refactor-for-testability (#3)

Co-authored-by: jdl <jdl@desktop>
Co-authored-by: jdl <jdl@crumpington.com>
Reviewed-on: #3

peer/cipher-control.go

@@ -0,0 +1,26 @@
+package peer
+
+import "golang.org/x/crypto/nacl/box"
+
+type controlCipher struct {
+	sharedKey [32]byte
+}
+
+func newControlCipher(privKey, pubKey []byte) *controlCipher {
+	shared := [32]byte{}
+	box.Precompute(&shared, (*[32]byte)(pubKey), (*[32]byte)(privKey))
+	return &controlCipher{shared}
+}
+
+func (cc *controlCipher) Encrypt(h header, data, out []byte) []byte {
+	const s = controlHeaderSize
+	out = out[:s+controlCipherOverhead+len(data)]
+	h.Marshal(out[:s])
+	box.SealAfterPrecomputation(out[s:s], data, (*[24]byte)(out[:s]), &cc.sharedKey)
+	return out
+}
+
+func (cc *controlCipher) Decrypt(encrypted, out []byte) (data []byte, ok bool) {
+	const s = controlHeaderSize
+	return box.OpenAfterPrecomputation(out[:0], encrypted[s:], (*[24]byte)(encrypted[:s]), &cc.sharedKey)
+}