diff --git a/md5.c b/md5.c
index b024e63..ba72eb9 100644
--- a/md5.c
+++ b/md5.c
@@ -270,3 +270,50 @@ void MD5_Final(unsigned char *result, MD5_CTX *ctx)
 
     memset(ctx, 0, sizeof(*ctx));
 }
+
+void MD5_Hmac(unsigned char *result, const void *data, unsigned long size, const void *key, unsigned long key_size)
+{
+    MD5_CTX ctx;
+    unsigned char block[MD5_BLOCK_SZ];
+
+    unsigned char short_key[MD5_DIGEST_SZ];
+    unsigned char temp[MD5_DIGEST_SZ];
+
+    const unsigned char *used_key;
+    int i;
+
+    if (key_size > MD5_BLOCK_SZ)
+    {
+	MD5_Init(&ctx);
+	MD5_Update(&ctx, key, key_size);
+	MD5_Final(short_key, &ctx);
+	used_key = short_key;
+	key_size = MD5_DIGEST_SZ;
+    }
+    else
+    {
+	used_key = key;
+    }
+
+    MD5_Init(&ctx);
+
+    memset(block, 0x36, MD5_BLOCK_SZ);
+    for (i = 0; i < key_size; i++)
+	block[i] ^= used_key[i];
+    MD5_Update(&ctx, block, MD5_BLOCK_SZ);
+
+    MD5_Update(&ctx, data, size);
+
+    MD5_Final(temp, &ctx);
+
+    MD5_Init(&ctx);
+
+    memset(block, 0x5c, MD5_BLOCK_SZ);
+    for (i = 0; i < key_size; i++)
+	block[i] ^= used_key[i];
+    MD5_Update(&ctx, block, MD5_BLOCK_SZ);
+
+    MD5_Update(&ctx, temp, sizeof(temp));
+
+    MD5_Final(result, &ctx);
+}
diff --git a/md5.h b/md5.h
index 971c3e6..823275e 100644
--- a/md5.h
+++ b/md5.h
@@ -25,5 +25,6 @@ typedef struct {
 extern void MD5_Init(MD5_CTX *ctx);
 extern void MD5_Update(MD5_CTX *ctx, const void *data, unsigned long size);
 extern void MD5_Final(unsigned char *result, MD5_CTX *ctx);
+extern void MD5_Hmac(unsigned char *result, const void *data, unsigned long size, const void *key, unsigned long key_size);
 
 #endif /* __MD5_H__ */