If your browser supports HTML5 file-api you can encrypt/decrypt local files without uploading to a server.
The upper Encrypt/Decrypt buttons are for file encryption. The file size is limited to a few megabytes because of the data-URL saving. The file is read directly with the FileAPI. The encrypted file is padded up to a multiple of 16 bytes, but the original file size is saved in the first 4 bytes, so at decryption will be truncated to the original size.
The lower buttons are for text encryption/decryption. The action is applied to the upper text box. The encrypted text is displayed in hexadecimal. The same format must be used for decryption. The text is padded with spaces up to a multiple of 16 characters.
The password (key) is padded with zeros up to 32 bytes.