var b cryptobyte.Builder
	b.AddUint8(0) // outer
	b.AddUint16(kdfID)
	b.AddUint16(aeadID)
	b.AddUint8(id)
	b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) { b.AddBytes(encodedKey) })
	b.AddUint16LengthPrefixed(func(b *cryptobyte.Builder) { b.AddBytes(payload) })
	return b.Bytes()
}

func computeAndUpdateOuterECHExtension(outer, inner *clientHelloMsg, ech *echContext, useKey bool) error {
	var encapKey []byte