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Allow bypasing signature hash key matching #2310

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Allow bypasing signature hash key matching #2310

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f2503a2
bootutil: Support for MCUBOOT_BYPASS_KEY_MATCH
de-nordic May 15, 2025
d9f52fa
zephyr: Add CONFIG_BOOT_BYPASS_KEY_MATCH
de-nordic May 15, 2025
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8 changes: 8 additions & 0 deletions boot/bootutil/src/image_validate.c
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Original file line number Diff line number Diff line change
Expand Up @@ -271,6 +271,8 @@ bootutil_img_hash(struct boot_loader_state *state,
# define KEY_BUF_SIZE (SIG_BUF_SIZE + 24)
#endif /* !MCUBOOT_HW_KEY */

#if !defined(MCUBOOT_BYPASS_KEY_MATCH)
/* Find functions are only needed when key is checked first */
#if !defined(MCUBOOT_HW_KEY)
static int
bootutil_find_key(uint8_t *keyhash, uint8_t keyhash_len)
Expand Down Expand Up @@ -337,6 +339,7 @@ bootutil_find_key(uint8_t image_index, uint8_t *key, uint16_t key_len)
#endif /* !MCUBOOT_HW_KEY */
#endif /* !MCUBOOT_BUILTIN_KEY */
#endif /* EXPECTED_SIG_TLV */
#endif /* !MCUBOOT_BYPASS_KEY_MATCH */

/**
* Reads the value of an image's security counter.
Expand Down Expand Up @@ -631,7 +634,12 @@ bootutil_img_validate(struct boot_loader_state *state,
if (rc) {
goto out;
}
#if !defined(MCUBOOT_BYPASS_KEY_MATCH)
key_id = bootutil_find_key(buf, len);
#else
/* There is only one key */
key_id = 0;
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Copilot AI May 28, 2025

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[nitpick] Consider expanding the comment to clarify that returning key_id 0 is intentional when MCUBOOT_BYPASS_KEY_MATCH is defined, which improves code readability and maintainability.

Copilot uses AI. Check for mistakes.

#endif
#else
rc = LOAD_IMAGE_DATA(hdr, fap, off, key_buf, len);
if (rc) {
Expand Down
13 changes: 13 additions & 0 deletions boot/zephyr/Kconfig
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Expand Up @@ -325,6 +325,19 @@ endif

endchoice

config BOOT_BYPASS_KEY_MATCH
bool "Do not match TLV key hash against built in key"
depends on !BOOT_SIGNATURE_TYPE_NONE
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depends on !hw key?

depends on !BOOT_HW_KEY
help
MCUboot reads, from TLV, hash of key thath should be used to verify
signature and tries to match it against list of keys, to select the
key from known keys. This pointless when there is only single key
compiled in, as the key can be used whether it is the right one
or not, the signature verification process will verify the key.
Enabling this option turns off key matching, slightly reducing
MCUboot code and boot time.

config BOOT_SIGNATURE_KEY_FILE
string "PEM key file"
default "root-ec-p256.pem" if BOOT_SIGNATURE_TYPE_ECDSA_P256
Expand Down
9 changes: 9 additions & 0 deletions boot/zephyr/include/mcuboot_config/mcuboot_config.h
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Expand Up @@ -153,6 +153,15 @@
#define MCUBOOT_ENCRYPT_X25519
#endif

/* Turn off check of public key hash against compiled in key
* before attempting signature verification. When there is only
* one key, matching is pointless, the signature may just be
* verified with the only key that there is.
*/
#ifdef CONFIG_BOOT_BYPASS_KEY_MATCH
#define MCUBOOT_BYPASS_KEY_MATCH
#endif

#ifdef CONFIG_BOOT_DECOMPRESSION
#define MCUBOOT_DECOMPRESS_IMAGES
#endif
Expand Down
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