CRX Package Format

Package header
Extension contents
Example
Packaging scripts
1. Ruby
2. Bash

CRX files are ZIP files with a special header and the .crx file extension.

Package header

The header contains the author's public key and the extension's signature. The signature is generated from the ZIP file using SHA-1 with the author's private key. The header requires a little-endian byte ordering with 4-byte alignment. The following table describes the fields of the .crx header in order:

Field	Type	Length	Value	Description
magic number	char[]	32 bits	Cr24	Chrome requires this constant at the beginning of every `.crx` package.
version	unsigned int	32 bits	2	The version of the `*.crx` file format used (currently 2).
public key length	unsigned int	32 bits	pubkey.length	The length of the RSA public key in bytes.
signature length	unsigned int	32 bits	sig.length	The length of the signature in bytes.
public key	byte[]	pubkey.length	pubkey.contents	The contents of the author's RSA public key, formatted as an X509 SubjectPublicKeyInfo block.
signature	byte[]	sig.length	sig.contents	The signature of the ZIP content using the author's private key. The signature is created using the RSA algorithm with the SHA-1 hash function.

Extension contents

The extension's ZIP file is appended to the *.crx package after the header. This should be the same ZIP file that the signature in the header was generated from.

Example

The following is an example hex dump from the beginning of a .crx file.

43 72 32 34   # "Cr24" -- the magic number
02 00 00 00   # 2 -- the crx format version number
A2 00 00 00   # 162 -- length of public key in bytes
80 00 00 00   # 128 -- length of signature in bytes
...........   # the contents of the public key
...........   # the contents of the signature
...........   # the contents of the zip file

Packaging scripts

Members of the community have written the following scripts to package .crx files.

Ruby

github: crxmake

Bash

#!/bin/bash -e
#
# Purpose: Pack a Chromium extension directory into crx format

if test $# -ne 2; then
  echo "Usage: crxmake.sh <extension dir> <pem path>"
  exit 1
fi

dir=$1
key=$2
name=$(basename "$dir")
crx="$name.crx"
pub="$name.pub"
sig="$name.sig"
zip="$name.zip"
trap 'rm -f "$pub" "$sig" "$zip"' EXIT

# zip up the crx dir
cwd=$(pwd -P)
(cd "$dir" && zip -qr -9 -X "$cwd/$zip" .)

# signature
openssl sha1 -sha1 -binary -sign "$key" < "$zip" > "$sig"

# public key
openssl rsa -pubout -outform DER < "$key" > "$pub" 2>/dev/null

byte_swap () {
  # Take "abcdefgh" and return it as "ghefcdab"
  echo "${1:6:2}${1:4:2}${1:2:2}${1:0:2}"
}

crmagic_hex="4372 3234" # Cr24
version_hex="0200 0000" # 2
pub_len_hex=$(byte_swap $(printf '%08x\n' $(ls -l "$pub" | awk '{print $5}')))
sig_len_hex=$(byte_swap $(printf '%08x\n' $(ls -l "$sig" | awk '{print $5}')))
(
  echo "$crmagic_hex $version_hex $pub_len_hex $sig_len_hex" | xxd -r -p
  cat "$pub" "$sig" "$zip"
) > "$crx"
echo "Wrote $crx"

Google Chrome Extensions