vendor: update buildkit to master@d5c1d785b042

Signed-off-by: Justin Chadwell <me@jedevc.com>

vendor/golang.org/x/crypto/curve25519/curve25519_go120.go

@@ -0,0 +1,46 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.20
+
+package curve25519
+
+import "crypto/ecdh"
+
+func x25519(dst *[32]byte, scalar, point []byte) ([]byte, error) {
+	curve := ecdh.X25519()
+	pub, err := curve.NewPublicKey(point)
+	if err != nil {
+		return nil, err
+	}
+	priv, err := curve.NewPrivateKey(scalar)
+	if err != nil {
+		return nil, err
+	}
+	out, err := priv.ECDH(pub)
+	if err != nil {
+		return nil, err
+	}
+	copy(dst[:], out)
+	return dst[:], nil
+}
+
+func scalarMult(dst, scalar, point *[32]byte) {
+	if _, err := x25519(dst, scalar[:], point[:]); err != nil {
+		// The only error condition for x25519 when the inputs are 32 bytes long
+		// is if the output would have been the all-zero value.
+		for i := range dst {
+			dst[i] = 0
+		}
+	}
+}
+
+func scalarBaseMult(dst, scalar *[32]byte) {
+	curve := ecdh.X25519()
+	priv, err := curve.NewPrivateKey(scalar[:])
+	if err != nil {
+		panic("curve25519: internal error: scalarBaseMult was not 32 bytes")
+	}
+	copy(dst[:], priv.PublicKey().Bytes())
+}