Handle cert fingerprints that contain '/'

[diracdeltas]

The cert fingerprint emitted by Muon/Electron is base64 encoded [1], so it may contain '/'. I think we were truncating the certificate fingerprint when this was the case, which may have caused the decoding error in #6524.

[1] https://cs.chromium.org/chromium/src/net/base/hash_value.h?q=net::HashValue&sq=package:chromium&l=58

Should fix #6524

Auditors: @darkdh

[darkdh]

lgtm!
Can we ask Eric Lawrence to provide the url of the site whose certificate contains "/" fingerprint in base64? Just wanna record it in this PR.

[diracdeltas]

^ pinging @ericlaw1979

[ericlaw1979]

Can we ask Eric Lawrence to provide the url of the site whose certificate contains "/" fingerprint in base64? Just wanna record it in this PR.

This wasn't a live site, but instead a self-generated certificate (like one a bad guy could generate for his own server).

My repro used Fiddler. Enable HTTPS decryption, then click
Rules > Customize Rules. Inside OnBeforeRequest, set the desired CN value, e.g.

static function OnBeforeRequest(oSession: Session) { oSession["X-overridecertCN"]= "<b>alert(1)";

Save the file, then visit any HTTPS site to have the self-generated certificate with the bad CN value returned to the client.

[bsclifton]

Last travis-ci run had an error; I re-ran this test locally and it's working as expected 😄 Merging...