Start sketching

box/box.go

@@ -0,0 +1,82 @@
+package box
+
+import (
+	"crypto/hmac"
+	"crypto/rand"
+	"crypto/sha512"
+	"encoding/binary"
+	"io"
+	"strconv"
+)
+
+type Ciphersuite interface {
+	Name() [24]byte
+	DHLen() int
+	CCLen() int
+	MACLen() int
+
+	DH(privkey, pubkey []byte) []byte
+	NewCipher(cv []byte) CipherContext
+}
+
+type CipherContext interface {
+	Encrypt(dst, authtext, plaintext []byte) []byte
+}
+
+func deriveKey(secret, extraData, info []byte, outputLen int) []byte {
+	output := make([]byte, 0, outputLen+sha512.Size)
+	t := make([]byte, 0, sha512.Size)
+	h := hmac.New(sha512.New, secret)
+
+	// info || (byte)c || t[0:32] || extra_data
+	data := make([]byte, len(info)+1+32+len(extraData))
+	copy(data, info)
+	copy(data[len(info)+1+32:], extraData)
+	var c byte
+	for len(output) < outputLen {
+		data[len(info)] = c
+		copy(data[len(info)+1:], t[:32])
+		h.Write(data)
+		t = h.Sum(t[:0])
+		h.Reset()
+		c++
+		output = append(output, t...)
+	}
+	return output[:outputLen]
+}
+
+const cvLen = 48
+
+type Key struct {
+	Public  []byte
+	Private []byte
+}
+
+func noiseBody(cc CipherContext, dst []byte, padLen int, appData, header []byte) []byte {
+	plaintext := make([]byte, len(appData)+padLen+4)
+	copy(plaintext, appData)
+	if _, err := io.ReadFull(rand.Reader, plaintext[len(appData):len(appData)+padLen]); err != nil {
+		panic(err)
+	}
+	binary.BigEndian.PutUint32(plaintext[len(appData)+padLen:], uint32(padLen))
+	return cc.Encrypt(dst, header, plaintext)
+}
+
+func NoiseBox(c Ciphersuite, dst []byte, ephKey, senderKey Key, recvrPubkey []byte, padLen int, appData []byte, kdfNum int, cv []byte) ([]byte, []byte) {
+	if len(cv) == 0 {
+		cv = make([]byte, cvLen)
+	}
+
+	dh1 := c.DH(ephKey.Private, recvrPubkey)
+	dh2 := c.DH(senderKey.Private, recvrPubkey)
+
+	name := c.Name()
+	cv1 := deriveKey(dh1, cv, strconv.AppendInt(name[:], int64(kdfNum), 10), cvLen+c.CCLen())
+	cv2 := deriveKey(dh2, cv1, strconv.AppendInt(name[:], int64(kdfNum+1), 10), cvLen+c.CCLen())
+
+	cc1 := c.NewCipher(cv1)
+	cc2 := c.NewCipher(cv2)
+
+	header := cc1.Encrypt(ephKey.Public, ephKey.Public, senderKey.Public)
+	return noiseBody(cc2, header, padLen, appData, header), cv2
+}