I'm having trouble logging into a server that runs minecraft 1.5.2, this issue has been noted by:
http://www.minecraftforum.net/topic/2160943-javalangruntimeexception-public-key-mismatch/

I have no idea who this person (the OP of that thread) is, the issue was affecting Windows clients, however the update to the new launcher fixes this for Windows users. I can't however 'update' because the 'legacy launcher' (hosted on s3) downloads MC 1.7.2, and doesn't allow me to connect to my friends server. I reverted, tried changing permissions on the folder/files but it gives me a permissions issue, to be expected.

I even went as far as trying to remove the key (and locate the new launcher key [non existant]) but this doesn't work.

I tried downloading the new launcher minecraft.jar from the official mojang website (s3), however the new launcher does not work as expected. The new features look amazing, but as soon as I try to run the launcher, it automatically starts downloading Minecraft 1.7.2. I hope to get there one day, but MODS are preventing the server operator from upgrading.

I'm running Java 1.6_45, and Java 1.7_45 as well, they're in separate directories in /opt/. However this is likely NOT the issue at all.