Method expandLabel

key_schedule.go:35–51

expandLabel implements HKDF-Expand-Label from RFC 8446, Section 7.1.

(secret []byte, label string, context []byte, length int)

// expandLabel implements HKDF-Expand-Label from RFC 8446, Section 7.1.
func (c *cipherSuiteTLS13) expandLabel(secret []byte, label string, context []byte, length int) []byte {
	var hkdfLabel cryptobyte.Builder
	hkdfLabel.AddUint16(uint16(length))
	hkdfLabel.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
		b.AddBytes([]byte("tls13 "))
		b.AddBytes([]byte(label))
	})
	hkdfLabel.AddUint8LengthPrefixed(func(b *cryptobyte.Builder) {
		b.AddBytes(context)
	})
	out := make([]byte, length)
	n, err := hkdf.Expand(c.hash.New, secret, hkdfLabel.BytesOrPanic()).Read(out)
	if err != nil || n != length {
		panic("tls: HKDF-Expand-Label invocation failed unexpectedly")
	}
	return out
}

// deriveSecret implements Derive-Secret from RFC 8446, Section 7.1.
func (c *cipherSuiteTLS13) deriveSecret(secret []byte, label string, transcript hash.Hash) []byte {