Forged Signaute or not Probability

Question

A document is signed with an ElGamal signature. The probability that a signature is forged is (.1). Find the probability that at least one of the three Rob’s signatures is forged.

Answer 1

The probability that a signautre is not forged is 1-0.1 = 0.9

 

The probability that all 3 signatures aren't forged is:

 

0.9 * 0.9 * 0.9

 

Therefore the probability that at least one signature is forged is:

 

1 - probability that no signatures are forged = 1 - 0.9*0.9*0.9 = 0.271