sslutil: issue warning when [hostfingerprint] is used

Mercurial 3.9 added the [hostsecurity] section, which is better
than [hostfingerprints] in every way.

One of the ways that [hostsecurity] is better is that it supports
SHA-256 and SHA-512 fingerprints, not just SHA-1 fingerprints.

The world is moving away from SHA-1 because it is borderline
secure. Mercurial should be part of that movement.

This patch adds a warning when a valid SHA-1 fingerprint from
the [hostfingerprints] section is being used. The warning informs
users to switch to [hostsecurity]. It even prints the config
option they should set. It uses the SHA-256 fingerprint because
recommending a SHA-1 fingerprint in 2017 would be ill-advised.

The warning will print itself on every connection to a server until
it is fixed. There is no way to suppress the warning. I admit this
is annoying. But given the security implications of sticking with
SHA-1, I think this is justified. If this patch is accepted,
I'll likely send a follow-up to start warning on SHA-1
certificates in [hostsecurity] as well. Then sometime down
the road, we can drop support for SHA-1 fingerprints.

Credit for this idea comes from timeless in issue 5466.

mercurial/sslutil.py

@@ -815,6 +815,16 b' def validatesocket(sock):'
             if peerfingerprints[hash].lower() == fingerprint:
                 ui.debug('%s certificate matched fingerprint %s:%s\n' %
                          (host, hash, fmtfingerprint(fingerprint)))
+                if settings['legacyfingerprint']:
+                    ui.warn(_('(SHA-1 fingerprint for %s found in legacy '
+                              '[hostfingerprints] section; '
+                              'if you trust this fingerprint, set the '
+                              'following config value in [hostsecurity] and '
+                              'remove the old one from [hostfingerprints] '
+                              'to upgrade to a more secure SHA-256 '
+                              'fingerprint: '
+                              '%s.fingerprints=%s)\n') % (
+                                  host, host, nicefingerprint))
                 return
 
         # Pinned fingerprint didn't match. This is a fatal error.

tests/test-https.t

@@ -382,6 +382,7 b' Fingerprints'
 - works without cacerts (hostfingerprints)
   $ hg -R copy-pull id https://localhost:$HGPORT/ --insecure --config hostfingerprints.localhost=ec:d8:7c:d6:b3:86:d0:4f:c1:b8:b4:1c:9d:8f:5e:16:8e:ef:1c:03
   warning: connecting to localhost using legacy security technology (TLS 1.0); see https://mercurial-scm.org/wiki/SecureConnections for more info (?)
+  (SHA-1 fingerprint for localhost found in legacy [hostfingerprints] section; if you trust this fingerprint, set the following config value in [hostsecurity] and remove the old one from [hostfingerprints] to upgrade to a more secure SHA-256 fingerprint: localhost.fingerprints=sha256:20:de:b3:ad:b4:cd:a5:42:f0:74:41:1c:a2:70:1e:da:6e:c0:5c:16:9e:e7:22:0f:f1:b7:e5:6e:e4:92:af:7e)
   5fed3813f7f5
 
 - works without cacerts (hostsecurity)
@@ -396,6 +397,7 b' Fingerprints'
 - multiple fingerprints specified and first matches
   $ hg --config 'hostfingerprints.localhost=ecd87cd6b386d04fc1b8b41c9d8f5e168eef1c03, deadbeefdeadbeefdeadbeefdeadbeefdeadbeef' -R copy-pull id https://localhost:$HGPORT/ --insecure
   warning: connecting to localhost using legacy security technology (TLS 1.0); see https://mercurial-scm.org/wiki/SecureConnections for more info (?)
+  (SHA-1 fingerprint for localhost found in legacy [hostfingerprints] section; if you trust this fingerprint, set the following config value in [hostsecurity] and remove the old one from [hostfingerprints] to upgrade to a more secure SHA-256 fingerprint: localhost.fingerprints=sha256:20:de:b3:ad:b4:cd:a5:42:f0:74:41:1c:a2:70:1e:da:6e:c0:5c:16:9e:e7:22:0f:f1:b7:e5:6e:e4:92:af:7e)
   5fed3813f7f5
 
   $ hg --config 'hostsecurity.localhost:fingerprints=sha1:ecd87cd6b386d04fc1b8b41c9d8f5e168eef1c03, sha1:deadbeefdeadbeefdeadbeefdeadbeefdeadbeef' -R copy-pull id https://localhost:$HGPORT/
@@ -405,6 +407,7 b' Fingerprints'
 - multiple fingerprints specified and last matches
   $ hg --config 'hostfingerprints.localhost=deadbeefdeadbeefdeadbeefdeadbeefdeadbeef, ecd87cd6b386d04fc1b8b41c9d8f5e168eef1c03' -R copy-pull id https://localhost:$HGPORT/ --insecure
   warning: connecting to localhost using legacy security technology (TLS 1.0); see https://mercurial-scm.org/wiki/SecureConnections for more info (?)
+  (SHA-1 fingerprint for localhost found in legacy [hostfingerprints] section; if you trust this fingerprint, set the following config value in [hostsecurity] and remove the old one from [hostfingerprints] to upgrade to a more secure SHA-256 fingerprint: localhost.fingerprints=sha256:20:de:b3:ad:b4:cd:a5:42:f0:74:41:1c:a2:70:1e:da:6e:c0:5c:16:9e:e7:22:0f:f1:b7:e5:6e:e4:92:af:7e)
   5fed3813f7f5
 
   $ hg --config 'hostsecurity.localhost:fingerprints=sha1:deadbeefdeadbeefdeadbeefdeadbeefdeadbeef, sha1:ecd87cd6b386d04fc1b8b41c9d8f5e168eef1c03' -R copy-pull id https://localhost:$HGPORT/
@@ -436,6 +439,7 b' Fingerprints'
 - ignores that certificate doesn't match hostname
   $ hg -R copy-pull id https://$LOCALIP:$HGPORT/ --config hostfingerprints.$LOCALIP=ecd87cd6b386d04fc1b8b41c9d8f5e168eef1c03
   warning: connecting to $LOCALIP using legacy security technology (TLS 1.0); see https://mercurial-scm.org/wiki/SecureConnections for more info (?)
+  (SHA-1 fingerprint for $LOCALIP found in legacy [hostfingerprints] section; if you trust this fingerprint, set the following config value in [hostsecurity] and remove the old one from [hostfingerprints] to upgrade to a more secure SHA-256 fingerprint: $LOCALIP.fingerprints=sha256:20:de:b3:ad:b4:cd:a5:42:f0:74:41:1c:a2:70:1e:da:6e:c0:5c:16:9e:e7:22:0f:f1:b7:e5:6e:e4:92:af:7e)
   5fed3813f7f5
 
 Ports used by next test. Kill servers.
@@ -574,6 +578,7 b' Test https with cacert and fingerprint t'
   $ http_proxy=http://localhost:$HGPORT1/ hg -R copy-pull pull https://localhost:$HGPORT/ --config hostfingerprints.localhost=ecd87cd6b386d04fc1b8b41c9d8f5e168eef1c03 --trace
   pulling from https://*:$HGPORT/ (glob)
   warning: connecting to localhost using legacy security technology (TLS 1.0); see https://mercurial-scm.org/wiki/SecureConnections for more info (?)
+  (SHA-1 fingerprint for localhost found in legacy [hostfingerprints] section; if you trust this fingerprint, set the following config value in [hostsecurity] and remove the old one from [hostfingerprints] to upgrade to a more secure SHA-256 fingerprint: localhost.fingerprints=sha256:20:de:b3:ad:b4:cd:a5:42:f0:74:41:1c:a2:70:1e:da:6e:c0:5c:16:9e:e7:22:0f:f1:b7:e5:6e:e4:92:af:7e)
   searching for changes
   no changes found