The Prediction of Serial Number in OpenSSL’s X.509 Certificate

Algorithm 4

Guess the serial number.

Step 1: Attacker applies for a certificate to target CA and records the submitting time in seconds.

Step 2: Attacker receives the certificate, checks the time Tb of “not before” in certificate and the time Ta of “not after”,

and computes and .

Step 3: According to the Algorithms 1 and 2, attacker brutes force the every 100-nanosecond (for Windows) or every

microsecond (for Linux) in to find which time seed is used to generate the certificate (totally or )

Step 4: Attacker selects a future time as the time of “not before” of the target forged certificate

Step 5: Attacker randomly selects a value of m, which satisfies the condition:

(for Windows XP)

(for Ubuntu)

Step 6: According to the Algorithms 1 and 2, attacker computes the candidate serial numbers

with the seed of (in seconds) and +100144m(in 100nanoseconds) or +1008m(in microseconds).

Step 7: Attacker uses the candidate serial numbers, as “not before”, and as “not after”,

to generate forged certificates according to the Stevens’s method [3].

Step 8: Attacker submits the application at the time and get the signature of the certificate.