# Verifying a Signature

Alice publishes $(n,e)$ where $n$ is exponential modulus and $e$ is the public key. We have $n=80$ digit number and $e=3$. Bob, in correspondence with Alice, asked Alice to prove she is really Alice, by making her signature on the following five numbers: $m1, m2, m3, m4, m5$. Then Alice sent 6 numbers $y1, y2, y3, y4, y5, y6$ where all the 6 numbers are really long, around 80 integers.

How do verify that these five numbers are indeed signed by Alice?

• Welcome to Cryptography Stack Exchange. Sorry, you need to be a bit more specific. Please read RSA signature, and then tell us what at what point you need clarifications. (You can edit your question.) – Paŭlo Ebermann Oct 18 '13 at 18:22

$y_{i} = m_{i}^{d} \,\mbox{mod}\, n$
$d_{i} = y_{i}^{e} \,\mbox{mod}\, n$
If Bob finds $d_{i} == m_{i}$ for every $i$, he will probably conclude that they were indeed signed by Alice. His conclusion however, is very unwise, since for this special case where $e=3$, the numbers $y_{i}$ can be easily forged by an attacker even if he does not possess the private key $d$ of Alice.