diff --git a/integrity_algo.cpp b/integrity_algo.cpp
new file mode 100644
index 0000000..b765e06
--- /dev/null
+++ b/integrity_algo.cpp
@@ -0,0 +1,30 @@
+#include <openssl/hmac.h>
+#include <openssl/sha.h>
+#include "integrity_algo.hpp"
+#include "message_parsers.hpp"
+
+namespace cipher
+{
+
+namespace integrity
+{
+
+Interface::Interface(const Buffer& sik, const Key& addKey, size_t authLength)
+{
+    unsigned int mdLen = 0;
+
+    // Generated K1 for the integrity algorithm with the additional key keyed
+    // with SIK.
+    if (HMAC(EVP_sha1(), sik.data(), sik.size(), addKey.data(),
+             addKey.size(), K1.data(), &mdLen) == NULL)
+    {
+        throw std::runtime_error("Generating Key1 for integrity "
+                                 "algorithm failed");
+    }
+
+    authCodeLength = authLength;
+}
+
+}// namespace integrity
+
+}// namespace cipher