Cepstrum: IFFT or FFT to enter the cepstral domain?
For Cepstrum I always have used to this steps:


Apply hamming windows in the signal
Apply FFT
Get magnitude
Convert to log scale
Apply IFFT


The equation for cepstrum:

IFFT(log(abs(FFT(s))))


But you can use FFT or IFFT, take a look:

IFFT(log(abs(FFT(s)))) == real(FFT(log(abs(FFT(s)))))


The difference is the scale representation, if do you end using FFT you need extract just the real information, for both above equations you will get the same shape:

For IFFT(log(abs(FFT(s)))):



For real(FFT(log(abs(FFT(s))))):



This is a cepstrum shape example from 4096 points sine in 440hz sampled at 44100hz