diff --git a/md5.c b/md5.c
index 184a876..b024e63 100644
--- a/md5.c
+++ b/md5.c
@@ -46,7 +46,7 @@
  * doesn't work.
  */
 #if defined(__i386__) || defined(__vax__)
-# define SET(n) (*(MD5_u32plus *)&ptr[(n) * 4])
+# define SET(n) (*(const MD5_u32plus *)&ptr[(n) * 4])
 # define GET(n) SET(n)
 #else
 # define SET(n) \
@@ -63,9 +63,9 @@
  * This processes one or more 64-byte data blocks, but does NOT update
  * the bit counters.  There're no alignment requirements.
  */
-static void *body(MD5_CTX *ctx, void *data, unsigned long size)
+static const void *body(MD5_CTX *ctx, const void *data, unsigned long size)
 {
-    unsigned char *ptr;
+    const unsigned char *ptr;
     MD5_u32plus a, b, c, d;
     MD5_u32plus saved_a, saved_b, saved_c, saved_d;
 
@@ -181,7 +181,7 @@ void MD5_Init(MD5_CTX *ctx)
     ctx->hi = 0;
 }
 
-void MD5_Update(MD5_CTX *ctx, void *data, unsigned long size)
+void MD5_Update(MD5_CTX *ctx, const void *data, unsigned long size)
 {
     MD5_u32plus saved_lo;
     unsigned long used, free;
@@ -205,7 +205,7 @@ void MD5_Update(MD5_CTX *ctx, void *data, unsigned long size)
 	}
 
 	memcpy(&ctx->buffer[used], data, free);
-	data = (unsigned char *)data + free;
+	data = (const unsigned char *)data + free;
 	size -= free;
 	body(ctx, ctx->buffer, MD5_BLOCK_SZ);
     }
diff --git a/md5.h b/md5.h
index 25a59e0..971c3e6 100644
--- a/md5.h
+++ b/md5.h
@@ -23,7 +23,7 @@ typedef struct {
 } MD5_CTX;
 
 extern void MD5_Init(MD5_CTX *ctx);
-extern void MD5_Update(MD5_CTX *ctx, void *data, unsigned long size);
+extern void MD5_Update(MD5_CTX *ctx, const void *data, unsigned long size);
 extern void MD5_Final(unsigned char *result, MD5_CTX *ctx);
 
 #endif /* __MD5_H__ */