Move public key format translations to dedicated functions

Author: Andrzej Kurek

atecc608a_se.c

@@ -122,6 +122,21 @@ psa_status_t atecc608a_to_psa_error(ATCA_STATUS ret)
     }
 }
 
+/* The driver works with pubkeys as concatenated x and y values, and the PSA
+ * format for pubkeys is 0x04 + x + y. Always use a pubkey buffer in PSA
+ * format, with enough space for the PSA format. To translate this buffer for
+ * use with cryptoauthlib, use pubkey_for_driver(). To ensure the buffer is in
+ * valid PSA format after cryptoauthlib operations, call pubkey_for_psa(). */
+static uint8_t *pubkey_for_driver(uint8_t *data)
+{
+    return &data[1];
+}
+
+static void pubkey_for_psa(uint8_t *data)
+{
+    data[0] = 0x4;
+}
+
 static psa_status_t is_public_key_slot(uint16_t key_slot)
 {
     /* Keys 8 to 15 can store public keys. Slots 1-7 are too small. */
@@ -156,11 +171,9 @@ static psa_status_t atecc608a_export_public_key(psa_key_slot_number_t key,
 
     ASSERT_SUCCESS_PSA(atecc608a_init());
 
-    /* atcab_get_pubkey returns concatenated x and y values, and the desired
-     * format is 0x04 + x + y. Start at &p_data[1] and add a 0x04 at p_data[0]. */
-    ASSERT_SUCCESS(atcab_get_pubkey(slot, &p_data[1]));
+    ASSERT_SUCCESS(atcab_get_pubkey(slot, pubkey_for_driver(p_data)));
+    pubkey_for_psa(p_data);
 
-    p_data[0] = 4;
     *p_data_length = key_data_len;
 
 #ifdef DEBUG_PRINT
@@ -206,9 +219,7 @@ static psa_status_t atecc608a_import_public_key(psa_key_slot_number_t key_slot,
 
     ASSERT_SUCCESS_PSA(atecc608a_init());
 
-    /* PSA public key format is {0x04, X, Y}, and the cryptoauthlib accepts
-     * raw {X,Y}. */
-    ASSERT_SUCCESS(atcab_write_pubkey(key_id, p_data + 1));
+    ASSERT_SUCCESS(atcab_write_pubkey(key_id, pubkey_for_driver(p_data)));
 exit:
     atecc608a_deinit();
     return status;
@@ -252,11 +263,8 @@ static psa_status_t atecc608a_generate_key(psa_key_slot_number_t key_slot,
 
     if (p_pubkey_out != NULL)
     {
-        /* atcab_genkey returns concatenated x and y values, and the desired
-         * format is 0x04 + x + y. Start at &p_pubkey_out[1] and add a 0x04
-         * at p_pubkey_out[0]. */
-        ASSERT_SUCCESS(atcab_genkey(key_id, &p_pubkey_out[1]));
-        p_pubkey_out[0] = 4;
+        ASSERT_SUCCESS(atcab_genkey(key_id, pubkey_for_driver(p_pubkey_out)));
+        pubkey_for_psa(p_pubkey_out);
     }
     else
     {