mbedtls_ecp_write_key(): deprecate the old function

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

ChangeLog.d/ecp_write_key.txt

@@ -2,3 +2,7 @@ Features
    * The new function mbedtls_ecp_write_key_ext() is similar to
      mbedtls_ecp_write_key(), but can be used without separately calculating
      the output length.
+
+New deprecations
+   * mbedtls_ecp_write_key() is deprecated in favor of
+     mbedtls_ecp_write_key_ext().

include/mbedtls/ecp.h

@@ -24,6 +24,7 @@
 #include "mbedtls/private_access.h"
 
 #include "mbedtls/build_info.h"
+#include "mbedtls/platform_util.h"
 
 #include "mbedtls/bignum.h"
 
@@ -1327,10 +1328,11 @@ int mbedtls_ecp_set_public_key(mbedtls_ecp_group_id grp_id,
 int mbedtls_ecp_read_key(mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
                          const unsigned char *buf, size_t buflen);
 
+#if !defined(MBEDTLS_DEPRECATED_REMOVED)
 /**
  * \brief           This function exports an elliptic curve private key.
  *
- * \note            Note that although this function accepts an output
+ * \deprecated      Note that although this function accepts an output
  *                  buffer that is smaller or larger than the key, most key
  *                  import interfaces require the output to have exactly
  *                  key's nominal length. It is generally simplest to
@@ -1340,6 +1342,8 @@ int mbedtls_ecp_read_key(mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
  *                  how to calculate the nominal length.
  *                  To avoid this difficulty, use mbedtls_ecp_write_key_ext()
  *                  instead.
+ *                  mbedtls_ecp_write_key() is deprecated and will be
+ *                  removed in a future version of the library.
  *
  * \note            If the private key was not set in \p key,
  *                  the output is unspecified. Future versions
@@ -1369,8 +1373,9 @@ int mbedtls_ecp_read_key(mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
  *                  representation is larger than the available space in \p buf.
  * \return          Another negative error code on different kinds of failure.
  */
-int mbedtls_ecp_write_key(mbedtls_ecp_keypair *key,
+int MBEDTLS_DEPRECATED mbedtls_ecp_write_key(mbedtls_ecp_keypair *key,
                                              unsigned char *buf, size_t buflen);
+#endif /* MBEDTLS_DEPRECATED_REMOVED */
 
 /**
  * \brief           This function exports an elliptic curve private key.

library/ecp.c

@@ -3302,6 +3302,7 @@ cleanup:
 /*
  * Write a private key.
  */
+#if !defined MBEDTLS_DEPRECATED_REMOVED
 int mbedtls_ecp_write_key(mbedtls_ecp_keypair *key,
                           unsigned char *buf, size_t buflen)
 {
@@ -3332,6 +3333,7 @@ cleanup:
 
     return ret;
 }
+#endif /* MBEDTLS_DEPRECATED_REMOVED */
 
 int mbedtls_ecp_write_key_ext(mbedtls_ecp_keypair *key,
                               size_t *olen, unsigned char *buf, size_t buflen)

tests/suites/test_suite_ecp.function

@@ -1213,10 +1213,12 @@ void mbedtls_ecp_read_key(int grp_id, data_t *in_key, int expected, int canonica
             TEST_MEMORY_COMPARE(in_key->x, in_key->len,
                                 buf, length);
 
+#if defined(MBEDTLS_TEST_DEPRECATED)
             memset(buf, 0, sizeof(buf));
             TEST_EQUAL(mbedtls_ecp_write_key(&key, buf, in_key->len), 0);
             TEST_MEMORY_COMPARE(in_key->x, in_key->len,
                                 buf, in_key->len);
+#endif /* MBEDTLS_TEST_DEPRECATED */
         } else {
             unsigned char export1[MBEDTLS_ECP_MAX_BYTES];
             unsigned char export2[MBEDTLS_ECP_MAX_BYTES];
@@ -1232,6 +1234,7 @@ void mbedtls_ecp_read_key(int grp_id, data_t *in_key, int expected, int canonica
             TEST_MEMORY_COMPARE(export1, length1,
                                 export2, length2);
 
+#if defined(MBEDTLS_TEST_DEPRECATED)
             memset(export1, 0, sizeof(export1));
             memset(export2, 0, sizeof(export2));
             TEST_EQUAL(mbedtls_ecp_write_key(&key, export1, in_key->len), 0);
@@ -1240,6 +1243,7 @@ void mbedtls_ecp_read_key(int grp_id, data_t *in_key, int expected, int canonica
             TEST_EQUAL(mbedtls_ecp_write_key(&key2, export2, in_key->len), 0);
             TEST_MEMORY_COMPARE(export1, in_key->len,
                                 export2, in_key->len);
+#endif /* MBEDTLS_TEST_DEPRECATED */
         }
     }
 
@@ -1249,7 +1253,7 @@ exit:
 }
 /* END_CASE */
 
-/* BEGIN_CASE */
+/* BEGIN_CASE depends_on:MBEDTLS_TEST_DEPRECATED */
 void ecp_write_key(int grp_id, data_t *in_key,
                    int exported_size, int expected_ret)
 {