Re: [PATCH v8 12/20] fscrypt: add an HKDF-SHA512 implementation

[Paul Crowley <paulcrowley@xxxxxxxxxx>]

On Mon, 5 Aug 2019 at 09:28, Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> Add an implementation of HKDF (RFC 5869) to fscrypt, for the purpose of
> deriving additional key material from the fscrypt master keys for v2
> encryption policies.  HKDF is a key derivation function built on top of
> HMAC.  We choose SHA-512 for the underlying unkeyed hash, and use an
> "hmac(sha512)" transform allocated from the crypto API.
>
> We'll be using this to replace the AES-ECB based KDF currently used to
> derive the per-file encryption keys.  While the AES-ECB based KDF is
> believed to meet the original security requirements, it is nonstandard
> and has problems that don't exist in modern KDFs such as HKDF:
>
> 1. It's reversible.  Given a derived key and nonce, an attacker can
>    easily compute the master key.  This is okay if the master key and
>    derived keys are equally hard to compromise, but now we'd like to be
>    more robust against threats such as a derived key being compromised
>    through a timing attack, or a derived key for an in-use file being
>    compromised after the master key has already been removed.
>
> 2. It doesn't evenly distribute the entropy from the master key; each 16
>    input bytes only affects the corresponding 16 output bytes.
>
> 3. It isn't easily extensible to deriving other values or keys, such as
>    a public hash for securely identifying the key, or per-mode keys.
>    Per-mode keys will be immediately useful for Adiantum encryption, for
>    which fscrypt currently uses the master key directly, introducing
>    unnecessary usage constraints.  Per-mode keys will also be useful for
>    hardware inline encryption, which is currently being worked on.
>
> HKDF solves all the above problems.
>
> Reviewed-by: Theodore Ts'o <tytso@xxxxxxx>
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

Looks good, feel free to add:

Reviewed-by: Paul Crowley <paulcrowley@xxxxxxxxxx>

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