-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi all,
I wonder if anyone on the list can help me to understand the purpose and correct use of HKDF's salt parameter. RFC 5869 has this to say: HKDF is defined to operate with and without random salt. This is done to accommodate applications where a salt value is not available. We stress, however, that the use of salt adds significantly to the strength of HKDF, ensuring independence between different uses of the hash function, supporting "source-independent" extraction, and strengthening the analytical results that back the HKDF design. Random salt differs fundamentally from the initial keying material in two ways: it is non-secret and can be re-used. As such, salt values are available to many applications. For example, a pseudorandom number generator (PRNG) that continuously produces outputs by applying HKDF to renewable pools of entropy (e.g., sampled system events) can fix a salt value and use it for multiple applications of HKDF without having to protect the secrecy of the salt. In a different application domain, a key agreement protocol deriving cryptographic keys from a Diffie-Hellman exchange can derive a salt value from public nonces exchanged and authenticated between communicating parties as part of the key agreement (this is the approach taken in [IKEv2]). My understanding of the above is that the salt doesn't increase the entropy of HKDF's output from the adversary's point of view, since the adversary knows the salt value. However, the salt prevents accidental collisions if identical initial keying material is used in multiple application domains. Is that right? Can anyone shed light on the meaning of "source-independent extraction"? The RFC continues: Ideally, the salt value is a random (or pseudorandom) string of the length HashLen. Yet, even a salt value of less quality (shorter in size or with limited entropy) may still make a significant contribution to the security of the output keying material; designers of applications are therefore encouraged to provide salt values to HKDF if such values can be obtained by the application. This doesn't sit well with my interpretation above, because it suggests that the salt contains entropy (from someone's point of view) that contributes to the security of HKDF's output. But how can the salt be said to contain entropy when its value is non-secret? It is worth noting that, while not the typical case, some applications may even have a secret salt value available for use; in such a case, HKDF provides an even stronger security guarantee. An example of such application is IKEv1 in its "public-key encryption mode", where the "salt" to the extractor is computed from nonces that are secret; similarly, the pre-shared mode of IKEv1 uses a secret salt derived from the pre-shared key. This seems unsurprising - if the salt value is unknown to the adversary then clearly it can contribute entropy to HKDF's output. Going back to the issue of non-secret salt, here's a thought experiment: we generate a random salt value, publish it in the New York Times, and use it for all calls to HKDF in a certain application domain. Is this somehow more secure than using no salt? If so, can you help me to understand how? Less extremely: each time we use HKDF, we generate a fresh random salt value and publish it in the New York Times. Is this more secure than using no salt? How? Thanks, Michael -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) iQEcBAEBAgAGBQJR+ieDAAoJEBEET9GfxSfMVsIH/3dsAhF4FukIcdVLa/Kw782A akbTjnYHAvwdvRi3fVBrXejM3csya9psSu2qVIgAUXWaMxRVcvPkUoTc7NF+MC65 xVS4j1YcmkEQL7L7LnUQVukISzBO3NgwmAKPrxdzeXLJlaiL9N51ecYmjC0jo9Ou dHs9108z2AQHYZ/n4PhRCVdSPjIA5/Z6kusu6cOQsZHTzeNbmoTuOafZTHFkESbX TmSVS4m54vgQWukTsjGsHDDoemvGzY4ahfZj8l+oOSr3OUP3MdYaxaQEXxq6ZQ3L fdNkdxnpOznz+e14RQzIOFjr8QbWBjwlGFp5CxaMPgKL9a5cKuU9JIxjLsUWyXs= =ZaC7 -----END PGP SIGNATURE----- _______________________________________________ cryptography mailing list cryptography@randombit.net http://lists.randombit.net/mailman/listinfo/cryptography
