Comment 3 for bug 1322892

We needed to apply a fix to curve25519-sha256 key exchange supplied by upstream, but that involved disabling that key exchange method for unpatched 6.6 and earlier versions. OpenSSH 6.7 will fix this when it's released, but in the meantime I suggest applying the same patch to your homebrew client (and perhaps getting the package maintainer to do so as well; I'm not familiar with how homebrew works).

  https://lists.mindrot.org/pipermail/openssh-unix-dev/2014-April/032494.html