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I have a Poly1305-implementation (not Poly1305-AES) in my library.

The difference: Poly1305-AES uses a key and a nonce and then uses AES to generate a one-time-key from key + nonce. This is necessary, because Poly1305 requires one-time-keys. However if you are generating a unique random key anyway, you can leave out the AES-part. (short source, original source)


My question: Is it secure to generate a unique one-time-key from a static key using another KDF with unique salt (like PBKDF2)?

(I think you could call this Poly1305-PBKDF2 or sth. like this)

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Yes this would be secure.

Why would it be secure?
Because you would be running HMAC (the underlying primitive of PBKDF) as a stream-cipher using some key-derivation function.
As this is a common approach (at least for integrated encryption schemes) using this would at least as secure as using the bare KDF and a salt.

However I'd strongly argue against using PBKDF2 for anything but password-based key derivation and as a "stream cipher" it would give people a wrong sense of security (because it's 1000x slower = 10 bits more security).

If you want to use Poly-1305 I'd recommend you using a dedicated stream cipher (e.g. ChaCha20) or a dedicated stream cipher mode (e.g. CTR).

  • Well ChaCha20 is also available in the lib, so I think I will use this. Thank you :) – K. Biermann May 3 '15 at 15:15

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