root@bt:~# apt-get install twofi
Reading package lists... Done
Building dependency tree       
Reading state information... Done
twofi is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 97 not upgraded.
1 not fully installed or removed.
After this operation, 0B of additional disk space will be used.
Setting up twofi (1.0-bt0) ...
Building native extensions.  This could take a while...
ERROR:  Error installing json:
        ERROR: Failed to build gem native extension.

/usr/bin/ruby1.9.2 extconf.rb
extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
        from extconf.rb:1:in `<main>'

Gem files will remain installed in /var/lib/gems/1.9.2/gems/json-1.7.5 for inspection.
Results logged to /var/lib/gems/1.9.2/gems/json-1.7.5/ext/json/ext/generator/gem_make.out
dpkg: error processing twofi (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
E: Sub-process /usr/bin/dpkg returned an error code (1)
I've googled for 30minutes, but i couldn't fix it. Please help me