diff --git a/hmacimpl.h b/hmacimpl.h
index 3d9fdee..d1c2c93 100644
--- a/hmacimpl.h
+++ b/hmacimpl.h
@@ -31,15 +31,18 @@
 #define hash_finalize concat(HASH_ALG,_finalize)
 
 #define hmac_init     concat3(hmac_,HASH_ALG,_init)
+#define hmac_update   concat3(hmac_,HASH_ALG,_update)
+#define hmac_finalize concat3(hmac_,HASH_ALG,_finalize)
 #define hmac_compute  concat3(hmac_,HASH_ALG,_compute)
 #define hmac_context  concat3(hmac_,HASH_ALG,_context)
 
+#define BLOCK_SIZE sizeof(context->u[0].ctx.block)
+#define HASH_SIZE  sizeof(context->u[0].ctx.hash)
+
 void hmac_init(struct hmac_context *context,
                const unsigned char *key,
                unsigned long key_length)
 {
-#define BLOCK_SIZE sizeof(context->u[0].ctx.block)
-#define HASH_SIZE  sizeof(context->u[0].ctx.hash)
 
     unsigned i;
 
@@ -67,6 +70,30 @@ void hmac_init(struct hmac_context *context,
     // Save outer hash context following initial block as context->u[1].ctx
     hash_init(&context->u[1].ctx);
     hash_update(&context->u[1].ctx, context->u[0].k, BLOCK_SIZE);
+
+    // initialize context for use with hmac_update
+    context->u[0].ctx = context->u[2].ctx;
+}
+
+
+void hmac_update(struct hmac_context *context,
+               const unsigned char *message_part,
+               unsigned long part_length)
+{
+    hash_update(&context->u[0].ctx, message_part, part_length);
+}
+
+
+void hmac_finalize(struct hmac_context *context)
+{
+    // finalize inner hash
+    hash_finalize(&context->u[0].ctx);
+    memcpy(context->inner_hash, context->u[0].ctx.hash, HASH_SIZE);
+    
+    // Compute outer hash
+    context->u[0].ctx = context->u[1].ctx;
+    hash_update(&context->u[0].ctx, context->inner_hash, HASH_SIZE);
+    hash_finalize(&context->u[0].ctx);
 }
 
 
diff --git a/hmactest.c b/hmactest.c
index d733837..577b26d 100644
--- a/hmactest.c
+++ b/hmactest.c
@@ -48,7 +48,6 @@ int main(void) {
         printf("%02x", hmac_sha256_context->u[0].ctx.hash[i]);
     }
     printf("\n");
-    
         
     hmac_sha1_context = (struct hmac_sha1_context *)*context_hndl;
     hmac_sha1_init(hmac_sha1_context, key, sizeof(key)-1);
@@ -60,7 +59,6 @@ int main(void) {
     }
     printf("\n");
 
-
     hmac_md5_context = (struct hmac_md5_context *)*context_hndl;
     hmac_md5_init(hmac_md5_context, key, sizeof(key)-1);
     hmac_md5_compute(hmac_md5_context, msg, sizeof(msg)-1);
@@ -81,4 +79,15 @@ int main(void) {
         printf("%02x", hmac_md4_context->u[0].ctx.hash[i]);
     }
     printf("\n");
+
+    hmac_md5_init(hmac_md5_context, key, sizeof(key)-1);
+    hmac_md5_update(hmac_md5_context, "T", 1);
+    hmac_md5_update(hmac_md5_context, msg+1, sizeof(msg)-2);
+    hmac_md5_finalize(hmac_md5_context);
+    
+    printf("HMAC-MD5 (incremental calculation): ");
+    for (int i = 0; i < sizeof(hmac_md5_context->u[0].ctx.hash); i++) {
+        printf("%02x", hmac_md5_context->u[0].ctx.hash[i]);
+    }
+    printf("\n");
 }
diff --git a/md5.h b/md5.h
index 3d40b16..282184d 100644
--- a/md5.h
+++ b/md5.h
@@ -60,18 +60,32 @@ void md5_processblock(struct md5_context *context);
 
 /*
  * Initialize a context for HMAC-MD5 computation with a specified key.
- * This must be called before any calls to hmac_md5_compute. After 
- * initialization, the context can be used to compute the HMAC for any
- * number of messages.
+ * This must be called before any other HMAC calls. After initialization,
+ * the context can be used with either hmac_md5_update/hmac_md5_finalize
+ * or hmac_md5_compute, but they should not be mixed.
  */
 void hmac_md5_init(struct hmac_md5_context *context,
                    const unsigned char *key,
                    unsigned long key_length);
 
 /*
- * Compute the HMAC-MD5 of a message, using an already-initialized context.
+ * Update an HMAC-MD5 context based on the specified data.
+ */
+void hmac_md5_update(struct hmac_md5_context *context,
+                     const unsigned char *message_part,
+                     unsigned long part_length);
+
+/*
+ * Finish HMAC-MD5 processing and generate the final HMAC.
  * The result will be in context->u[0].ctx.hash.
  */
+void hmac_md5_finalize(struct hmac_md5_context *context);
+
+/*
+ * Compute the HMAC-MD5 of a message as a single operation.
+ * The result will be in context->u[0].ctx.hash.
+ * The context can be reused for multiple hmac_md5_compute operations.
+ */
 void hmac_md5_compute(struct hmac_md5_context *context,
                       const unsigned char *message,
                       unsigned long message_length);