Encryption Tool

By Alex • Published: 6 November 2025 • Updated: 6 November 2025

Client-Side File Encryption

Encrypt and decrypt files locally in your browser using a password-derived key. Files and passwords never leave this device. If you forget the password, the data cannot be recovered.

Cryptography details: AES-256-GCM with a key derived from your password using PBKDF2 (SHA-256) and a high iteration count. A random salt and IV are generated for every encryption. The encrypted file format is: [salt (16 bytes)][iv (12 bytes)][ciphertext].

Context, threat model and limitations

This tool sits in the “practical crypto” space: it shows how password-based encryption can be implemented with modern browser APIs, and it is suitable for small personal files or demonstrations where you want to keep data off a server. It is not a replacement for a full enterprise key management system, an audited product, or a structured backup process.

The threat model assumed here is that your main risk is someone gaining access to a copy of the encrypted file – for example, through a lost USB stick or cloud account – and trying to decrypt it offline. Using a strong, unique passphrase makes that offline attack impractical. What this tool does not protect you from is a compromised device, malware with keylogging capabilities, or someone looking over your shoulder while you type the password.

In short: if your device is already compromised, browser-based encryption cannot save you. Equally, if you choose a weak or reused password, you are giving an attacker a much easier job. Treat the password as seriously as you would for your primary email or password manager account.

When this tool is a good fit

• Encrypting a small number of personal files before storing them somewhere you don’t fully control.
• Demonstrating password-based encryption and decryption in teaching, workshops or talks.
• Testing how AES-GCM and password-derived keys behave in a zero-knowledge, browser-only setup.

When you should look for something else

• You need centralised key management, access control, or audit trails.
• You are handling highly regulated data with formal compliance requirements.
• You need multi-user sharing, automatic backups, or integration with other systems.

Cryptographic design choices

Under the hood, this page uses the window.crypto.subtle API provided by your browser. A random salt and initialisation vector (IV) are generated for every encryption. The password is turned into a key using PBKDF2 with SHA-256 and a high iteration count, and that key is then used with AES-256-GCM to encrypt the file.

The encrypted file format is deliberately simple: [salt (16 bytes)][iv (12 bytes)][ciphertext]. This makes it easier to explain, inspect, or port to another environment if you want to write a compatible decryptor in a different language.

As with any small browser tool, this implementation relies on the security of your browser, operating system and hardware. Subtle cryptographic flaws or implementation bugs are beyond the scope of what a single static page can hope to detect, but the approach here follows well-understood patterns from modern cryptography rather than inventing anything new.

My background and why I built this

I built this tool as part of a broader set of CyberAlex projects to make security concepts concrete and transparent. My background is in Computer Security and Cyber Security, Risk & Resilience, with formal study covering cryptography, secure software design, and how real attacks play out in practice rather than just in theory.

The goal here is not to claim this page is a silver bullet, but to give you a clear, inspectable example of client-side file encryption done in a way that lines up with modern guidance. If you have ideas for improvements, want to sanity-check the approach, or would like to see other tools in this space, feel free to reach out via the contact link below.

Questions, change log and sharing