refactor-for-testability (#3)

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

peer/cipher-data.go

@@ -0,0 +1,61 @@
+package peer
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"log"
+)
+
+type dataCipher struct {
+	key  [32]byte
+	aead cipher.AEAD
+}
+
+func newDataCipher() *dataCipher {
+	key := [32]byte{}
+	if _, err := rand.Read(key[:]); err != nil {
+		log.Fatalf("Failed to read random data: %v", err)
+	}
+	return newDataCipherFromKey(key)
+}
+
+func newDataCipherFromKey(key [32]byte) *dataCipher {
+	block, err := aes.NewCipher(key[:])
+	if err != nil {
+		log.Fatalf("Failed to create new cipher: %v", err)
+	}
+
+	aead, err := cipher.NewGCM(block)
+	if err != nil {
+		log.Fatalf("Failed to create new GCM: %v", err)
+	}
+
+	return &dataCipher{key: key, aead: aead}
+}
+
+func (sc *dataCipher) Key() [32]byte {
+	return sc.key
+}
+
+func (sc *dataCipher) Encrypt(h header, data, out []byte) []byte {
+	const s = dataHeaderSize
+	out = out[:s+dataCipherOverhead+len(data)]
+	h.Marshal(out[:s])
+	sc.aead.Seal(out[s:s], out[:s], data, nil)
+	return out
+}
+
+func (sc *dataCipher) Decrypt(encrypted, out []byte) (data []byte, ok bool) {
+	const s = dataHeaderSize
+	if len(encrypted) < s+dataCipherOverhead {
+		ok = false
+		return
+	}
+
+	var err error
+
+	data, err = sc.aead.Open(out[:0], encrypted[:s], encrypted[s:], nil)
+	ok = err == nil
+	return
+}