Gets the {@code serialNumber} value from the certificate.
The serial number is an integer assigned by the certification
authority to each certificate. It must be unique for each
certificate issued by a given CA (i.e., the issuer name and
serial number identify a unique certificate).
The ASN.1 definition for this is:
<pre>
serialNumber CertificateSerialNumber
Gets the {@code serialNumber} value from the certificate. The serial number is an integer assigned by the certification authority to each certificate. It must be unique for each certificate issued by a given CA (i.e., the issuer name and serial number identify a unique certificate). The ASN.1 definition for this is: <pre> serialNumber CertificateSerialNumber
CertificateSerialNumber ::= INTEGER </pre>
@return the serial number.