implement HKDF expansion

internal/crypto/hkdf.go

@@ -3,6 +3,7 @@ package crypto
 import (
 	"crypto"
 	"crypto/hmac"
+	"encoding/binary"
 )
 
 // copied from https://github.com/cloudflare/tls-tris/blob/master/hkdf.go
@@ -17,3 +18,39 @@ func hkdfExtract(hash crypto.Hash, secret, salt []byte) []byte {
 	extractor.Write(secret)
 	return extractor.Sum(nil)
 }
+
+// copied from https://github.com/cloudflare/tls-tris/blob/master/hkdf.go
+func hkdfExpand(hash crypto.Hash, prk, info []byte, l int) []byte {
+	var (
+		expander = hmac.New(hash.New, prk)
+		res      = make([]byte, l)
+		counter  = byte(1)
+		prev     []byte
+	)
+
+	if l > 255*expander.Size() {
+		panic("hkdf: requested too much output")
+	}
+
+	p := res
+	for len(p) > 0 {
+		expander.Reset()
+		expander.Write(prev)
+		expander.Write(info)
+		expander.Write([]byte{counter})
+		prev = expander.Sum(prev[:0])
+		counter++
+		n := copy(p, prev)
+		p = p[n:]
+	}
+
+	return res
+}
+
+func qhkdfExpand(secret []byte, label string, length int) []byte {
+	qlabel := make([]byte, 2+1+5+len(label))
+	binary.BigEndian.PutUint16(qlabel[0:2], uint16(length))
+	qlabel[2] = uint8(5 + len(label))
+	copy(qlabel[3:], []byte("QUIC "+label))
+	return hkdfExpand(crypto.SHA256, secret, qlabel, length)
+}