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Jun
21
awarded  Tumbleweed
Jun
12
comment Preimage resistance hash in digital signature
$z$ in less than $n $ ! I missed to write this, sorry.
Jun
12
comment Preimage resistance hash in digital signature
C to make the attack is started from computing $y = z^e \mod n $ and then finds $m'$ such that.. My question is C can start from $m'$ and compute $z$ such that $y = z^e \mod n $ fixed $y = h(m')$ ? Thanks and sorry for my bad explaining and my bad English
Jun
12
comment Preimage resistance hash in digital signature
Let's see if C is right: Since the digital signature,for a generic message $k$ is in the form: $ H(k)^d \mod n $ where $d$ is the exponent of BOB we will have in our case: $H(m') ^d \mod n = y ^d \mod n = (z^e \mod n) ^d \mod n = z $ . So we can conclude that because hash function $h()$ is not preimage resistance Bob was screwed.
Jun
12
comment Preimage resistance hash in digital signature
Hi,thanks for your answer. I'm a student, I started to study Security a week ago, so I don't think to be able to find attack. However, Can you say what is wrong in my argument? Attacker would like to state that Bob has signed $m'$, so compute $ y = z^e \mod n $ ($z$ random ),then he finds a $m'$ such that $h(m') = y $. C states "BOB you have sent ${ m', z }$ , you have signed $m'$ with $z$ ! "
Jun
11
awarded  Editor
Jun
11
revised Preimage resistance hash in digital signature
added 7 characters in body
Jun
11
awarded  Custodian
Jun
11
reviewed Approve suggested edit on Preimage resistance hash in digital signature
Jun
11
asked Preimage resistance hash in digital signature
Jun
10
awarded  Student