mbedtls: use CSPRNG whenever possible as PSA random source

The main problem of MBEDTLS_PSA_CRYPTO_LEGACY_RNG is that it brings in some legacy modules (entropy + ctr_drbg/hmac_drbg) which means extra ROM/RAM footprint. MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG instead simply calls to the CSPRNG which makes it definitely smaller. Signed-off-by: Valerio Setti <[email protected]>
zephyrproject-rtos · Oct 31, 2024 · a8737af · a8737af
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diff --git a/doc/releases/migration-guide-4.0.rst b/doc/releases/migration-guide-4.0.rst
@@ -75,6 +75,12 @@ Mbed TLS
   corresponding build symbol was removed in Mbed TLS 3.1.0 and is now assumed to
   be enabled. (:github:`77657`)
 
+* If a platform has a CSPRNG source available (i.e. :kconfig:option:`CONFIG_CSPRNG_ENABLED`
+  is set), then the Kconfig option :kconfig:option:`CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG`
+  is the default choice for random number source instead of
+  :kconfig:option:`CONFIG_MBEDTLS_PSA_CRYPTO_LEGACY_RNG`. This helps in reducing
+  ROM/RAM footprint of the Mbed TLS library.
+
 TinyCrypt
 =========
 

diff --git a/modules/mbedtls/Kconfig.tls-generic b/modules/mbedtls/Kconfig.tls-generic
@@ -481,6 +481,7 @@ config MBEDTLS_SSL_EXTENDED_MASTER_SECRET
 choice MBEDTLS_PSA_CRYPTO_RNG_SOURCE
 	prompt "Select random source for built-in PSA crypto"
 	depends on MBEDTLS_PSA_CRYPTO_C
+	default MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG if CSPRNG_ENABLED
 	default MBEDTLS_PSA_CRYPTO_LEGACY_RNG
 
 config MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG

diff --git a/samples/psa/its/overlay-entropy_driver.conf b/samples/psa/its/overlay-entropy_driver.conf
@@ -1,4 +1,3 @@
 # SPDX-License-Identifier: Apache-2.0
 
 CONFIG_ENTROPY_GENERATOR=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
diff --git a/samples/psa/persistent_key/overlay-entropy_driver.conf b/samples/psa/persistent_key/overlay-entropy_driver.conf
@@ -1,4 +1,3 @@
 # SPDX-License-Identifier: Apache-2.0
 
 CONFIG_ENTROPY_GENERATOR=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
diff --git a/tests/bsim/bluetooth/host/gatt/caching/psa_overlay.conf b/tests/bsim/bluetooth/host/gatt/caching/psa_overlay.conf
@@ -4,4 +4,3 @@ CONFIG_MBEDTLS_PSA_CRYPTO_C=y
 CONFIG_PSA_CRYPTO_ENABLE_ALL=y
 
 CONFIG_ENTROPY_GENERATOR=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
diff --git a/tests/bsim/bluetooth/ll/conn/psa_overlay.conf b/tests/bsim/bluetooth/ll/conn/psa_overlay.conf
@@ -4,4 +4,3 @@ CONFIG_MBEDTLS_PSA_CRYPTO_C=y
 CONFIG_PSA_CRYPTO_ENABLE_ALL=y
 
 CONFIG_ENTROPY_GENERATOR=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
diff --git a/tests/crypto/mbedtls_psa/prj.conf b/tests/crypto/mbedtls_psa/prj.conf
@@ -3,4 +3,3 @@ CONFIG_ZTEST=y
 
 CONFIG_MBEDTLS=y
 CONFIG_MBEDTLS_PSA_CRYPTO_C=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
diff --git a/tests/crypto/mbedtls_psa/testcase.yaml b/tests/crypto/mbedtls_psa/testcase.yaml
@@ -11,8 +11,6 @@
 # - no TF-M enabled devices because we assume that the TF-M implementation
 #   of PSA crypto is working fine on the platforms that support TF-M.
 # - platform should be testable by the CI.
-# - enable CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG in order to reduce as much
-#   as possible usage of legacy modules in Mbed TLS.
 # - pick 1 platform which supports entropy driver and 1 which does not. The
 #   latter case will allow to test
 #   CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG_ALLOW_NON_CSPRNG.
@@ -34,8 +32,11 @@ tests:
     # Pick a platform which does not have an entropy driver. In this case we
     # enable the timer random generator because it's always available on all
     # platforms.
+    # Explicitly select CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG because this is
+    # not "automatically selected" when there is CSPRNG available.
     integration_platforms:
       - qemu_x86
     extra_configs:
+      - CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
       - CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG_ALLOW_NON_CSPRNG=y
       - CONFIG_TEST_RANDOM_GENERATOR=y
diff --git a/tests/crypto/secp256r1/mbedtls.conf b/tests/crypto/secp256r1/mbedtls.conf
@@ -1,5 +1,4 @@
 CONFIG_MBEDTLS=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
 CONFIG_MBEDTLS_PSA_CRYPTO_C=y
 CONFIG_MBEDTLS_PSA_P256M_DRIVER_ENABLED=y
 

diff --git a/tests/crypto/secp256r1/p256-m_raw.conf b/tests/crypto/secp256r1/p256-m_raw.conf
@@ -1,5 +1,4 @@
 CONFIG_MBEDTLS=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
 CONFIG_MBEDTLS_PSA_CRYPTO_C=y
 CONFIG_MBEDTLS_PSA_P256M_DRIVER_ENABLED=y
 CONFIG_MBEDTLS_PSA_P256M_DRIVER_RAW=y
diff --git a/tests/net/socket/tls_configurations/prj.conf b/tests/net/socket/tls_configurations/prj.conf
@@ -28,7 +28,6 @@ CONFIG_MBEDTLS_SSL_MAX_CONTENT_LEN=2048
 CONFIG_MBEDTLS_PEM_CERTIFICATE_FORMAT=y
 # Build the PSA Crypto core so that the TLS stack uses the PSA crypto API.
 CONFIG_MBEDTLS_PSA_CRYPTO_C=y
-CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
 CONFIG_ENTROPY_GENERATOR=y
 
 # Disable some Kconfigs that are implied by CONFIG_NET_SOCKETS_SOCKOPT_TLS.

diff --git a/tests/subsys/jwt/testcase.yaml b/tests/subsys/jwt/testcase.yaml
@@ -17,6 +17,9 @@ tests:
     extra_configs:
       - CONFIG_JWT_SIGN_ECDSA=y
       - CONFIG_JWT_USE_PSA=y
+      # Explicitly select CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG because this
+      # is not automatically selected on platforms that do not have a CSPRNG
+      # source.
       - CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
       - CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG_ALLOW_NON_CSPRNG=y
   libraries.encoding.jwt.rsa.legacy:
@@ -28,5 +31,8 @@ tests:
     extra_configs:
       - CONFIG_JWT_SIGN_RSA=y
       - CONFIG_JWT_USE_PSA=y
+      # Explicitly select CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG because this
+      # is not automatically selected on platforms that do not have a CSPRNG
+      # source.
       - CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
       - CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG_ALLOW_NON_CSPRNG=y
Original file line number	Diff line number	Diff line change
Expand Up		@@ -4,4 +4,3 @@ CONFIG_MBEDTLS_PSA_CRYPTO_C=y
		CONFIG_PSA_CRYPTO_ENABLE_ALL=y

		CONFIG_ENTROPY_GENERATOR=y
		CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y
Original file line number	Diff line number	Diff line change
Expand Up		@@ -3,4 +3,3 @@ CONFIG_ZTEST=y

		CONFIG_MBEDTLS=y
		CONFIG_MBEDTLS_PSA_CRYPTO_C=y
		CONFIG_MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG=y