to encrypt the data.
4. **Ciphertext**: Encrypted data.

### Decryption
- **Definition**: Decryption is the process of converting ciphertext back into plaintext using an algorithm and a key.
- **Purpose**: To restore the original readable data from its encrypted form.
- **Process**:
1. **Ciphertext**: Encrypted data.
2. **Decryption Algorithm**: The method used to transform the ciphertext back into plaintext.
3. **Key**: A value used by the algorithm to decrypt the data.
4. **Plaintext**: Original readable data restored.

### Cryptanalysis
- **Definition**: Cryptanalysis is the study and practice of analyzing information systems to understand hidden aspects of the systems.
- **Purpose**: To find weaknesses or vulnerabilities in cryptographic algorithms and protocols, potentially allowing for the breaking of encryption.
- **Methods**:
- **Brute Force Attack**: Trying all possible keys until the correct one is found.
- **Ciphertext-only Attack**: The attacker has only ciphertext and tries to deduce the plaintext or key.
- **Known-plaintext Attack**: The attacker has access to both plaintext and its corresponding ciphertext.
- **Chosen-plaintext Attack**: The attacker can encrypt plaintexts of their choice to study the ciphertext.
- **Chosen-ciphertext Attack**: The attacker can decrypt ciphertexts of their choice to study the plaintext.
- **Side-channel Attack**: Exploiting physical implementations (e.g., timing information, power consumption) rather than theoretical weaknesses.
- **Objectives**:
- **Breaking Encryption**: Successfully decrypting ciphertext without access to the key.
- **Finding Weaknesses**: Identifying flaws in cryptographic algorithms or protocols.
- **Improving Security**: Enhancing cryptographic methods by addressing identified weaknesses.

### Summary
- **Encryption**: Protects data by converting it into an unreadable format using algorithms and keys.
- **Decryption**: Restores encrypted data to its original readable format using algorithms and keys.
- **Cryptanalysis**: Analyzes and attempts to break encryption methods to find weaknesses and improve security.