Add a round-trip test

src/sframe.cpp

diff --git a/src/sframe.cpp b/src/sframe.cpp
index 3ef9d23..2ecdeec 100644
--- a/src/sframe.cpp
+++ b/src/sframe.cpp
@@ -225,7 +225,7 @@
 {
   uint64_t val = 0;
   for (size_t i = 0; i < size; i++) {
-    val = (val << 8) + data[size - i - 1];
+    val = (val << 8) + static_cast<uint64_t>(data[i]);
   }
   return val;
 }

test/sframe.cpp

diff --git a/test/sframe.cpp b/test/sframe.cpp
index 1877caa..cc9955c 100644
--- a/test/sframe.cpp
+++ b/test/sframe.cpp
@@ -1,10 +1,10 @@
 #include <doctest/doctest.h>
 #include <sframe/sframe.h>
 
-#include <iostream>    // for string, operator<<
-#include <map>         // for map
-#include <stdexcept>   // for invalid_argument
-#include <string>      // for basic_string, operator==
+#include <iostream>  // for string, operator<<
+#include <map>       // for map
+#include <stdexcept> // for invalid_argument
+#include <string>    // for basic_string, operator==
 
 using namespace sframe;
 
@@ -114,3 +114,33 @@
     CHECK(plaintext == ctx.unprotect(ct2));
   }
 }
+
+TEST_CASE("SFrame Round-Trip")
+{
+  const auto rounds = 1 << 9;
+  const auto kid = KeyID(0x42);
+  const auto plaintext = from_hex("00010203");
+  const std::map<CipherSuite, bytes> keys{
+    { CipherSuite::AES_GCM_128, from_hex("101112131415161718191a1b1c1d1e1f") },
+    { CipherSuite::AES_GCM_256,
+      from_hex("202122232425262728292a2b2c2d2e2f"
+               "303132333435363738393a3b3c3d3e3f") },
+  };
+
+  for (auto& pair : keys) {
+    auto& suite = pair.first;
+    auto& key = pair.second;
+
+    auto send = Context(suite);
+    send.add_key(kid, key);
+
+    auto recv = Context(suite);
+    recv.add_key(kid, key);
+
+    for (int i = 0; i < rounds; i++) {
+      auto encrypted = send.protect(kid, plaintext);
+      auto decrypted = recv.unprotect(encrypted);
+      CHECK(decrypted == plaintext);
+    }
+  }
+}