diff --git a/integrity_algo.cpp b/integrity_algo.cpp
index 3a6c34d..62c2653 100644
--- a/integrity_algo.cpp
+++ b/integrity_algo.cpp
@@ -1,3 +1,4 @@
+#include <openssl/evp.h>
 #include <openssl/hmac.h>
 #include <openssl/sha.h>
 #include "integrity_algo.hpp"
@@ -9,21 +10,10 @@
 namespace integrity
 {
 
-Interface::Interface(const std::vector<uint8_t>& sik,
-                     const Key& addKey, size_t authLength)
+AlgoSHA1::AlgoSHA1(const std::vector<uint8_t>& sik)
+    : Interface(SHA1_96_AUTHCODE_LENGTH)
 {
-    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;
+    k1 = generateKn(sik, rmcp::const_1);
 }
 
 std::vector<uint8_t> AlgoSHA1::generateHMAC(const uint8_t* input,
@@ -32,7 +22,7 @@
     std::vector<uint8_t> output(SHA_DIGEST_LENGTH);
     unsigned int mdLen = 0;
 
-    if (HMAC(EVP_sha1(), K1.data(), K1.size(), input, len,
+    if (HMAC(EVP_sha1(), k1.data(), k1.size(), input, len,
              output.data(), &mdLen) == NULL)
     {
         throw std::runtime_error("Generating integrity data failed");
@@ -70,6 +60,23 @@
             packet.size() - message::parser::RMCP_SESSION_HEADER_SIZE);
 }
 
+std::vector<uint8_t> AlgoSHA1::generateKn(const std::vector<uint8_t>& sik,
+        const rmcp::Const_n& const_n) const
+{
+    unsigned int mdLen = 0;
+    std::vector<uint8_t> Kn(sik.size());
+
+    // Generated Kn for the integrity algorithm with the additional key keyed
+    // with SIK.
+    if (HMAC(EVP_sha1(), sik.data(), sik.size(), const_n.data(),
+             const_n.size(), Kn.data(), &mdLen) == NULL)
+    {
+        throw std::runtime_error("Generating KeyN for integrity "
+                                 "algorithm failed");
+    }
+    return Kn;
+}
+
 }// namespace integrity
 
 }// namespace cipher