diff --git a/Source/codec.cpp b/Source/codec.cpp
index 8e0882eb7..7977720cd 100644
--- a/Source/codec.cpp
+++ b/Source/codec.cpp
@@ -80,10 +80,10 @@ void codec_init_key(int unused, char *pszPassword)
 // 4035DB: using guessed type char var_58[64];
 // 4035DB: using guessed type char dst[20];
 
-int codec_get_encoded_len(int dwSrcBytes)
+DWORD codec_get_encoded_len(DWORD dwSrcBytes)
 {
-	if (dwSrcBytes & 0x3F)
-		dwSrcBytes += 64 - (dwSrcBytes & 0x3F);
+	if (dwSrcBytes % 64 != 0)
+		dwSrcBytes += 64 - (dwSrcBytes % 64);
 	return dwSrcBytes + 8;
 }
 
diff --git a/Source/codec.h b/Source/codec.h
index 0df61c1e8..c801e0a9e 100644
--- a/Source/codec.h
+++ b/Source/codec.h
@@ -4,7 +4,7 @@
 
 int codec_decode(BYTE *pbSrcDst, DWORD size, char *pszPassword);
 void codec_init_key(int unused, char *pszPassword);
-int codec_get_encoded_len(int dwSrcBytes);
+DWORD codec_get_encoded_len(DWORD dwSrcBytes);
 void codec_encode(void *pbSrcDst, int size, int size_64, char *pszPassword);
 
 #endif /* __CODEC_H__ */