pkparse: add new function for deriving public key from private using PSA

Instead of using the legacy mbedtls_ecp_mul() function which makes use of
ECP's math, this commit adds a new function named pk_derive_public_key()
which implements the same behavior using PSA functions.
The flow is simple:
- import the private key into PSA
- export its public part

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

library/pkparse.c

@@ -48,6 +48,14 @@
 #include "mbedtls/pkcs12.h"
 #endif
 
+#if defined(MBEDTLS_PSA_CRYPTO_C)
+#include "mbedtls/psa_util.h"
+#endif
+
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
+#include "psa/crypto.h"
+#endif
+
 #include "mbedtls/platform.h"
 
 #if defined(MBEDTLS_FS_IO)
@@ -868,6 +876,56 @@ cleanup:
 }
 #endif /* MBEDTLS_RSA_C */
 
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
+/*
+ * Helper function for deriving a public key from its private counterpart by
+ * using PSA functions
+ */
+static int pk_derive_public_key(mbedtls_ecp_group *grp, mbedtls_ecp_point *Q,
+                                const mbedtls_mpi *d)
+{
+    psa_status_t status;
+    psa_key_attributes_t key_attr = PSA_KEY_ATTRIBUTES_INIT;
+    size_t curve_bits;
+    psa_ecc_family_t curve = mbedtls_ecc_group_to_psa(grp->id, &curve_bits);
+    unsigned char key_buf[MBEDTLS_PSA_MAX_EC_KEY_PAIR_LENGTH];
+    size_t key_len = PSA_BITS_TO_BYTES(curve_bits);
+    mbedtls_svc_key_id_t key_id = MBEDTLS_SVC_KEY_ID_INIT;
+    int ret;
+
+    psa_set_key_type(&key_attr, PSA_KEY_TYPE_ECC_KEY_PAIR(curve));
+    psa_set_key_usage_flags(&key_attr, PSA_KEY_USAGE_EXPORT);
+
+    ret = mbedtls_mpi_write_binary(d, key_buf, key_len);
+    if (ret != 0) {
+        return ret;
+    }
+
+    status = psa_import_key(&key_attr, key_buf, key_len, &key_id);
+    if (status != PSA_SUCCESS) {
+        ret = psa_pk_status_to_mbedtls(status);
+        return ret;
+    }
+
+    mbedtls_platform_zeroize(key_buf, sizeof(key_buf));
+    status = psa_export_public_key(key_id, key_buf, sizeof(key_buf), &key_len);
+    if (status != PSA_SUCCESS) {
+        ret = psa_pk_status_to_mbedtls(status);
+        status = psa_destroy_key(key_id);
+        return (status != PSA_SUCCESS) ? psa_pk_status_to_mbedtls(status) : ret;
+    }
+
+    ret = mbedtls_ecp_point_read_binary(grp, Q, key_buf, key_len);
+
+    status = psa_destroy_key(key_id);
+    if (status != PSA_SUCCESS) {
+        return psa_pk_status_to_mbedtls(status);
+    }
+
+    return ret;
+}
+#endif /* MBEDTLS_USE_PSA_CRYPTO */
+
 #if defined(MBEDTLS_ECP_C)
 /*
  * Parse a SEC1 encoded private EC key
@@ -975,11 +1033,21 @@ static int pk_parse_key_sec1_der(mbedtls_ecp_keypair *eck,
         }
     }
 
-    if (!pubkey_done &&
+    if (!pubkey_done) {
-        (ret = mbedtls_ecp_mul(&eck->grp, &eck->Q, &eck->d, &eck->grp.G,
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
-                               f_rng, p_rng)) != 0) {
+        (void) f_rng;
-        mbedtls_ecp_keypair_free(eck);
+        (void) p_rng;
-        return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret);
+        if ((ret = pk_derive_public_key(&eck->grp, &eck->Q, &eck->d)) != 0) {
+            mbedtls_ecp_keypair_free(eck);
+            return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret);
+        }
+#else /* MBEDTLS_USE_PSA_CRYPTO */
+        if ((ret = mbedtls_ecp_mul(&eck->grp, &eck->Q, &eck->d, &eck->grp.G,
+                                   f_rng, p_rng)) != 0) {
+            mbedtls_ecp_keypair_free(eck);
+            return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_PK_KEY_INVALID_FORMAT, ret);
+        }
+#endif /* MBEDTLS_USE_PSA_CRYPTO */
     }
 
     if ((ret = mbedtls_ecp_check_privkey(&eck->grp, &eck->d)) != 0) {

tests/suites/test_suite_pkparse.function

@@ -101,6 +101,10 @@ void pk_parse_keyfile_ec(char *key_file, char *password, int result)
     mbedtls_pk_context ctx;
     int res;
 
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
+    PSA_INIT();
+#endif
+
     mbedtls_pk_init(&ctx);
 
     res = mbedtls_pk_parse_keyfile(&ctx, key_file, password,
@@ -117,6 +121,9 @@ void pk_parse_keyfile_ec(char *key_file, char *password, int result)
 
 exit:
     mbedtls_pk_free(&ctx);
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
+    PSA_DONE();
+#endif
 }
 /* END_CASE */