Sign a XAR with productsign is corrupting the archive file

Question

I am trying to use productsign to sign a XAR archive containing 2 pkg files created using productbuild . The xar tool is creating the file correctly, but I think the signing is corrupting the content, even though the file obtained is signed. Some relevant command outputs below (I replaced the sensitive information with INFO):

$ ls .
file1.pkg file2.pkg
$
$ xar -cf '../_file.xar' .
$
$ cd ..
$ /usr/bin/productsign --sign 'Developer ID Installer: INFO' '_file.xar' 'file.xar'
productsign: using timestamp authority for signature
productsign: signing product with identity "Developer ID Installer: INFO" from keychain /Users/INFO/Library/Keychains/login.keychain
productsign: adding certificate "Developer ID Certification Authority"
productsign: adding certificate "Apple Root CA"
productsign: Wrote signed product archive to file.xar
$
$ /usr/sbin/pkgutil --check-signature 'file.xar'
Package "file.xar":
Status: signed by a certificate trusted by Mac OS X
Certificate Chain:
1. Developer ID Installer: INFO
SHA1 fingerprint: INFO
-----------------------------------------------------------------------------
2. Developer ID Certification Authority
SHA1 fingerprint: INFO
-----------------------------------------------------------------------------
3. Apple Root CA
SHA1 fingerprint: INFO

$ file file.xar
file.xar: xar archive - version 1
$
$ xar -xf file.xar -D /tmp
Error while extracting archive:(file1.pkg): Error decompressing file
$

Is the productsign command intended to work in some other way? I don't understand what is wrong with my approach.

Answer 1

After some testing, I found out xar is using relative paths inside the archive, so in my case there was a problem because I used "../" inside a path name.