package handshake import ( "crypto" "encoding/binary" "golang.org/x/crypto/hkdf" ) // hkdfExpandLabel HKDF expands a label. // Since this implementation avoids using a cryptobyte.Builder, it is about 15% faster than the // hkdfExpandLabel in the standard library. func hkdfExpandLabel(hash crypto.Hash, secret, context []byte, label string, length int) []byte { b := make([]byte, 3, 3+6+len(label)+1+len(context)) binary.BigEndian.PutUint16(b, uint16(length)) b[2] = uint8(6 + len(label)) b = append(b, []byte("tls13 ")...) b = append(b, []byte(label)...) b = b[:3+6+len(label)+1] b[3+6+len(label)] = uint8(len(context)) b = append(b, context...) out := make([]byte, length) n, err := hkdf.Expand(hash.New, secret, b).Read(out) if err != nil || n != length { panic("quic: HKDF-Expand-Label invocation failed unexpectedly") } return out }