I'm trying to compute the entropy of a passphrase of the following pattern:
.... .... .... ....
. represent lowercase characters.
The passphrase follows the rule that the two parts of each 4-character sequence have a vowel and a consonant, e.g.:
anno domi arip keto
Which has the pattern
:: :: :: ::
: represents a vowel-consonant pair.
Assuming an attacker knows all this, is the following entropy calculation correct?
log_2((2*5*21)^8) = 61.7 bits
Since there are 5*21 ways to create a vowel-consonant and 21*5 ways to create a consonant-vowel, we get 2*5*21, and because there are 8 of these the value is raised to the 8th power.