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ASN.1 Decoding bypass that allows to reduce size of flash when ED25519 is used #347

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ASN.1 Decoding bypass that allows to reduce size of flash when ED25519 is used #347

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4441664
[nrf fromtree] bootutil: Allow bypassing ASN.1 encoding for ED25519 k…
de-nordic Oct 8, 2024
aeb8714
[nrf fromtree] zephyr: Add Kconfig option CONFIG_BOOT_KEY_IMPORT_BYPA…
de-nordic Oct 8, 2024
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21 changes: 18 additions & 3 deletions boot/bootutil/src/image_ed25519.c
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Original file line number Diff line number Diff line change
Expand Up @@ -17,10 +17,12 @@
#ifdef MCUBOOT_SIGN_ED25519
#include "bootutil/sign_key.h"

#if !defined(MCUBOOT_KEY_IMPORT_BYPASS_ASN)
/* We are not really using the MBEDTLS but need the ASN.1 parsing functions */
#define MBEDTLS_ASN1_PARSE_C
#include "mbedtls/oid.h"
#include "mbedtls/asn1.h"
#endif

#include "bootutil_priv.h"
#include "bootutil/crypto/common.h"
Expand All @@ -34,12 +36,12 @@ extern int ED25519_verify(const uint8_t *message, size_t message_len,
const uint8_t public_key[NUM_ED25519_BYTES]);

#if !defined(CONFIG_BOOT_SIGNATURE_USING_KMU)

static const uint8_t ed25519_pubkey_oid[] = MBEDTLS_OID_ISO_IDENTIFIED_ORG "\x65\x70";

#if !defined(MCUBOOT_KEY_IMPORT_BYPASS_ASN)
/*
* Parse the public key used for signing.
*/
static const uint8_t ed25519_pubkey_oid[] = MBEDTLS_OID_ISO_IDENTIFIED_ORG "\x65\x70";

static int
bootutil_import_key(uint8_t **cp, uint8_t *end)
{
Expand Down Expand Up @@ -97,11 +99,24 @@ bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig, size_t slen,
pubkey = (uint8_t *)bootutil_keys[key_id].key;
end = pubkey + *bootutil_keys[key_id].len;

#if !defined(MCUBOOT_KEY_IMPORT_BYPASS_ASN)
rc = bootutil_import_key(&pubkey, end);
if (rc) {
FIH_SET(fih_rc, FIH_FAILURE);
goto out;
}
#else
/* Directly use the key contents from the ASN stream,
* these are the last NUM_ED25519_BYTES.
* There is no check whether this is the correct key,
* here, by the algorithm selected.
*/
if (*bootutil_keys[key_id].len < NUM_ED25519_BYTES) {
FIH_SET(fih_rc, FIH_FAILURE);
goto out;
}

pubkey = end - NUM_ED25519_BYTES;
#endif

rc = ED25519_verify(hash, IMAGE_HASH_SIZE, sig, pubkey);
Expand Down
9 changes: 9 additions & 0 deletions boot/zephyr/Kconfig
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Original file line number Diff line number Diff line change
Expand Up @@ -299,6 +299,15 @@ config BOOT_ED25519_PSA
select BOOT_X25519_PSA_DEPENDENCIES if BOOT_ENCRYPT_IMAGE

endchoice

config BOOT_KEY_IMPORT_BYPASS_ASN
bool "Directly access key value without ASN.1 parsing"
help
Originally, public keys compiled into MCUboot were
stored in ASN.1 encoded format. Enabling this option
bypasses the ASN.1 decoding and directly accesses the key
in ASN.1 bitstream; this reduces MCUboot code by removing
the ASN.1 processing.
endif

endchoice
Expand Down
4 changes: 4 additions & 0 deletions boot/zephyr/include/mcuboot_config/mcuboot_config.h
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Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,10 @@
# error "One crypto library implementation allowed at a time."
#endif

#if defined(CONFIG_BOOT_KEY_IMPORT_BYPASS_ASN)
#define MCUBOOT_KEY_IMPORT_BYPASS_ASN
#endif

#ifdef CONFIG_BOOT_USE_MBEDTLS
#define MCUBOOT_USE_MBED_TLS
#elif defined(CONFIG_BOOT_USE_TINYCRYPT)
Expand Down
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