Great question—let’s unpack this clearly to resolve your confusion.

Is the HMAC Nonce a confidential piece of information? #

Short answer: No.

HMAC’s security model is rooted entirely in the confidentiality of your pre-shared symmetric key, not the nonce. The nonce is designed to be sent in plaintext alongside your message and HMAC. Even if an attacker intercepts or obtains the nonce, they can’t use it to forge a valid HMAC for a new message without knowing the secret key.

What happens if an attacker can predict or obtain the plaintext nonce? #

This doesn’t break HMAC’s core anti-forgery security, but it does undermine a critical secondary purpose of the nonce: preventing replay attacks.

• If an attacker can predict or reuse a nonce, they can resend a previously valid message+HMAC pair. Since the same nonce, message, and key will produce identical HMAC values, your system might accept the replayed message as legitimate.
• However, the attacker still can’t generate a new valid HMAC for a fake message—they’d still need the secret key for that. The risk here is replay, not direct forgery of new messages.

Why use a Cryptographically Secure PRNG (CSPRNG) for nonce generation? #

This is where the confusion often lies—CSPRNGs aren’t required because their algorithm or seed is part of the pre-shared key. Instead, they solve two key problems for nonces:

• Unpredictability: A CSPRNG generates nonces that an attacker can’t guess or predict. This makes it impossible for them to precompute potential HMAC values for future messages or prepare replay attacks in advance.
• Uniqueness: CSPRNGs ensure that nonces are highly unlikely to repeat (within a reasonable operational window). Repeating nonces paired with identical message content would produce identical HMACs, which directly enables replay attacks.

The seed for the CSPRNG doesn’t need to be tied to your pre-shared key—it just needs to be a secure, unique starting point to ensure the nonces meet these two criteria.

Final Takeaways #

• Nonces for HMAC do not need to be kept confidential; they can be transmitted in plaintext.
• Predictable or exposed nonces don’t break HMAC’s core security, but they eliminate its anti-replay protections.
• CSPRNGs are used to generate nonces that are unpredictable and unique, not because their internal details are part of your pre-shared key set.

内容的提问来源于stack exchange，提问作者wondra