1. Introduction to PDF Cryptography
PDF security relies on encryption to protect document contents, using a password to generate a cryptographic key. The type of algorithm used determines how secure your document is from brute-force attacks and decryption tools.
2. The Legacy: RC4 128-bit Encryption
Introduced in early PDF versions, **RC4** is a stream cipher that was commonly used for 40-bit and 128-bit encryption. While compatible with older software, RC4 has known security weaknesses and is no longer recommended for sensitive documents.
3. The Standard: AES-256 Encryption
**AES-256** (Advanced Encryption Standard) is a block cipher that is the industry standard for document security. It uses a 256-bit key and is designed to resist brute-force attacks, making it the recommended choice for modern PDF security.
4. Cryptographic Comparison Matrix
| Metric | RC4 (128-bit) | AES-256 (256-bit) |
|---|---|---|
| Cipher Type | Stream Cipher | Block Cipher |
| Key Length | 128 bits | 256 bits |
| Security Status | Deprecated / Vulnerable | Strong / Recommended |
| PDF Standard | PDF 1.4+ (Legacy) | PDF 2.0 (Modern) |
5. Which Encryption Standard Should You Use?
For almost all cases, **AES-256** is the recommended choice because of its strong security. Only use **RC4** if you need compatibility with older legacy software that does not support modern PDF security standards.
