Returns the authority key identifier extension.
The authority key identifier extension provides means of identifying the public key corresponding to the private key used to sign a certificate.
This extension is used where an issuer has multiple signing keys.
Usually, the key identifier is the result of some hash function applied to the issuer's public key. The obtained fingerprint is later used to identify the public key which certificate was signed with.
For more information about this extension, see AuthorityKeyIdentifierExtension class description.