1 files changed, 38 insertions, 0 deletions
diff --git a/src/crypto/internal/randutil/randutil.go b/src/crypto/internal/randutil/randutil.go
new file mode 100644
index 0000000..84b1295
--- /dev/null
+++ b/src/crypto/internal/randutil/randutil.go
@@ -0,0 +1,38 @@
+// Copyright 2018 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 randutil contains internal randomness utilities for various
+// crypto packages.
+package randutil
+
+import (
+	"io"
+	"sync"
+)
+
+var (
+	closedChanOnce sync.Once
+	closedChan     chan struct{}
+)
+
+// MaybeReadByte reads a single byte from r with ~50% probability. This is used
+// to ensure that callers do not depend on non-guaranteed behaviour, e.g.
+// assuming that rsa.GenerateKey is deterministic w.r.t. a given random stream.
+//
+// This does not affect tests that pass a stream of fixed bytes as the random
+// source (e.g. a zeroReader).
+func MaybeReadByte(r io.Reader) {
+	closedChanOnce.Do(func() {
+		closedChan = make(chan struct{})
+		close(closedChan)
+	})
+
+	select {
+	case <-closedChan:
+		return
+	case <-closedChan:
+		var buf [1]byte
+		r.Read(buf[:])
+	}
+}