What’s So Special About PKCS#1 v1.5? And The Attack That Just Won’t Go Away!
Published in
5 min readJan 29, 2020
RSA has been with us for many decades, and it’s still going strong. But it has weaknesses. The first is where we have a short message and the second is where we have a small value of e. To recap, in traditional RSA, we take the message of m, and an encryption key value e, along with a modulus of N, to give a cipher message of:
c = m^e (mod N)
To decrypt we use a value of d, to recover the message:
m = c^d (mod N)
The value of N is the product to two large prime numbers (p and q).
Short messages
If I use a small value of m, and which is less than N, we will end up with a discrete log problem, and where:
m^e = c
log (m^e) = log(c)
e log (m) = log ( c)
log (m) = log (c ) /e
m = Inv Log (log ( c) /e)
Here is the demo of this cracking:
