diff --git a/src/util/HMAC.cpp b/src/util/HMAC.cpp
new file mode 100644
index 0000000..538b784
--- /dev/null
+++ b/src/util/HMAC.cpp
@@ -0,0 +1,38 @@
+#include "util/HMAC.hpp"
+#include <cstring>
+#include <common/SHA1.hpp>
+
+#define HMAC_SHA1_IPAD 0x36
+#define HMAC_SHA1_OPAD 0x5C
+#define HMAC_SHA1_BLOCK_SIZE 64
+
+void HMAC_SHA1(const uint8_t* key, uint32_t keyLen, const uint8_t* data, uint32_t dataLen, uint8_t* out) {
+    uint8_t ibuf[HMAC_SHA1_BLOCK_SIZE], obuf[HMAC_SHA1_BLOCK_SIZE];
+
+    for (uint32_t i = 0; i < keyLen; i++) {
+        ibuf[i] = key[i] ^ HMAC_SHA1_IPAD;
+        obuf[i] = key[i] ^ HMAC_SHA1_OPAD;
+    }
+
+    for (uint32_t i = keyLen; i < HMAC_SHA1_BLOCK_SIZE; i++) {
+        ibuf[i] = HMAC_SHA1_IPAD;
+        obuf[i] = HMAC_SHA1_OPAD;
+    }
+
+    SHA1_CONTEXT ctx;
+    uint8_t idigest[SHA1_DIGEST_SIZE];
+
+    // Inner
+
+    SHA1_Init(&ctx);
+    SHA1_Update(&ctx, ibuf, HMAC_SHA1_BLOCK_SIZE);
+    SHA1_Update(&ctx, data, dataLen);
+    SHA1_Final(idigest, &ctx);
+
+    // Outer
+
+    SHA1_Init(&ctx);
+    SHA1_Update(&ctx, obuf, HMAC_SHA1_BLOCK_SIZE);
+    SHA1_Update(&ctx, idigest, SHA1_DIGEST_SIZE);
+    SHA1_Final(out, &ctx);
+}
diff --git a/src/util/HMAC.hpp b/src/util/HMAC.hpp
new file mode 100644
index 0000000..c154145
--- /dev/null
+++ b/src/util/HMAC.hpp
@@ -0,0 +1,8 @@
+#ifndef UTIL_HMAC_HPP
+#define UTIL_HMAC_HPP
+
+#include <cstdint>
+
+void HMAC_SHA1(const uint8_t* key, uint32_t keyLen, const uint8_t* data, uint32_t dataLen, uint8_t* out);
+
+#endif