[arch-general] Packages Verified with MD5

Jelle van der Waa jelle at vdwaa.nl
Sun Jan 12 12:27:49 EST 2014

On 01/12/14 at 09:58am, Taylor Hornby wrote:
> On 01/12/2014 02:58 AM, Rashif Ray Rahman wrote:
> > On 12 January 2014 14:09, Taylor Hornby <havoc at defuse.ca> wrote:
> >> Are there other packages still being verified with MD5? Can we fix them
> >> too? I'll gladly donate my time if it's not something that can be automated.
> > 
> > Of the 4890 base packages shown by ABS, 2988 are MD5-only. That is
> > 61%, or more than half.
> > 
> Wow, that's quite a lot.
> Do I understand correctly that the hashes are relied on for security? In
> other words, is it the package (containing the PKGBUILD) that's signed,
> and once it's verified, it's the PKGBUILD's responsibility to check the
> integrity of the files it needs?
> If so, this should be fixed as soon as possible. How feasible would it
> be? Could it be as simple as making a script that:
> 1. Finds the 'source' and 'md5sums' lines.
> 2. Downloads the packages and checks the md5sums.
> 3. Computes the SHA256sums, and adds them to the file.
> If there's anything I can do to help, let me know.
> -- 
> Taylor Hornby

No, you don't rely on hashes for security, hashes are for integrity
checks. Signatures are for the verification of a file or message, since
anyone can replace the hash on the server and upload a new tarball.

Signatures can only be created by the developers private key, it hashes
a file or messages, then encrypts this hash with his private key. Then
the developer puts the signature and tarball on a server.

Everyone who has somehow obtained the developers public key, can verify
that the tarball hasn't been tampered with by creating a hash from the
tarball and comparing it with the decrypted signature (original hash).

If a hacker uploaded a malicious tarball, he would be able to create a
new hash, but wouldn't be able to create a new valid signature.

PS: the explanation of signing isn't exactly correct, since I didn't
explain that there hash is actually not encrypted with the private key.
A nice explanation of PGP can be found here:

PS2: You may raise more concerns about the truecrypts code.

Jelle van der Waa
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