How Can Nonces Secure Web Requests Against Replay Attacks?

How to Secure Web Requests with Nonces

Issue

A user has found a way to exploit the request validation system of a website's scoring system by duplicating high-value HTTP requests. This compromises the integrity and reliability of the system.

Solution: Implementing a Nonce System

Nonces (number used once) are values that prevent request replay attacks by ensuring that a particular request has not been made before. Here's a common and secure way to implement a nonce system:

Server-Side Nonce Generation and Verification

getNonce() Function

• Identifies the client making the request (e.g., by username, session).
• Generates a random nonce using a secure hash function (e.g., SHA512).
• Stores the nonce in a database associated with the client's ID.
• Returns the nonce to the client.

verifyNonce() Function

• Fetches the previously stored nonce for the client ID.
• Removes the nonce from the database (to prevent it from being reused).
• Generates a hash with the client-provided nonce (cnonce), the request data, and a secret salt.
• Compares the generated hash to the hash provided by the client.
• Returns true if the hashes match, indicating a valid nonce.

Client-Side Nonce Usage

sendData() Function

• Retrieves the nonce from the server using the getNonce() function.
• Generates a client-specific nonce (cnonce) using a secure hash function.
• Concatenates the server nonce, client nonce, and request data.
• Generates a hash from the concatenated value.
• Sends the request to the server, including the data, cnonce, and hash.

Security Considerations

• Random Nonce Generation: The makeRandomString() function should generate highly unpredictable random numbers to enhance security.
• Secure Hash Function: Utilize a strong hash function like SHA512 or bcrypt for nonce-related hash computations.
• Single-Use Per Request: Nonces should only be used once and removed from storage to prevent replay attacks.

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