psa: Support RSA signature without MBEDTLS_GENPRIME

On space-constrained platforms, it is a useful configuration to be able
to import/export and perform RSA key pair operations, but to exclude RSA
key generation, potentially saving flash space. It is not possible to
express this with the PSA_WANT_ configuration system at the present
time. However, in previous versions of Mbed TLS (v2.24.0 and earlier) it
was possible to configure a software PSA implementation which was
capable of making RSA signatures but not capable of generating RSA keys.
To do this, one unset MBEDTLS_GENPRIME.

Since the addition of MBEDTLS_PSA_BUILTIN_KEY_TYPE_RSA_KEY_PAIR, this
expressivity was lost. Expressing that you wanted to work with RSA key
pairs forced you to include the ability to generate key pairs as well.

Change psa_crypto_rsa.c to only call mbedtls_rsa_gen_key() if
MBEDTLS_GENPRIME is also set. This restores the configuration behavior
present in Mbed TLS v2.24.0 and earlier versions.

It left as a future exercise to add the ability to PSA to be able to
express a desire for a software or accelerator configuration that
includes RSA key pair operations, like signature, but excludes key pair
generation.

Without this change, linker errors will occur when attempts to call,
which doesn't exist when MBEDTLS_GENPRIME is unset.
    psa_crypto_rsa.c.obj: in function `rsa_generate_key':
    psa_crypto_rsa.c:320: undefined reference to `mbedtls_rsa_gen_key'

Fixes Mbed-TLS#4512

Signed-off-by: Jaeden Amero <[email protected]>

ChangeLog.d/psa-without-genprime-fix.txt

@@ -0,0 +1,5 @@
+Bugfix
+   * Restore the ability to configure PSA via Mbed TLS options to support RSA
+     key pair operations but exclude RSA key generation. When MBEDTLS_GENPRIME
+     is not defined PSA will no longer attempt to use mbedtls_rsa_gen_key().
+     Fixes #4512.

library/psa_crypto_rsa.c

@@ -274,7 +274,10 @@ static psa_status_t rsa_export_public_key(
 #endif /* defined(BUILTIN_KEY_TYPE_RSA_KEY_PAIR) ||
         * defined(BUILTIN_KEY_TYPE_RSA_PUBLIC_KEY) */
 
-#if defined(BUILTIN_KEY_TYPE_RSA_KEY_PAIR)
+/* XXX Does MBEDTLS_PSA_ACCEL_KEY_TYPE_RSA_KEY_PAIR use these functions? Both
+ * of them or just rsa_generate_key? */
+#if defined(BUILTIN_KEY_TYPE_RSA_KEY_PAIR) && \
+    defined(MBEDTLS_GENPRIME)
 static psa_status_t psa_rsa_read_exponent( const uint8_t *domain_parameters,
                                            size_t domain_parameters_size,
                                            int *exponent )
@@ -332,7 +335,8 @@ static psa_status_t rsa_generate_key(
 
     return( status );
 }
-#endif /* defined(BUILTIN_KEY_TYPE_RSA_KEY_PAIR) */
+#endif /* defined(BUILTIN_KEY_TYPE_RSA_KEY_PAIR)
+        * defined(MBEDTLS_GENPRIME) */
 
 /****************************************************************/
 /* Sign/verify hashes */
@@ -565,15 +569,17 @@ psa_status_t mbedtls_psa_rsa_export_public_key(
 #endif /* defined(MBEDTLS_PSA_BUILTIN_KEY_TYPE_RSA_KEY_PAIR) ||
         * defined(MBEDTLS_PSA_BUILTIN_KEY_TYPE_RSA_PUBLIC_KEY) */
 
-#if defined(MBEDTLS_PSA_BUILTIN_KEY_TYPE_RSA_KEY_PAIR)
+#if defined(MBEDTLS_PSA_BUILTIN_KEY_TYPE_RSA_KEY_PAIR) && \
+    defined(MBEDTLS_GENPRIME)
 psa_status_t mbedtls_psa_rsa_generate_key(
     const psa_key_attributes_t *attributes,
     uint8_t *key_buffer, size_t key_buffer_size, size_t *key_buffer_length )
 {
     return( rsa_generate_key( attributes, key_buffer, key_buffer_size,
                               key_buffer_length ) );
 }
-#endif /* defined(MBEDTLS_PSA_BUILTIN_KEY_TYPE_RSA_KEY_PAIR) */
+#endif /* defined(MBEDTLS_PSA_BUILTIN_KEY_TYPE_RSA_KEY_PAIR)
+        * defined(MBEDTLS_GENPRIME) */
 
 #if defined(MBEDTLS_PSA_BUILTIN_ALG_RSA_PKCS1V15_SIGN) || \
     defined(MBEDTLS_PSA_BUILTIN_ALG_RSA_PSS)

tests/scripts/all.sh

@@ -1306,6 +1306,12 @@ component_build_crypto_baremetal () {
   if_build_succeeded are_empty_libraries library/libmbedx509.* library/libmbedtls.*
 }
 
+component_build_psa_crypto_rsa_no_genprime() {
+    msg "build: default config minus MBEDTLS_GENPRIME"
+    scripts/config.py unset MBEDTLS_GENPRIME
+    make
+}
+
 component_test_depends_curves () {
     msg "test/build: curves.pl (gcc)" # ~ 4 min
     record_status tests/scripts/curves.pl