AES CBC 256 encryption on nrf52840

RE: AES CBC 256 encryption on nrf52840

imtiz_ahmed — Tue, 01 Apr 2025 11:14:35 GMT

Thanks a lot, this does it!

RE: AES CBC 256 encryption on nrf52840

Einar Thorsrud — Tue, 01 Apr 2025 10:23:03 GMT

Hi,

[quote user="imtiz_ahmed"]1) Does CONFIG_PSA_CRYPTO_DRIVER_OBERON=y mean that all crypto related work then be run in software and hardware cell will no longer be used for the app?[/quote]

CC310 will still be used and preferred where it can be used.

[quote user="imtiz_ahmed"]I get following error when using 2.4.2
[/quote]

I see. You can do something similar in NCS v2.4. Also there, make sure that both Oberon and CC310 are enabled:

CONFIG_PSA_CRYPTO_DRIVER_OBERON=y
CONFIG_PSA_CRYPTO_DRIVER_CC3XX=y

And additionally, disable the CC310 driver for CBC explicitly:

CONFIG_PSA_CRYPTO_DRIVER_ALG_CBC_NO_PADDING_CC3XX=n

RE: AES CBC 256 encryption on nrf52840

imtiz_ahmed — Fri, 28 Mar 2025 12:30:49 GMT

I get following error when using 2.4.2

error: PSA_WANT_KEY_TYPE_AES (defined at modules/mbedtls/Kconfig.psa:55,
/opt/nordic/ncs/v2.4.2/nrf/modules/trusted-firmware-m/Kconfig.mbedtls_minimal.defconfig:89,
/opt/nordic/ncs/v2.4.2/nrf/modules/trusted-firmware-m/Kconfig.psa.defconfig:11,
modules/mbedtls/Kconfig.psa:55, modules/mbedtls/Kconfig.psa:55) is assigned in a configuration file,
but is not directly user-configurable (has no prompt). It gets its value indirectly from other
symbols. See docs.zephyrproject.org/.../kconfig.html and/or
look up PSA_WANT_KEY_TYPE_AES in the menuconfig/guiconfig interface. The Application Development
Primer, Setting Configuration Values, and Kconfig - Tips and Best Practices sections of the manual
might be helpful too.

I can make it work if I set the following value in ./aes_cbc/boards/nrf52840dk_nrf52840.conf, but I guess that disables the hardware crypto (not something I want).

CONFIG_PSA_CRYPTO_DRIVER_CC3XX=n

RE: AES CBC 256 encryption on nrf52840

imtiz_ahmed — Fri, 28 Mar 2025 11:08:17 GMT

Thanks, I could make it run in the sample. I have couple of questions though.

1) Does CONFIG_PSA_CRYPTO_DRIVER_OBERON=y mean that all crypto related work then be run in software and hardware cell will no longer be used for the app?

2) I could make it work in aes_cbc sample based on 2.9.0 but the project I have to port this to is based on 2.4.2. If I upgrade that project, it will open up new work scope, so I tried aes_cbc sample based on 2.4.2 and followed exact steps, but over there I still got -134 at psa_cipher_encrypt_setup. Any guesses what could be wrong?

RE: AES CBC 256 encryption on nrf52840

Einar Thorsrud — Fri, 28 Mar 2025 09:52:12 GMT

Hi,

I would recommend that you use the PSA crypto APIs, as the legacy APIs are deprecated and will be removed. You can use the Oberon backend fro AES256. To test it you can use the AES CBC sample, modify nrf/samples/crypto/aes_cbc/boards/nrf52840dk_nrf52840.conf so that CONFIG_PSA_CRYPTO_DRIVER_OBERON is enabled isntead of being disabled, and add CONFIG_PSA_WANT_AES_KEY_SIZE_256=y to prj.conf. Lastly, modify the call to psa_set_key_bits() in the sample to set 256 bit key size instead of 128.

This is a full diff of my modifications from SDK 2.9.1 that works for me on the nRF52840 DK:

diff --git a/samples/crypto/aes_cbc/boards/nrf52840dk_nrf52840.conf b/samples/crypto/aes_cbc/boards/nrf52840dk_nrf52840.conf
index 1506df21e8..60a0917f5f 100644
--- a/samples/crypto/aes_cbc/boards/nrf52840dk_nrf52840.conf
+++ b/samples/crypto/aes_cbc/boards/nrf52840dk_nrf52840.conf
@@ -4,7 +4,7 @@
 # SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
 #
 # Using hardware crypto accelerator
-CONFIG_PSA_CRYPTO_DRIVER_OBERON=n
+CONFIG_PSA_CRYPTO_DRIVER_OBERON=y
 CONFIG_PSA_CRYPTO_DRIVER_CC3XX=y
 
 # Mbedtls configuration
diff --git a/samples/crypto/aes_cbc/prj.conf b/samples/crypto/aes_cbc/prj.conf
index 8b4ddbcf9f..934b132e36 100644
--- a/samples/crypto/aes_cbc/prj.conf
+++ b/samples/crypto/aes_cbc/prj.conf
@@ -16,7 +16,10 @@ CONFIG_LOG=y
 # Enable nordic security backend and PSA APIs
 CONFIG_NRF_SECURITY=y
 CONFIG_MBEDTLS_PSA_CRYPTO_C=y
+CONFIG_PSA_CRYPTO_DRIVER_OBERON=y
 
 CONFIG_PSA_WANT_GENERATE_RANDOM=y
 CONFIG_PSA_WANT_KEY_TYPE_AES=y
 CONFIG_PSA_WANT_ALG_CBC_NO_PADDING=y
+
+CONFIG_PSA_WANT_AES_KEY_SIZE_256=y
\ No newline at end of file
diff --git a/samples/crypto/aes_cbc/src/main.c b/samples/crypto/aes_cbc/src/main.c
index 37ab449d8a..2b851c628d 100644
--- a/samples/crypto/aes_cbc/src/main.c
+++ b/samples/crypto/aes_cbc/src/main.c
@@ -87,7 +87,7 @@ int generate_key(void)
 	psa_set_key_lifetime(&key_attributes, PSA_KEY_LIFETIME_VOLATILE);
 	psa_set_key_algorithm(&key_attributes, PSA_ALG_CBC_NO_PADDING);
 	psa_set_key_type(&key_attributes, PSA_KEY_TYPE_AES);
-	psa_set_key_bits(&key_attributes, 128);
+	psa_set_key_bits(&key_attributes, 256);
 
 	/* Generate a random key. The key is not exposed to the application,
 	 * we can use it to encrypt/decrypt using the key handle