c1144c9b8a
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 <paulcrowley@google.com> Reviewed-by: Theodore Ts'o <tytso@mit.edu> Signed-off-by: Eric Biggers <ebiggers@google.com>
19 lines
540 B
Plaintext
19 lines
540 B
Plaintext
# SPDX-License-Identifier: GPL-2.0-only
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config FS_ENCRYPTION
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bool "FS Encryption (Per-file encryption)"
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select CRYPTO
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select CRYPTO_AES
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select CRYPTO_CBC
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select CRYPTO_ECB
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select CRYPTO_XTS
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select CRYPTO_CTS
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select CRYPTO_SHA512
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select CRYPTO_HMAC
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select KEYS
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help
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Enable encryption of files and directories. This
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feature is similar to ecryptfs, but it is more memory
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efficient since it avoids caching the encrypted and
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decrypted pages in the page cache. Currently Ext4,
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F2FS and UBIFS make use of this feature.
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