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[nrf fromlist] zephyr: Add support for AES256 #481

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[nrf fromlist] zephyr: Add support for AES256 #481

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e0c1113
[nrf fromtree] imgtool: Allow for AES256 encryption
ahasztag Jul 29, 2025
08a5bf5
[nrf fromtree] zephyr: Add support for AES256
ahasztag Jul 29, 2025
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16 changes: 16 additions & 0 deletions boot/zephyr/Kconfig
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Original file line number Diff line number Diff line change
Expand Up @@ -724,6 +724,22 @@ config BOOT_ENCRYPT_X25519
help
Hidden option selecting x25519 encryption.

if BOOT_ENCRYPT_IMAGE

choice BOOT_ENCRYPT_ALG
prompt "Algorithm used for image encryption"
default BOOT_ENCRYPT_ALG_AES_128

config BOOT_ENCRYPT_ALG_AES_128
bool "Use AES-128 for image encryption"

config BOOT_ENCRYPT_ALG_AES_256
bool "Use AES-256 for image encryption"

endchoice # BOOT_ENCRYPT_ALG

endif # BOOT_ENCRYPT_IMAGE

if BOOT_ENCRYPT_X25519 && BOOT_USE_PSA_CRYPTO

choice BOOT_HMAC_SHA
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8 changes: 8 additions & 0 deletions boot/zephyr/include/mcuboot_config/mcuboot_config.h
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Expand Up @@ -163,6 +163,14 @@
#define MCUBOOT_ENCRYPT_X25519
#endif

#ifdef CONFIG_BOOT_ENCRYPT_ALG_AES_128
#define MCUBOOT_AES_128
#endif

#ifdef CONFIG_BOOT_ENCRYPT_ALG_AES_256
#define MCUBOOT_AES_256
#endif

/* Support for HMAC/HKDF using SHA512; this is used in key exchange where
* HKDF is used for key expansion and HMAC is used for key verification.
*/
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15 changes: 12 additions & 3 deletions scripts/imgtool/image.py
100644 → 100755
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Expand Up @@ -443,14 +443,23 @@ def ecies_hkdf(self, enckey, plainkey, hmac_sha_alg):
else:
newpk = X25519PrivateKey.generate()
shared = newpk.exchange(enckey._get_public())

# Detect AES key length from plainkey size
key_len = len(plainkey) # 16 for AES-128, 32 for AES-256

# Generate derived key with appropriate length (key_len + 32 bytes for HMAC)
derived_key = HKDF(
algorithm=hmac_sha_alg, length=16 + hmac_sha_alg.digest_size, salt=None,
algorithm=hmac_sha_alg, length=key_len + hmac_sha_alg.digest_size, salt=None,
info=b'MCUBoot_ECIES_v1', backend=default_backend()).derive(shared)
encryptor = Cipher(algorithms.AES(derived_key[:16]),

# Use appropriate key length for AES encryption
encryptor = Cipher(algorithms.AES(derived_key[:key_len]),
modes.CTR(bytes([0] * 16)),
backend=default_backend()).encryptor()
cipherkey = encryptor.update(plainkey) + encryptor.finalize()
mac = hmac.HMAC(derived_key[16:], hmac_sha_alg,

# Use remaining bytes for HMAC (after the AES key)
mac = hmac.HMAC(derived_key[key_len:], hmac_sha_alg,
backend=default_backend())
mac.update(cipherkey)
ciphermac = mac.finalize()
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