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>

4 files changed, 113 insertions, 35 deletions

diff --git a/board/cr50/u2f.c b/board/cr50/u2f.c
index 7c6273823c..7fe1f6484f 100644
--- a/board/cr50/u2f.c
+++ b/board/cr50/u2f.c
@@ -201,7 +201,8 @@ int u2f_origin_user_keyhandle(const uint8_t *origin, const uint8_t *user,
 
 int u2f_origin_user_versioned_keyhandle(
 	const uint8_t *origin, const uint8_t *user, const uint8_t *origin_seed,
-	uint8_t version, struct u2f_versioned_key_handle *key_handle)
+	uint8_t version,
+	struct u2f_versioned_key_handle_header *key_handle_header)
 {
 	LITE_HMAC_CTX ctx;
 	struct u2f_state *state = get_state();
@@ -209,16 +210,17 @@ int u2f_origin_user_versioned_keyhandle(
 	if (!state)
 		return EC_ERROR_UNKNOWN;
 
-	key_handle->version = version;
-	memcpy(key_handle->origin_seed, origin_seed, P256_NBYTES);
+	key_handle_header->version = version;
+	memcpy(key_handle_header->origin_seed, origin_seed, P256_NBYTES);
 
 	DCRYPTO_HMAC_SHA256_init(&ctx, state->salt_kek, SHA256_DIGEST_SIZE);
 	HASH_update(&ctx.hash, origin, P256_NBYTES);
 	HASH_update(&ctx.hash, user, P256_NBYTES);
 	HASH_update(&ctx.hash, origin_seed, P256_NBYTES);
-	HASH_update(&ctx.hash, &version, sizeof(key_handle->version));
+	HASH_update(&ctx.hash, &version, sizeof(key_handle_header->version));
 
-	memcpy(key_handle->hmac, DCRYPTO_HMAC_final(&ctx), SHA256_DIGEST_SIZE);
+	memcpy(key_handle_header->kh_hmac, DCRYPTO_HMAC_final(&ctx),
+	       SHA256_DIGEST_SIZE);
 
 	return EC_SUCCESS;
 }
@@ -250,6 +252,27 @@ int u2f_origin_user_keypair(const uint8_t *key_handle, size_t key_handle_size,
 	return EC_SUCCESS;
 }
 
+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)
+{
+	LITE_HMAC_CTX ctx;
+	struct u2f_state *state = get_state();
+
+	if (!state)
+		return EC_ERROR_UNKNOWN;
+
+	DCRYPTO_HMAC_SHA256_init(&ctx, state->salt_kek, SHA256_DIGEST_SIZE);
+	HASH_update(&ctx.hash, authorization_salt, U2F_AUTHORIZATION_SALT_SIZE);
+	HASH_update(&ctx.hash, (uint8_t *)header,
+		    sizeof(struct u2f_versioned_key_handle_header));
+	HASH_update(&ctx.hash, auth_time_secret_hash, SHA256_DIGEST_SIZE);
+
+	memcpy(hmac, DCRYPTO_HMAC_final(&ctx), SHA256_DIGEST_SIZE);
+
+	return EC_SUCCESS;
+}
+
 int u2f_gen_kek(const uint8_t *origin, uint8_t *kek, size_t key_len)
 {
 	uint32_t buf[P256_NDIGITS];
diff --git a/common/u2f.c b/common/u2f.c
index eaeb38b08c..91cb4ab121 100644
--- a/common/u2f.c
+++ b/common/u2f.c
@@ -10,6 +10,7 @@
 #include "cryptoc/sha256.h"
 #include "dcrypto.h"
 #include "extension.h"
+#include "fips_rand.h"
 #include "system.h"
 #include "u2f_impl.h"
 #include "u2f.h"
@@ -127,8 +128,10 @@ static enum vendor_cmd_rc u2f_generate(enum vendor_cmd_cc code, void *buf,
 		struct u2f_key_handle kh;
 		struct u2f_versioned_key_handle vkh;
 	} kh_buf;
-	size_t kh_size = (kh_version == 0) ? sizeof(kh_buf.kh) :
-					     sizeof(kh_buf.vkh);
+	size_t keypair_input_size =
+		(kh_version == 0) ?
+			sizeof(kh_buf.kh) :
+			sizeof(struct u2f_versioned_key_handle_header);
 
 	/* Whether key handle generation succeeded */
 	int generate_kh_rc;
@@ -168,13 +171,14 @@ static enum vendor_cmd_rc u2f_generate(enum vendor_cmd_cc code, void *buf,
 		else
 			generate_kh_rc = u2f_origin_user_versioned_keyhandle(
 				req->appId, req->userSecret, od_seed,
-				kh_version, &kh_buf.vkh);
+				kh_version, &kh_buf.vkh.header);
 
 		if (generate_kh_rc != EC_SUCCESS)
 			return VENDOR_RC_INTERNAL_ERROR;
 
 		generate_keypair_rc = u2f_origin_user_keypair(
-			(uint8_t *)&kh_buf, kh_size, &od, &opk_x, &opk_y);
+			(uint8_t *)&kh_buf, keypair_input_size, &od, &opk_x,
+			&opk_y);
 	} while (generate_keypair_rc == EC_ERROR_TRY_AGAIN);
 
 	if (generate_keypair_rc != EC_SUCCESS)
@@ -188,6 +192,17 @@ static enum vendor_cmd_rc u2f_generate(enum vendor_cmd_cc code, void *buf,
 		copy_kh_pubkey_out(&opk_x, &opk_y, &kh_buf.kh, buf);
 		*response_size = sizeof(struct u2f_generate_resp);
 	} else {
+		if (!fips_rand_bytes(kh_buf.vkh.authorization_salt,
+				     U2F_AUTHORIZATION_SALT_SIZE))
+			return VENDOR_RC_INTERNAL_ERROR;
+
+		if (u2f_authorization_hmac(kh_buf.vkh.authorization_salt,
+					   &kh_buf.vkh.header,
+					   req->authTimeSecretHash,
+					   kh_buf.vkh.authorization_hmac) !=
+		    EC_SUCCESS)
+			return VENDOR_RC_INTERNAL_ERROR;
+
 		copy_versioned_kh_pubkey_out(&opk_x, &opk_y, &kh_buf.vkh, buf);
 		*response_size = sizeof(struct u2f_generate_versioned_resp);
 	}
@@ -242,14 +257,14 @@ static int verify_kh_owned(const uint8_t *user_secret, const uint8_t *app_id,
 	return rc;
 }
 
-static int
-verify_versioned_kh_owned(const uint8_t *user_secret, const uint8_t *app_id,
-			  const struct u2f_versioned_key_handle *key_handle,
-			  int *owned)
+static int verify_versioned_kh_owned(
+	const uint8_t *user_secret, const uint8_t *app_id,
+	const struct u2f_versioned_key_handle_header *key_handle_header,
+	int *owned)
 {
 	int rc;
 	/* Re-created key handle. */
-	struct u2f_versioned_key_handle recreated_kh;
+	struct u2f_versioned_key_handle_header recreated_kh_header;
 
 	/*
 	 * Re-create the key handle and compare against that which
@@ -258,13 +273,13 @@ verify_versioned_kh_owned(const uint8_t *user_secret, const uint8_t *app_id,
 	 */
 
 	rc = u2f_origin_user_versioned_keyhandle(app_id, user_secret,
-						 key_handle->origin_seed,
-						 key_handle->version,
-						 &recreated_kh);
+						 key_handle_header->origin_seed,
+						 key_handle_header->version,
+						 &recreated_kh_header);
 
 	if (rc == EC_SUCCESS)
-		*owned = safe_memcmp(&recreated_kh, key_handle,
-				     sizeof(recreated_kh)) == 0;
+		*owned = safe_memcmp(&recreated_kh_header, key_handle_header,
+				     sizeof(recreated_kh_header)) == 0;
 
 	return rc;
 }
@@ -321,8 +336,8 @@ static enum vendor_cmd_rc u2f_sign(enum vendor_cmd_cc code, void *buf,
 	/* Version of KH; 0 if KH is not versioned. */
 	uint8_t version;
 
-	/* Size of KH in bytes. */
-	size_t kh_size;
+	/* Size of the part of KH used to derive keypair, in bytes. */
+	size_t keypair_input_size;
 
 	int verify_owned_rc;
 
@@ -334,18 +349,19 @@ static enum vendor_cmd_rc u2f_sign(enum vendor_cmd_cc code, void *buf,
 		key_handle = (uint8_t *)&req->keyHandle;
 		hash = req->hash;
 		flags = req->flags;
-		kh_size = sizeof(struct u2f_key_handle);
+		keypair_input_size = sizeof(struct u2f_key_handle);
 		verify_owned_rc = verify_kh_owned(req->userSecret, req->appId,
 						  &req->keyHandle, &kh_owned);
 	} else if (input_size == sizeof(struct u2f_sign_versioned_req)) {
-		version = req_versioned->keyHandle.version;
+		version = req_versioned->keyHandle.header.version;
 		key_handle = (uint8_t *)&req_versioned->keyHandle;
 		hash = req_versioned->hash;
 		flags = req_versioned->flags;
-		kh_size = sizeof(struct u2f_versioned_key_handle);
+		keypair_input_size =
+			sizeof(struct u2f_versioned_key_handle_header);
 		verify_owned_rc = verify_versioned_kh_owned(
 			req_versioned->userSecret, req_versioned->appId,
-			&req_versioned->keyHandle, &kh_owned);
+			&req_versioned->keyHandle.header, &kh_owned);
 	} else {
 		return VENDOR_RC_BOGUS_ARGS;
 	}
@@ -378,17 +394,28 @@ static enum vendor_cmd_rc u2f_sign(enum vendor_cmd_cc code, void *buf,
 	if ((flags & U2F_AUTH_CHECK_ONLY) == U2F_AUTH_CHECK_ONLY)
 		return VENDOR_RC_SUCCESS;
 
-	/* Always enforce user presence, with optional consume. */
-	if (pop_check_presence(flags & G2F_CONSUME) != POP_TOUCH_YES)
-		return VENDOR_RC_NOT_ALLOWED;
+	/*
+	 * Enforce user presence for version 0 KHs, with optional consume.
+	 */
+	if (pop_check_presence(flags & G2F_CONSUME) != POP_TOUCH_YES) {
+		if (version != U2F_KH_VERSION_1)
+			return VENDOR_RC_NOT_ALLOWED;
+		if ((flags & U2F_AUTH_FLAG_TUP) != 0)
+			return VENDOR_RC_NOT_ALLOWED;
+		/*
+		 * TODO(yichengli): When auth-time secrets is ready, enforce
+		 * authorization hmac when no power button press.
+		 */
+	}
 
 	/* Re-create origin-specific key. */
 	if (legacy_kh) {
 		if (u2f_origin_key(legacy_origin_seed, &origin_d) != EC_SUCCESS)
 			return VENDOR_RC_INTERNAL_ERROR;
 	} else {
-		if (u2f_origin_user_keypair(key_handle, kh_size, &origin_d,
-					    NULL, NULL) != EC_SUCCESS)
+		if (u2f_origin_user_keypair(key_handle, keypair_input_size,
+					    &origin_d, NULL,
+					    NULL) != EC_SUCCESS)
 			return VENDOR_RC_INTERNAL_ERROR;
 	}
 
diff --git a/include/u2f.h b/include/u2f.h
index 61b9677185..6680ef5300 100644
--- a/include/u2f.h
+++ b/include/u2f.h
@@ -31,6 +31,8 @@ extern "C" {
 #define U2F_MAX_ATTEST_SIZE   256 /* Size of largest blob to sign */
 #define U2F_P256_SIZE	      32
 
+#define SHA256_DIGEST_SIZE    32
+
 #define ENC_SIZE(x) ((x + 7) & 0xfff8)
 
 /* EC (uncompressed) point */
@@ -53,15 +55,24 @@ struct u2f_ec_point {
 
 #define U2F_KH_VERSION_1 0x01
 
+#define U2F_AUTHORIZATION_SALT_SIZE 16
+
 struct u2f_key_handle {
 	uint8_t origin_seed[U2F_P256_SIZE];
-	uint8_t hmac[U2F_P256_SIZE];
+	uint8_t hmac[SHA256_DIGEST_SIZE];
 };
 
-struct u2f_versioned_key_handle {
+struct u2f_versioned_key_handle_header {
 	uint8_t version;
 	uint8_t origin_seed[U2F_P256_SIZE];
-	uint8_t hmac[U2F_P256_SIZE];
+	uint8_t kh_hmac[SHA256_DIGEST_SIZE];
+};
+
+struct u2f_versioned_key_handle {
+	struct u2f_versioned_key_handle_header header;
+	/* Optionally checked in u2f_sign. */
+	uint8_t authorization_salt[U2F_AUTHORIZATION_SALT_SIZE];
+	uint8_t authorization_hmac[SHA256_DIGEST_SIZE];
 };
 
 /* TODO(louiscollard): Add Descriptions. */
@@ -70,6 +81,11 @@ struct u2f_generate_req {
 	uint8_t appId[U2F_APPID_SIZE]; /* Application id */
 	uint8_t userSecret[U2F_P256_SIZE];
 	uint8_t flags;
+	/*
+	 * If generating versioned KH, derive an hmac from it and append to
+	 * the key handle. Otherwise unused.
+	 */
+	uint8_t authTimeSecretHash[SHA256_DIGEST_SIZE];
 };
 
 struct u2f_generate_resp {
@@ -93,6 +109,7 @@ struct u2f_sign_req {
 struct u2f_sign_versioned_req {
 	uint8_t appId[U2F_APPID_SIZE]; /* Application id */
 	uint8_t userSecret[U2F_P256_SIZE];
+	uint8_t authTimeSecret[U2F_P256_SIZE];
 	uint8_t hash[U2F_P256_SIZE];
 	uint8_t flags;
 	struct u2f_versioned_key_handle keyHandle;
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.
  *