gpgv: Untrust SHA1, RIPE-MD/160, but allow downgrading to weak

Change the trust level check to allow downgrading an Untrusted
option to weak (APT::Hashes::SHA1::Weak "yes";), so it prints
a warning instead of an error; and change the default values
for SHA1 and RIPE-MD/160 from Weak to Untrusted.

debian/NEWS

@@ -1,3 +1,21 @@
+apt (1.4~beta1) UNRELEASED; urgency=medium
+
+  Support for GPG signatures using the SHA1 or RIPE-MD/160 hash
+  algorithms has been disabled. Repositories using Release files
+  signed in such a way will stop working. This change has been made
+  due to security considerations, especially with regards to possible
+  further breakthroughs in SHA1 breaking during the lifetime
+  of this APT release series.
+
+  It is possible (but STRONGLY ADVISED AGAINST) to revert to the previous
+  behaviour by setting the options
+    APT::Hashes::SHA1::Weak "yes";
+    APT::Hashes::RIPE-MD/160::Weak "yes";
+  Note that setting these options only affects the verification of the overall
+  repository signature.
+
+ -- Julian Andres Klode <jak@debian.org>  Fri, 25 Nov 2016 13:19:32 +0100
+
 apt (1.2~exp1) experimental; urgency=medium
 
   [ Automatic removal of debs after install ]

methods/gpgv.cc

@@ -55,9 +55,9 @@ struct Digest {
       std::string optionWeak;
       strprintf(optionUntrusted, "APT::Hashes::%s::Untrusted", name);
       strprintf(optionWeak, "APT::Hashes::%s::Weak", name);
-      if (_config->FindB(optionUntrusted, state == State::Untrusted) == true)
+      if (_config->FindB(optionUntrusted, false) == true)
 	 return State::Untrusted;
-      if (_config->FindB(optionWeak, state == State::Weak) == true)
+      if (_config->FindB(optionWeak, false) == true)
 	 return State::Weak;
 
       return state;
@@ -67,8 +67,8 @@ struct Digest {
 static constexpr Digest Digests[] = {
    {Digest::State::Untrusted, "Invalid digest"},
    {Digest::State::Untrusted, "MD5"},
-   {Digest::State::Weak, "SHA1"},
-   {Digest::State::Weak, "RIPE-MD/160"},
+   {Digest::State::Untrusted, "SHA1"},
+   {Digest::State::Untrusted, "RIPE-MD/160"},
    {Digest::State::Trusted, "Reserved digest"},
    {Digest::State::Trusted, "Reserved digest"},
    {Digest::State::Trusted, "Reserved digest"},