.CRX File Format

Introduction

A .crx file packages an extension for Google Chrome and other Chromium-based browsers. It is a ZIP archive of the extension’s files, with a header in front that holds the public keys and signatures. The extension’s description, permissions and scripts are in a manifest.json at the top of the archive.

The format has had two versions. CRX3 is the current one. CRX2 was simpler, a single key and a signature. Chromium’s verifier today accepts only version 3.

Structure

Both versions start with the four bytes Cr24 and a 32-bit little-endian version number. A .crx dropped on any page of this site opens in the explorer, which shows the header field by field, the manifest, and every member of the archive.

Offsets inside the ZIP archive count from the start of the archive, not from the start of the file. So a ZIP reader that looks for the end record finds the archive, and the header in front looks like a prefix.

The rest of the format is described on these pages:

• CRX3 header – the current format, byte by byte
• CRX2 header – the old format, byte by byte
• Signing and IDs – what is signed, and how the extension ID comes from the key

Links