U2F for Encryption: Does it Work?

TL;DR

No, a FIDO U2F (Universal 2nd Factor) device cannot be directly used for symmetric encryption. U2F is designed for strong authentication – proving you *are* who you say you are – not for encrypting and decrypting data. While it can secure keys used in encryption systems, the U2F device itself doesn’t perform the encryption.

Understanding the Difference

It’s easy to get confused because both authentication and encryption involve security. Here’s a breakdown:

• Authentication: Verifying your identity (e.g., logging in). U2F excels at this.
• Encryption: Scrambling data so only authorized parties can read it. This requires an encryption algorithm and a key.

Why U2F Doesn’t Encrypt

U2F devices are hardware security keys that follow a specific protocol. They perform cryptographic operations, but these operations are limited to:

• Generating signatures.
• Verifying signatures.
• Key storage (securely).

They don’t have the functionality to take arbitrary data and encrypt it using a symmetric key.

How U2F *Can* Be Used with Encryption

U2F can play a vital role in securing the keys used for encryption. Here’s how:

1. Key Generation and Storage

1. Generate a Symmetric Key: Use a standard tool (like OpenSSL) to create your symmetric key. For example:

   openssl rand -base64 32

   This creates a 256-bit AES key.

2. Securely Store the Key: Instead of storing the key directly on your computer, you can store it encrypted *on* the U2F device. This is typically done through a higher-level security system or application that supports FIDO.
   Note: Direct storage of symmetric keys on U2F devices isn’t common due to protocol limitations and potential vulnerabilities.

2. Authentication Before Decryption

1. Encryption with a Key: Encrypt your data using the symmetric key.
2. Authentication Required: Before decrypting, require U2F authentication to prove the user is authorized.
   This means the decryption process only happens if the correct U2F device and its registered owner are present.
3. Key Retrieval (if stored on U2F): If the key is stored encrypted on the U2F device, the authentication process unlocks access to it.
4. Decryption: Decrypt the data using the retrieved symmetric key.

Example Scenario

Imagine a password manager:

• The password manager generates a unique AES encryption key for each user account.
• This key is encrypted and stored on the U2F device, protected by the U2F authentication mechanism.
• When you try to access a password, the application requires U2F authentication.
• Only after successful authentication does the application retrieve the decryption key from the U2F device and decrypt your password.

Alternatives for Symmetric Encryption

If you need to directly encrypt/decrypt data using hardware, consider these options:

• Hardware Security Modules (HSMs): Dedicated devices designed specifically for cryptographic operations.
• Trusted Platform Modules (TPMs): Chips built into motherboards that can perform encryption and secure key storage.