[crypto] Add an abstraction of an elliptic curve

Define an abstraction of an elliptic curve with a fixed generator and
one supported operation (scalar multiplication of a curve point).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/x25519.c

@@ -61,6 +61,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <assert.h>
 #include <errno.h>
 #include <ipxe/init.h>
+#include <ipxe/crypto.h>
 #include <ipxe/x25519.h>
 
 /** X25519 reduction constant
@@ -300,6 +301,11 @@ static const uint8_t x25519_121665_raw[] = { 0x01, 0xdb, 0x41 };
 /** Constant 121665 (used in the Montgomery ladder) */
 static union x25519_oct258 x25519_121665;
 
+/** Constant g=9 (the group generator) */
+static struct x25519_value x25519_generator = {
+	.raw = { 9, }
+};
+
 /**
  * Initialise constants
  *
@@ -811,3 +817,28 @@ int x25519_key ( const struct x25519_value *base,
 	/* Fail if result was all zeros (as required by RFC8422) */
 	return ( bigint_is_zero ( &point.value ) ? -EPERM : 0 );
 }
+
+/**
+ * Multiply scalar by curve point
+ *
+ * @v base		Base point (or NULL to use generator)
+ * @v scalar		Scalar multiple
+ * @v result		Result point to fill in
+ * @ret rc		Return status code
+ */
+static int x25519_curve_multiply ( const void *base, const void *scalar,
+				   void *result ) {
+
+	/* Use base point if applicable */
+	if ( ! base )
+		base = &x25519_generator;
+
+	return x25519_key ( base, scalar, result );
+}
+
+/** X25519 elliptic curve */
+struct elliptic_curve x25519_curve = {
+	.name = "x25519",
+	.keysize = sizeof ( struct x25519_value ),
+	.multiply = x25519_curve_multiply,
+};