Take the 2-minute tour ×
Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It's 100% free, no registration required.

I have the following password with a prescribed format: it starts with a fixed value of 111, followed by 2 random lower case letters and then lastly a random digit:

"111""2 Random lowercase letters""random digit"

Is my below calculation correct?

fixed value 111 -> log2(1) = 0 bits

2 random lowercase letters -> log2(26)*2 = 9.4 bits

random digit -> log2(10) = 3.3 bits

Total entropy of the password would be 0 + 9.4 + 3.3 bits = 12.7 bits

share|improve this question
add comment

1 Answer

up vote 2 down vote accepted

Yes, the computation is correct: the passwords will have an entropy of $log_2(26^2\cdot10)\simeq {12.7}$, which means that they are weaker than a randomly chosen secret key with 13 bits.

share|improve this answer
1  
I think you meant 12.7 and not $2^{12.7}$ in your answer :) –  Thomas Oct 14 '12 at 9:10
    
@bob Thanks! :) –  null Oct 14 '12 at 9:12
    
@Thomas: indeed, corrected. –  bob Oct 14 '12 at 9:16
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.