1 files changed, 50 insertions, 0 deletions
diff --git a/vendor/filippo.io/edwards25519/field/fe_extra.go b/vendor/filippo.io/edwards25519/field/fe_extra.go
new file mode 100644
index 00000000..699238e9
--- /dev/null
+++ b/vendor/filippo.io/edwards25519/field/fe_extra.go
@@ -0,0 +1,50 @@
+// Copyright (c) 2021 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.
+
+package field
+
+import "errors"
+
+// This file contains additional functionality that is not included in the
+// upstream crypto/ed25519/internal/edwards25519/field package.
+
+// SetWideBytes sets v to x, where x is a 64-byte little-endian encoding, which
+// is reduced modulo the field order. If x is not of the right length,
+// SetWideBytes returns nil and an error, and the receiver is unchanged.
+//
+// SetWideBytes is not necessary to select a uniformly distributed value, and is
+// only provided for compatibility: SetBytes can be used instead as the chance
+// of bias is less than 2⁻²⁵⁰.
+func (v *Element) SetWideBytes(x []byte) (*Element, error) {
+	if len(x) != 64 {
+		return nil, errors.New("edwards25519: invalid SetWideBytes input size")
+	}
+
+	// Split the 64 bytes into two elements, and extract the most significant
+	// bit of each, which is ignored by SetBytes.
+	lo, _ := new(Element).SetBytes(x[:32])
+	loMSB := uint64(x[31] >> 7)
+	hi, _ := new(Element).SetBytes(x[32:])
+	hiMSB := uint64(x[63] >> 7)
+
+	// The output we want is
+	//
+	//   v = lo + loMSB * 2²⁵⁵ + hi * 2²⁵⁶ + hiMSB * 2⁵¹¹
+	//
+	// which applying the reduction identity comes out to
+	//
+	//   v = lo + loMSB * 19 + hi * 2 * 19 + hiMSB * 2 * 19²
+	//
+	// l0 will be the sum of a 52 bits value (lo.l0), plus a 5 bits value
+	// (loMSB * 19), a 6 bits value (hi.l0 * 2 * 19), and a 10 bits value
+	// (hiMSB * 2 * 19²), so it fits in a uint64.
+
+	v.l0 = lo.l0 + loMSB*19 + hi.l0*2*19 + hiMSB*2*19*19
+	v.l1 = lo.l1 + hi.l1*2*19
+	v.l2 = lo.l2 + hi.l2*2*19
+	v.l3 = lo.l3 + hi.l3*2*19
+	v.l4 = lo.l4 + hi.l4*2*19
+
+	return v.carryPropagate(), nil
+}