1 files changed, 49 insertions, 1 deletions

diff --git a/inform/crypto.go b/inform/crypto.go
index 90118c1..4e82741 100644
--- a/inform/crypto.go
+++ b/inform/crypto.go
@@ -5,6 +5,7 @@ import (
         "crypto/aes"
         "crypto/cipher"
         "crypto/rand"
+        "encoding/binary"
         "encoding/hex"
         "errors"
 )
@@ -68,7 +69,54 @@ func Encrypt(payload []byte, key string) ([]byte, []byte, error) {
         return ct, iv, nil
 }
 
-func Decrypt(payload, iv []byte, key string) ([]byte, error) {
+func Decrypt(payload, iv []byte, key string, w *InformWrapper) ([]byte, error) {
+        if !w.IsEncrypted() {
+                return nil, errors.New("payload is not encrypted")
+        }
+
+        if w.IsGCMEncrypted() {
+                return decryptGCM(payload, iv, key, w)
+        } else {
+                return decryptCBC(payload, iv, key)
+        }
+
+        return nil, nil
+}
+
+func buildAuthData(w *InformWrapper, iv []byte) []byte {
+        ad := &bytes.Buffer{}
+        binary.Write(ad, binary.BigEndian, int32(PROTOCOL_MAGIC))
+        binary.Write(ad, binary.BigEndian, int32(w.Version))
+        ad.Write(w.MacAddr)
+        binary.Write(ad, binary.BigEndian, int16(w.Flags))
+        ad.Write(iv)
+        binary.Write(ad, binary.BigEndian, int32(w.DataVersion))
+        binary.Write(ad, binary.BigEndian, int32(w.DataLength))
+        return ad.Bytes()
+}
+
+func decryptGCM(payload, iv []byte, key string, w *InformWrapper) ([]byte, error) {
+        block, err := decodeHexKey(key)
+        if err != nil {
+                return nil, err
+        }
+
+        mode, err := cipher.NewGCMWithNonceSize(block, 16)
+        if err != nil {
+                return nil, err
+        }
+
+        _, err = mode.Open(payload[:0], iv, payload, buildAuthData(w, iv))
+        if err != nil {
+                return nil, err
+        }
+
+        // The last block always seems to be garbage, maybe it's padding or
+        // something else. I have not looked carefully at it.
+        return payload[:len(payload)-aes.BlockSize], nil
+}
+
+func decryptCBC(payload, iv []byte, key string) ([]byte, error) {
         b := make([]byte, len(payload))
 
         block, err := decodeHexKey(key)