internal/tony: Initial import

This is a series of wrappers around fiat-crypto's curve25519 code, along
with working around braindamage caused by the lack of `//go:inline`.

Upstream commit: f1951b3d80f1d0e402a832682875667ae52e8028

go.mod

@@ -3,6 +3,7 @@ module github.com/oasisprotocol/curve25519-voi
 go 1.16
 
 require (
+	github.com/mit-plv/fiat-crypto/fiat-go v0.0.0-20210807234606-f1951b3d80f1
 	golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad
 	golang.org/x/sys v0.0.0-20191026070338-33540a1f6037
 )

go.sum

@@ -1,3 +1,5 @@
+github.com/mit-plv/fiat-crypto/fiat-go v0.0.0-20210807234606-f1951b3d80f1 h1:tclc3vLlQfZi9scV1zBCXboNWiG3bpbz7O1gwybPyNQ=
+github.com/mit-plv/fiat-crypto/fiat-go v0.0.0-20210807234606-f1951b3d80f1/go.mod h1:59UI5/2yBTcSl1/+qCCOTsfXYy290H670oWjGFRqOLs=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad h1:DN0cp81fZ3njFcrLCytUHRSUkqBjfTo4Tx9RJTWs0EY=
 golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad/go.mod h1:jdWPYTVW3xRLrWPugEBEK3UY2ZEsg3UU495nc5E+M+I=

internal/tony/tony.go

@@ -0,0 +1,38 @@
+// Copyright (c) 2021 Oasis Labs Inc. All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+// 1. Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright
+// notice, this list of conditions and the following disclaimer in the
+// documentation and/or other materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
+// IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+// TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+// PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
+// TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// Package tony wraps fiat-crypto and provides some useful helpers and
+// fixes.
+package tony
+
+// RelaxCast converts from a TightFieldElement to a LooseFieldElement
+// by casting.
+func (tfe *TightFieldElement) RelaxCast() *LooseFieldElement {
+	return (*LooseFieldElement)(tfe)
+}