I have to ask: how are the PGP keys verified?
https://en.wikipedia.org/wiki/Web_of_trust
http://www.pgpi.org/doc/pgpintro/
http://www.rubin.ch/pgp/weboftrust.en.html
Essentially: with cryptography, you aren't concerned about the transport, you're concerned about the crypto.
Signatures of the source or binaries will tell you if they've been changed.
Signatures on keys will tell you who trusts those keys. If someone you trust trusts a key, then you have a transitive trust of a key. This does mean, though, trusting people not to sign bogus keys.
In TLS/SSL, you have a hierarchical trust. Your session (and its contents) are encrypted and authenticated by the host key, which is signed by the CA's key. The theory is that we can trust CAs. Practice shows otherwise.
It's not cryptographically viable to fake a signature (of a key or data), though if someone manages to lose control of their private key, that's possible. PGP/GPG doesn't handle expired keys well.
But the upshot is that HTTP transport of PuTTY is fine from an integrity standpoint, so long as you verify the binary and signatures. And if you don't mind others being able to see what you're fetching as you're fetching it (the only real benefit to HTTPS in this case).
Even an MITM rewrite of data isn't a critical issue since you can catch that in PGP validation.
I understand all those things in general, but I don't know the specific mechanism by which my local PGP install recognizes who else trusts this PGP key. I grant you that if the key is protected from MITM then all is well. I just still don't know this part:
What mechanism do I use, I who have no prior encounter with that key and no existing PGP setup or connection to any web, to validate that key?
I'm sure this is just a lack of familiarity with PGPs web of trust implementation, but lacking this info, I too just opted to trust the plain HTTP download (until I switched to Cygwin/OpenSSH to make it a moot point anyway).
In general, I fetch signatures of keys and may add some of these as partially trusted if they're very well known keys.
A bit of bash scripting that helps with this:
gpg --list-sigs <key ID> | grep 'not found' |
cut -c 13-22 | sort -u |
xargs --max-args 10 gpg --recv-keys
You cannot run parallel processes as your local gpg instance cannot do concurrent updates to the keyring.
Which, arguably, might be safer than being overly trusting of my "trusted CAs", which is verified by a flawed system.
I guess I'm still not thrilled wih my options overall, but thank you for your time explaining how to use PGP.