mbedtls_ecp_write_key: document and test larger output buffer

Signed-off-by: Gilles Peskine <[email protected]>

include/mbedtls/ecp.h

@@ -1286,11 +1286,28 @@ int mbedtls_ecp_read_key(mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
 /**
  * \brief           This function exports an elliptic curve private key.
  *
+ * \note            Note that although this function accepts an output
+ *                  buffer that is larger than the key, most key import
+ *                  interfaces require the output to be trimmed to the
+ *                  key's nominal length. It is generally simplest to
+ *                  pass the key's nominal length as \c buflen, after
+ *                  checking that the output buffer is large enough.
+ *                  See the description of the \p buflen parameter for
+ *                  how to calculate the nominal length.
+ *
  * \param key       The private key.
  * \param buf       The output buffer for containing the binary representation
- *                  of the key. (Big endian integer for Weierstrass curves, byte
- *                  string for Montgomery curves.)
+ *                  of the key.
+ *                  For Weierstrass curves, this is the big-endian
+ *                  representation, padded with null bytes at the beginning
+ *                  to reach \p buflen bytes.
+ *                  For Montgomery curves, this is the standard byte string
+ *                  representation (which is little-endian), padded with
+ *                  null bytes at the end to reach \p buflen bytes.
  * \param buflen    The total length of the buffer in bytes.
+ *                  The length of the output is always
+ *                  (`grp->nbits` + 7) / 8 bytes
+ *                  where `grp->nbits` is the private key size in bits.
  *
  * \return          \c 0 on success.
  * \return          #MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL if the \p key

tests/suites/test_suite_ecp.data

@@ -550,6 +550,54 @@ ECP read key #16 (Curve25519 RFC, OK)
 depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
 mbedtls_ecp_read_key:MBEDTLS_ECP_DP_CURVE25519:"70076d0a7318a57d3c16c17251b26645df4c2f87ebc0992ab177fba51db92c6a":0:1
 
+ECP write key: secp256r1, nominal
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_SECP256R1:"f12a1320760270a83cbffd53f6031ef76a5d86c8a204f2c30ca9ebf51f0f0ea7":32:0
+
+ECP write key: secp256r1, output longer by 1
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_SECP256R1:"f12a1320760270a83cbffd53f6031ef76a5d86c8a204f2c30ca9ebf51f0f0ea7":33:0
+
+ECP write key: secp256r1, output longer by 32
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_SECP256R1:"f12a1320760270a83cbffd53f6031ef76a5d86c8a204f2c30ca9ebf51f0f0ea7":64:0
+
+ECP write key: secp256r1, output longer by 33
+depends_on:MBEDTLS_ECP_DP_SECP256R1_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_SECP256R1:"f12a1320760270a83cbffd53f6031ef76a5d86c8a204f2c30ca9ebf51f0f0ea7":65:0
+
+ECP write key: secp384r1, nominal
+depends_on:MBEDTLS_ECP_DP_SECP384R1_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_SECP384R1:"d27335ea71664af244dd14e9fd1260715dfd8a7965571c48d709ee7a7962a156d706a90cbcb5df2986f05feadb9376f1":48:0
+
+ECP write key: secp384r1, output longer by 1
+depends_on:MBEDTLS_ECP_DP_SECP384R1_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_SECP384R1:"d27335ea71664af244dd14e9fd1260715dfd8a7965571c48d709ee7a7962a156d706a90cbcb5df2986f05feadb9376f1":49:0
+
+ECP write key: secp384r1, output longer by 48
+depends_on:MBEDTLS_ECP_DP_SECP384R1_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_SECP384R1:"d27335ea71664af244dd14e9fd1260715dfd8a7965571c48d709ee7a7962a156d706a90cbcb5df2986f05feadb9376f1":96:0
+
+ECP write key: secp384r1, output longer by 49
+depends_on:MBEDTLS_ECP_DP_SECP384R1_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_SECP384R1:"d27335ea71664af244dd14e9fd1260715dfd8a7965571c48d709ee7a7962a156d706a90cbcb5df2986f05feadb9376f1":97:0
+
+ECP write key: Curve25519, nominal
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_CURVE25519:"a046e36bf0527c9d3b16154b82465edd62144c0ac1fc5a18506a2244ba449a44":32:0
+
+ECP write key: Curve25519, output longer by 1
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_CURVE25519:"a046e36bf0527c9d3b16154b82465edd62144c0ac1fc5a18506a2244ba449a44":33:0
+
+ECP write key: Curve25519, output longer by 32
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_CURVE25519:"a046e36bf0527c9d3b16154b82465edd62144c0ac1fc5a18506a2244ba449a44":64:0
+
+ECP write key: Curve25519, output longer by 33
+depends_on:MBEDTLS_ECP_DP_CURVE25519_ENABLED
+ecp_write_key:MBEDTLS_ECP_DP_CURVE25519:"a046e36bf0527c9d3b16154b82465edd62144c0ac1fc5a18506a2244ba449a44":65:0
+
 ECP mod p192 small (more than 192 bits, less limbs than 2 * 192 bits)
 depends_on:MBEDTLS_ECP_DP_SECP192R1_ENABLED
 ecp_fast_mod:MBEDTLS_ECP_DP_SECP192R1:"0100000000000103010000000000010201000000000001010100000000000100"

tests/suites/test_suite_ecp.function

@@ -1383,6 +1383,53 @@ exit:
 }
 /* END_CASE */
 
+/* BEGIN_CASE */
+void ecp_write_key(int grp_id, data_t *in_key,
+                   int exported_size, int expected_ret)
+{
+    mbedtls_ecp_keypair key;
+    mbedtls_ecp_keypair_init(&key);
+    unsigned char *exported = NULL;
+
+    TEST_EQUAL(mbedtls_ecp_read_key(grp_id, &key, in_key->x, in_key->len), 0);
+
+    TEST_CALLOC(exported, exported_size);
+    TEST_EQUAL(mbedtls_ecp_write_key(&key, exported, exported_size),
+               expected_ret);
+
+    if (expected_ret == 0) {
+        size_t length = (key.grp.nbits + 7) / 8;
+        TEST_LE_U(length, exported_size);
+
+        const unsigned char *key_start = NULL;
+        const unsigned char *zeros_start = NULL;
+        switch (mbedtls_ecp_get_type(&key.grp)) {
+            case MBEDTLS_ECP_TYPE_SHORT_WEIERSTRASS:
+                key_start = exported + exported_size - length;
+                zeros_start = exported;
+                break;
+            case MBEDTLS_ECP_TYPE_MONTGOMERY:
+                key_start = exported;
+                zeros_start = exported + length;
+                break;
+            default:
+                TEST_FAIL("Unknown ECP curve type");
+                break;
+        }
+        TEST_MEMORY_COMPARE(in_key->x, in_key->len,
+                            key_start, length);
+        for (size_t i = 0; i < exported_size - length; i++) {
+            mbedtls_test_set_step(i);
+            TEST_EQUAL(zeros_start[i], 0);
+        }
+    }
+
+exit:
+    mbedtls_ecp_keypair_free(&key);
+    mbedtls_free(exported);
+}
+/* END_CASE */
+
 /* BEGIN_CASE depends_on:HAVE_FIX_NEGATIVE */
 void fix_negative(data_t *N_bin, int c, int bits)
 {