GenerateCircID

circuit.go

@@ -2,12 +2,24 @@ package pearl
 
 import (
 	"crypto/cipher"
+	"encoding/binary"
 	"errors"
 	"sync"
 
 	"github.com/mmcloughlin/pearl/torcrypto"
 )
 
+// GenerateCircID generates a circuit ID with the given most significant bit.
+func GenerateCircID(f CellFormat, msb uint32) CircID {
+	b := torcrypto.Rand(4)
+	x := binary.BigEndian.Uint32(b)
+	x = (x >> 1) | (msb << 31)
+	if f.CircIDLen() == 2 {
+		x >>= 16
+	}
+	return CircID(x)
+}
+
 type CircuitDirectionState struct {
 	digest []byte
 	cipher.Stream

circuit_test.go

@@ -0,0 +1,24 @@
+package pearl
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGenerateCircID2(t *testing.T) {
+	f := CircID2Format{}
+	assert.Equal(t, CircID(0), GenerateCircID(f, 0)>>15)
+	assert.Equal(t, CircID(1), GenerateCircID(f, 1)>>15)
+}
+
+func TestGenerateCircID4(t *testing.T) {
+	f := CircID4Format{}
+	assert.Equal(t, CircID(0), GenerateCircID(f, 0)>>31)
+	assert.Equal(t, CircID(1), GenerateCircID(f, 1)>>31)
+}
+
+func TestGenerateCircID4Rand(t *testing.T) {
+	f := CircID4Format{}
+	assert.NotEqual(t, GenerateCircID(f, 0), GenerateCircID(f, 0))
+}

torcrypto/core.go

@@ -0,0 +1,14 @@
+// Package torcrypto provides cryptographic functions useful in tor.
+package torcrypto
+
+import "crypto/rand"
+
+// Rand generates n bytes of cryptographic random. Panics if the read fails.
+func Rand(n int) []byte {
+	x := make([]byte, n)
+	_, err := rand.Read(x)
+	if err != nil {
+		panic(err)
+	}
+	return x
+}