Reland "u2f: Append hmac of auth time secret to versioned KH"stabilize-quickfix-13421.74.B-cr50_stabstabilize-quickfix-13421.100.B-cr50_stabstabilize-13421.99.B-cr50_stabstabilize-13421.96.B-cr50_stabstabilize-13421.89.B-cr50_stabstabilize-13421.80.B-cr50_stabstabilize-13421.73.B-cr50_stabstabilize-13421.53.B-cr50_stabstabilize-13421.42.B-cr50_stabstabilize-13421.103.B-cr50_stabstabilize-13421.102.B-cr50_stabrelease-R86-13421.B-cr50_stabfirmware-zork-13421.B-cr50_stab

This is a reland of d2627d12bb21308f49a72cadaf47a0a86730a960 with one
modification: The versioned key handle header (the old "key handle"
concept) is now used in the derivation of authorization_hmac. This is
to tie the key handle to the authorization secret.

Original change's description:
> u2f: Append hmac of auth time secret to versioned KH
>
> When generating versioned KHs, u2fd should send a public derivative
> (sha256) of the user's auth time secret to cr50. Cr50 derives an
> hmac of it and appends this authorization_hmac to the KH.
>
> When signing versioned KHs, u2fd may supply the unhashed auth time
> secret. Cr50 will check the authorization_hmac if no power button press.
> If the reconstructed hmac matches authorization_hmac, power button press
> is waived.
>
> Currently for v1, we will just prepare the authorization_hmac but not
> enforce it. This is because fingerprint and PIN are unable to unlock
> the same secret.
>
> While we waive power button press for v1, we can enforce
> authorization_hmac whenever auth-time secrets is ready.
>
> BUG=b:144861739
> TEST=- Use a known 32-byte "auth-time secret"
>      - Compute the sha256 of the auth-time secret (this is public)
>      - u2f_generate with the computed "authTimeSecretHash"
>      - Add code to u2f_sign command handler such that cr50 computes
>        the sha256 of the supplied auth-time secret at u2f_sign time
>        and require power button press if the hmac doesn't match.
>      - u2f_sign with the true auth-time secret -> observe in logging
>        that hmac matches, and no power button press required.
>      - u2f_sign with a wrong auth-time secret -> observe in logging
>        that hmac doesn't match, and power button press is required
>        for signing.
>
> Cq-Depend: chromium:2321731
> Change-Id: Ib9ae913667f8178ac7a4790f861d7dada972c4a0
> Signed-off-by: Yicheng Li <yichengli@chromium.org>
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2317047
> Reviewed-by: Andrey Pronin <apronin@chromium.org>
> Reviewed-by: Vadim Sukhomlinov <sukhomlinov@chromium.org>

BUG=b:144861739
TEST=See original CL's TEST above

Cq-Depend: chromium:2327865
Change-Id: Ia1b0b4a585ec604398cfa730354ae1a91e7bc00b
Signed-off-by: Yicheng Li <yichengli@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2355177
Reviewed-by: Andrey Pronin <apronin@chromium.org>

1 files changed, 13 insertions, 2 deletions

diff --git a/include/u2f_impl.h b/include/u2f_impl.h
index 5bd69309c6..2c63d11d87 100644
--- a/include/u2f_impl.h
+++ b/include/u2f_impl.h
@@ -76,13 +76,14 @@ int u2f_origin_user_keyhandle(const uint8_t *origin, const uint8_t *user,
  * @param user pointer to user secret
  * @param seed pointer to origin-specific random seed
  * @param version the version byte to pack; should be greater than 0.
- * @param key_handle buffer to hold the output key handle
+ * @param key_handle_header buffer to hold the output key handle header
  *
  * @return EC_SUCCESS if a valid keypair was created.
  */
 int u2f_origin_user_versioned_keyhandle(
 	const uint8_t *origin, const uint8_t *user, const uint8_t *seed,
-	uint8_t version, struct u2f_versioned_key_handle *key_handle);
+	uint8_t version,
+	struct u2f_versioned_key_handle_header *key_handle_header);
 
 /**
  * Generate an origin and user-specific ECDSA keypair from the specified
@@ -101,6 +102,16 @@ int u2f_origin_user_versioned_keyhandle(
 int u2f_origin_user_keypair(const uint8_t *key_handle, size_t key_handle_size,
 			    p256_int *d, p256_int *pk_x, p256_int *pk_y);
 
+/**
+ * Derive an hmac from the given salt, key handle and hash. The salt is to make
+ * sure the hmac is different for different key handles of one user. The key
+ * handle header is encoded into the authorization hmac to protect against
+ * swapping auth time secret.
+ */
+int u2f_authorization_hmac(const uint8_t *authorization_salt,
+			   const struct u2f_versioned_key_handle_header *header,
+			   const uint8_t *auth_time_secret_hash, uint8_t *hmac);
+
 /***
  * Generate a hardware derived 256b private key.
  *