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Signing using private key?



There is no need to sign. Just keep a cryptographic hash (SHA256 is a good bet) of the package in the dependencies manifest, and check it after download.

Using a git repository gives you that for free.


Attacker changes package, then changes hash in manifest to match. Checks pass, users are compromised.

Attacker clones git repo, creates new commit with trojan, pushes to repo with compromised credentials. New users clone compromised repo, others pull and fast-forward. All hashes are valid.

You need to read the Strong Distribution HOWTO: http://www.cryptnet.net/fdp/crypto/strong_distro.html


I don't get it. If I keep a hash of every dependency in my project, and an attacker change a dependency, I will detect the attack by computing the dependency hash and comparing it with the one stored in my project (for example in the dependencies lock file), which cannot be changed by the attacker.


> ...which cannot be changed by the attacker.

That's fine on your personal system, where you manually update your package's dependencies and manually update the hashes of your package's dependencies.

Now, what happens if an attacker compromises the repo where people download your package from? Or what if he executes a MitM attack on the repo or a user who downloads from it? He can change the entire package, including all manifests, all hashes, etc. Users who download it will be none the wiser. By the time someone notices and corrects it, people will have downloaded the compromised versions and be infected.

The only thing that protects against this is strong crypto signatures. For example, if a Debian mirror were compromised and an attacker uploaded compromised packages, users would be safe, because apt would refuse to install the packages, because they would fail signature verification.

Please read the link I gave. It explains everything in detail.


SHA1 collisions are affordable for large actors now and getting cheaper all the time, so git, with its SHA1 assumptions hard-coded, unfortunately doesn't protect you anymore. But the local hash approach is a good one, proven in pip 8 (Python) and npm-lockdown (JS).


More for new versions of same software where the account might be highjacked but hopefully the key wasnt. I dont think a hash would help.




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