If you're using Python you can use the music21 toolkit to do this:
import music21
score = music21.converter.parse('filename.mid')
key = score.analyze('key')
print(key.tonic.name, key.mode)
if you care about specific algorithms for key finding, you can use them instead of the generic "key":
key1 = score.analyze('Krumhansl')
key2 = score.analyze('AardenEssen')
etc. Any of these methods will work for chords also.
(Disclaimer: music21 is my project, so of course I have a vested interest in promoting it; but you can look at the music21.analysis.discrete module to take ideas from there for other projects/languages. If you have a MIDI parser, the Krumhansl algorithm is not hard to implement).