Fast handled security tokens

Hi there!

We are programming a tool for more interaction in the lecture hall.
To participate, students use their Smartphone, iPad, NoteBook, etc.
For identification a new token for each lecture is presented by the teacher=
 on the front screen.
Students then have to enter this token in their device.
For security reasons the token shall have a variability of at least 1 in on=
e hundred million.
We have three types of token in mind.
For each token spaces are included for better visualization, which need not=
 to be entered.
Letters are NOT case-sensitive, too.

1: 8 numbers
   Examples: 746 495 37; 662 785 28; 968 367 76

2: 6 letters
   Examples: duj wjs, hpl qux, gie fvt

3: 4 syllables (a syllable is built by one letter each of the following set=
s: 'bcdfghjklmnpqrstvwxyz' und 'aeiou')
   Examples: ma pe ci ko, zu ku pa wi, yu vo do mi

We would like to know, which token will be grasped and entered fastest by t=
he students?
What are the opinions on that?

-Didi

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