Refactor random string stuff and seeding

Make sure we have a good random seed on the default RNG, that the
predictable RNG is clearly marked as such, that random strings are
actually the length requested, and that they contain a restricted set of
characters only.

cmd/syncthing/random.go

@@ -0,0 +1,67 @@
+// Copyright (C) 2014 The Syncthing Authors.
+//
+// This program is free software: you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// This program is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+// more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program. If not, see <http://www.gnu.org/licenses/>.
+
+package main
+
+import (
+	"crypto/md5"
+	cryptoRand "crypto/rand"
+	"encoding/binary"
+	mathRand "math/rand"
+)
+
+// randomCharset contains the characters that can make up a randomString().
+const randomCharset = "01234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-"
+
+// predictableRandom is an RNG that will always have the same sequence. It
+// will be seeded with the device ID during startup, so that the sequence is
+// predictable but varies between instances.
+var predictableRandom = mathRand.New(mathRand.NewSource(42))
+
+func init() {
+	// The default RNG should be seeded with something good.
+	mathRand.Seed(randomInt64())
+}
+
+// randomString returns a string of random characters (taken from
+// randomCharset) of the specified length.
+func randomString(l int) string {
+	bs := make([]byte, l)
+	for i := range bs {
+		bs[i] = randomCharset[mathRand.Intn(len(randomCharset))]
+	}
+	return string(bs)
+}
+
+// randomInt64 returns a strongly random int64, slowly
+func randomInt64() int64 {
+	var bs [8]byte
+	n, err := cryptoRand.Reader.Read(bs[:])
+	if n != 8 || err != nil {
+		panic("randomness failure")
+	}
+	return seedFromBytes(bs[:])
+}
+
+// seedFromBytes calculates a weak 64 bit hash from the given byte slice,
+// suitable for use a predictable random seed.
+func seedFromBytes(bs []byte) int64 {
+	h := md5.New()
+	h.Write(bs)
+	s := h.Sum(nil)
+	// The MD5 hash of the byte slice is 16 bytes long. We interpret it as two
+	// uint64s and XOR them together.
+	return int64(binary.BigEndian.Uint64(s[0:]) ^ binary.BigEndian.Uint64(s[8:]))
+}