Not even close. This technique only works when the relative energy of different frequency buckets remains the same, and the same time periods apart (in milliseconds). You are very unlikely to have the same fingerprints when repeating the same words/phrases.

Try using an app that shows the FFT and see if you can get it to show the same thing twice when speaking. For example on Android this works https://play.google.com/store/apps/details?id=org.hermit.aud...

You can use CMUSphinx http://cmusphinx.sourceforge.net for keyphrase verification. For example you can find Android demo for keyphrase spotting at http://cmusphinx.sourceforge.net/wiki/tutorialandroid

Nope! Dejavu is meant for recovery of perfectly maintained signals with additive noise. That is to say, detecting a signal or song played slightly slower than the original hashed version is completely outside what Dejavu can do. Detecting a signal played back at the same speed with a lot of background noise is just fine - that's what the algorithm is meant for.

I think this approach is probably a little too rigid for something like that, where there would be significant pitch, amplitude, and timing differences each time you say your passphrase.