fix: use cryptographically secure RNG for IV generation

[michaelcheers]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows Conventional Commits
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

The IvGenerator class uses a predictable pseudo-random number generator seeded with DateTime.Now.Ticks for generating initialization vectors (IVs) used in PDF encryption. This is a critical security vulnerability.

Security Impact:

• An attacker who knows the approximate PDF creation time (within hours/days) can predict the IV
• Predictable IVs allow decryption of encrypted PDF content
• Affects anyone using the library to create encrypted PDFs

The vulnerable code:

static IvGenerator()
{
    _rc4 = new ArcfourEncryption();
    var val = DateTime.Now.Ticks;  // Predictable seed!
    // Seeds RC4 with timestamp...
}

What is the new behavior?

Replaced the predictable RC4-based PRNG with .NET's cryptographically secure RandomNumberGenerator:

public static byte[] GetIv(int len)
{
    var b = new byte[len];
    using (var rng = RandomNumberGenerator.Create())
    {
        rng.GetBytes(b);
    }
    return b;
}

This ensures:

• ✅ Cryptographically secure random IVs
• ✅ No predictability based on creation time
• ✅ Proper entropy for encryption security (prevents chosen plaintext attacks)
• ✅ Simpler, more maintainable code (removed RC4 complexity)

Does this PR introduce a breaking change?

• Yes
• No

The change is a drop-in replacement with the same API. All existing code continues to work unchanged, with improved security.

[gemini-code-assist]

Summary of Changes

Hello @michaelcheers, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical security vulnerability in the IvGenerator by replacing a predictable, timestamp-seeded pseudo-random number generator with .NET's cryptographically secure RandomNumberGenerator. This ensures that Initialization Vectors (IVs) used for PDF encryption are truly random and unpredictable, significantly enhancing the security of encrypted PDFs and preventing potential decryption by attackers. The change also simplifies the underlying code structure.

Highlights

• Security Vulnerability Fix: Addressed a critical security vulnerability where the IvGenerator used a predictable pseudo-random number generator (RC4 seeded with DateTime.Now.Ticks) for PDF encryption Initialization Vectors (IVs), making them guessable.
• Cryptographically Secure IV Generation: Replaced the insecure RC4-based PRNG with .NET's System.Security.Cryptography.RandomNumberGenerator to ensure cryptographically secure and unpredictable IVs for enhanced PDF encryption.
• Code Simplification: Simplified the IvGenerator class by removing the ArcfourEncryption dependency and its associated static constructor logic, resulting in more maintainable and secure code.

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[what-the-diff]

PR Summary

• Enhanced Security Through Random Number Generation
  This change removes the previous method of random number generation which was based on the RC4 algorithm, a less secure technique. Instead, a feature called RandomNumberGenerator has been employed, known for its superior strength in generating numbers for cryptography, rendering our system more secure.

• Updated Comments for Better Understanding
  To help future developers easily understand the changes, the comments explaining the generation of the random initialization vector have been revised.

• Simplified Initialization Process
  The initialization process has been streamlined, with the removal of the prior RC4 instance setup from the static constructor. This makes our codebase cleaner and easier to manage.

[gemini-code-assist]

Code Review

This pull request effectively addresses a critical security vulnerability by replacing the predictable RC4-based pseudo-random number generator with .NET's cryptographically secure RandomNumberGenerator for IV generation. The changes are well-implemented, significantly enhancing the security of PDF encryption by ensuring IVs are truly random and unpredictable. The removal of the outdated RC4 implementation also simplifies the code and improves maintainability. This is a crucial and well-executed security fix.