Activated Curve25519 support for ECDHE cipher suites.

inc/bearssl_ec.h

@@ -65,6 +65,11 @@
  *      Callback method that returns a pointer to the subgroup order for
  *      that curve. That value uses unsigned big-endian encoding.
  *
+ *   - `xoff()`
+ *
+ *      Callback method that returns the offset and length of the X
+ *      coordinate in an encoded point.
+ *
  *   - `mul()`
  *
  *      Multiply a curve point with an integer.
@@ -294,6 +299,18 @@ typedef struct {
 	 */
 	const unsigned char *(*order)(int curve, size_t *len);
 
+	/**
+	 * \brief Get the offset and length for the X coordinate.
+	 *
+	 * This function returns the offset and length (in bytes) of
+	 * the X coordinate in an encoded non-zero point.
+	 *
+	 * \param curve   curve identifier.
+	 * \param len     receiver for the X coordinate length (in bytes).
+	 * \return  the offset for the X coordinate (in bytes).
+	 */
+	size_t (*xoff)(int curve, size_t *len);
+
 	/**
 	 * \brief Multiply a curve point by an integer.
 	 *
@@ -423,8 +440,13 @@ extern const br_ec_impl br_ec_p256_m15;
  * \brief EC implementation "i15" (generic code) for Curve25519.
  *
  * This implementation uses the generic code for modular integers (with
- * 15-bit words) to support Curve25519. The `muladd()` method is not
- * implemented.
+ * 15-bit words) to support Curve25519. Due to the specificities of the
+ * curve definition, the following applies:
+ *
+ *   - `muladd()` is not implemented (the function returns 0 systematically).
+ *   - `order()` returns 2^255-1, since the point multiplication algorithm
+ *     accepts any 32-bit integer as input (it clears the top bit and low
+ *     three bits systematically).
  */
 extern const br_ec_impl br_ec_c25519_i15;
 
@@ -432,10 +454,27 @@ extern const br_ec_impl br_ec_c25519_i15;
  * \brief EC implementation "m15" (specialised code) for Curve25519.
  *
  * This implementation uses custom code relying on multiplication of
- * integers up to 15 bits. The `muladd()` method is not implemented.
+ * integers up to 15 bits. Due to the specificities of the curve
+ * definition, the following applies:
+ *
+ *   - `muladd()` is not implemented (the function returns 0 systematically).
+ *   - `order()` returns 2^255-1, since the point multiplication algorithm
+ *     accepts any 32-bit integer as input (it clears the top bit and low
+ *     three bits systematically).
  */
 extern const br_ec_impl br_ec_c25519_m15;
 
+/**
+ * \brief Aggregate EC implementation "m15".
+ *
+ * This implementation is a wrapper for:
+ *
+ *   - `br_ec_c25519_m15` for Curve25519
+ *   - `br_ec_p256_m15` for NIST P-256
+ *   - `br_ec_prime_i15` for other curves (NIST P-384 and NIST-P512)
+ */
+extern const br_ec_impl br_ec_all_m15;
+
 /**
  * \brief Convert a signature from "raw" to "asn1".
  *