At the moment, most maintainers do the cryptographic check manually, or maybe even don't bother to do it at all. For the common case of detached OpenPGP signatures, though, uscan can now do it for you automatically (as of devscripts version 2.13.3). You just need to tell uscan what keys you expect upstream to be signing with, and how to find the detached signature.
So, for example, Damien Miller recently announced his new key that he will be using to sign OpenSSH releases (his new key has OpenPGP fingerprint 59C2 118E D206 D927 E667 EBE3 D3E5 F56B 6D92 0D30 -- you can verify it has been cross-signed by his older key, and his older key has been revoked with the indication that it was superceded by this one). Having done a reasonable verification of Damien's key, if i was the openssh package maintainer, i'd do the following:
cd ~/src/openssh/ mkdir -p debian/upstream gpg --export-options export-minimal --armor --export '59C2 118E D206 D927 E667 EBE3 D3E5 F56B 6D92 0D30' >> debian/upstream/signing-key.ascAnd then upon noticing that the signature files are named with a simple
.ascsuffix on the upstream distribution site, we can use the following pgpsigurlmangle option in debian/watch:
version=3 opts=pgpsigurlmangle=s/$/.asc/ ftp://ftp.openbsd.org/pub/OpenBSD/OpenSSH/portable/openssh-(.*)\.tar\.gzI've filed this specific example as debian bug #732441. If you notice a package with upstream signatures that aren't currently being checked by uscan (or if you are upstream, you sign your packages, and you want your debian maintainer to verify them), you can file similar bugs. Or, if you maintain a package for debian, you can just fix up your package so that this check is there on the next upload.
If you maintain a package whose upstream doesn't sign their releases, ask them why not -- wouldn't upstream prefer that their downstream users can verify that each release wasn't tampered with?
Of course, none of these checks take the the place of the real work of a debian package maintainer: reviewing the code and the changelogs, thinking about what changes have happened, and how they fit into the broader distribution. But it helps to automate one of the basic safeguards we should all be using. Let's eliminate the possibility that the file was tampered with at the upstream distribution mirror or while in transit over the network. That way, the maintainer's time and energy can be spent where they're more needed.
UPDATED 2015-05-03: use the currently-preferred location for the signing key.