Switch nonce generation to purely random

The old implementation of counting up from a random starting
point had the problem that is allowed an attacker to find out
the write order of the blocks.

cryptfs/nonce.go

@@ -1,56 +1,36 @@
 package cryptfs
 
 import (
+	"bytes"
+	"fmt"
 	"crypto/rand"
-	"encoding/binary"
 	"encoding/hex"
-	"sync"
 )
 
-type nonce96 struct {
-	lock   sync.Mutex
-	high32 uint32
-	low64  uint64
-	ready  int
-}
-
-var gcmNonce nonce96
-
 // Get "n" random bytes from /dev/urandom or panic
 func RandBytes(n int) []byte {
 	b := make([]byte, n)
 	_, err := rand.Read(b)
 	if err != nil {
-		panic("Could not get random bytes for nonce")
+		panic("Failed to read random bytes: " + err.Error())
 	}
 	return b
 }
 
-func (n *nonce96) init() {
-	b := RandBytes(8)
-	n.low64 = binary.BigEndian.Uint64(b)
-	b = RandBytes(4)
-	n.high32 = binary.BigEndian.Uint32(b)
-	n.ready = 1
-	return
+var gcmNonce nonce96
+
+type nonce96 struct {
+	lastNonce []byte
 }
 
+// Get a random 96 bit nonce
 func (n *nonce96) Get() []byte {
-	n.lock.Lock()
-	if n.ready == 0 {
-		n.init()
+	nonce := RandBytes(12)
+	Debug.Printf("nonce96.Get(): %s\n", hex.EncodeToString(nonce))
+	if bytes.Equal(nonce, n.lastNonce) {
+		m := fmt.Sprintf("Got the same nonce twice: %s. This should never happen!", hex.EncodeToString(nonce))
+		panic(m)
 	}
-	n.low64++
-	if n.low64 == 0 {
-		// Counter has wrapped
-		n.high32++
-	}
-	r := make([]byte, 12)
-	binary.BigEndian.PutUint32(r[0:4], n.high32)
-	binary.BigEndian.PutUint64(r[4:12], n.low64)
-	n.lock.Unlock()
-
-	Debug.Printf("nonce96.Get(): %s\n", hex.EncodeToString(r))
-
-	return r
+	n.lastNonce = nonce
+	return nonce
 }