diff --git a/mozilla-sha1/sha1.c b/mozilla-sha1/sha1.c
index 7f6fc05e06..847531d19f 100644
--- a/mozilla-sha1/sha1.c
+++ b/mozilla-sha1/sha1.c
@@ -56,8 +56,8 @@ void SHA1_Init(SHA_CTX *ctx) {
 }
 
 
-void SHA1_Update(SHA_CTX *ctx, void *_dataIn, int len) {
-  unsigned char *dataIn = _dataIn;
+void SHA1_Update(SHA_CTX *ctx, const void *_dataIn, int len) {
+  const unsigned char *dataIn = _dataIn;
   int i;
 
   /* Read the data into W and process blocks as they get full
diff --git a/mozilla-sha1/sha1.h b/mozilla-sha1/sha1.h
index f5decbf43b..5d82afa3bd 100644
--- a/mozilla-sha1/sha1.h
+++ b/mozilla-sha1/sha1.h
@@ -41,5 +41,5 @@ typedef struct {
 } SHA_CTX;
 
 void SHA1_Init(SHA_CTX *ctx);
-void SHA1_Update(SHA_CTX *ctx, void *dataIn, int len);
+void SHA1_Update(SHA_CTX *ctx, const void *dataIn, int len);
 void SHA1_Final(unsigned char hashout[20], SHA_CTX *ctx);