Messages in this thread | | | Subject | Re: [PATCH 1/5 v2] PM / hibernate: Create snapshot keys handler | From | Andy Lutomirski <> | Date | Tue, 8 Jan 2019 15:54:22 -0800 |
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> On Jan 7, 2019, at 11:09 PM, Stephan Mueller <smueller@chronox.de> wrote: > > Am Dienstag, 8. Januar 2019, 06:03:58 CET schrieb Herbert Xu: > > Hi Herbert, > >> Are we going to have multiple implementations for the same KDF? >> If not then the crypto API is not a good fit. To consolidate >> multiple implementations of the same KDF, simply provide helpers >> for them. > > It is unlikely to have multiple implementations of a KDF. However, KDFs relate > to hashes like block chaining modes to raw block ciphers. Thus a KDF can be > applied with different hashes. > > My idea was to add template support to RNGs (because KDFs are effectively a > type of RNG since they produce an arbitrary output from a fixed input). The > KDFs would be a template wrapping hashes. For example, the CTR-KDF from > SP800-108 could be instantiated like kdf-ctr(sha256). > >
I think that, if the crypto API is going to grow a KDF facility, it should be done right. Have a key type or flag or whatever that says “this key may *only* be used to derive keys using such-and-such algorithm”, and have a helper to derive a key. That helper should take some useful parameters and mix them in:
- What type of key is being derived? ECDSA signing key? HMAC key? AES key?
- Can user code access the derived key?
- What is the key’s purpose? “Encrypt and authenticate a hibernation image” would be a purpose.
- Number of bytes.
All of these parameters should be mixed in to the key derivation.
Also, an AE key, even for AES+HMAC, should be just one derived key. If you need 512 bits, ask for a 512-bit key, not two 256-bit keys.
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