From c1144c9b8ad94d8c11809d75c1f322a853cdfc4a Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Sun, 4 Aug 2019 19:35:47 -0700 Subject: fscrypt: add an HKDF-SHA512 implementation 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: Paul Crowley Reviewed-by: Theodore Ts'o Signed-off-by: Eric Biggers --- fs/crypto/Kconfig | 2 ++ 1 file changed, 2 insertions(+) (limited to 'fs/crypto/Kconfig') diff --git a/fs/crypto/Kconfig b/fs/crypto/Kconfig index 5fdf24877c17..ff5a1746cbae 100644 --- a/fs/crypto/Kconfig +++ b/fs/crypto/Kconfig @@ -7,6 +7,8 @@ config FS_ENCRYPTION select CRYPTO_ECB select CRYPTO_XTS select CRYPTO_CTS + select CRYPTO_SHA512 + select CRYPTO_HMAC select KEYS help Enable encryption of files and directories. This -- cgit v1.2.3