code: func getAuthorityKeyIdentifierFromCrl(crl *pkix.CertificateList) ([]byte, error) {
  	aki := authorityKeyIdentifier{}
  
  	for _, ext := range crl.TBSCertList.Extensions {
  		// Authority Key Identifier is identified by the following ASN.1 tag
  		// authorityKeyIdentifier (2 5 29 35) (see https://tools.ietf.org/html/rfc3280.html)
  		if reflect.DeepEqual(ext.Id, asn1.ObjectIdentifier{2, 5, 29, 35}) {
  			_, err := asn1.Unmarshal(ext.Value, &aki)
  			if err != nil {
  				return nil, errors.Wrap(err, "failed to unmarshal AKI")
  			}
  
  			return aki.KeyIdentifier, nil
  		}
  	}
  
  	return nil, errors.New("authorityKeyIdentifier not found in certificate")
  }
docs: // getAuthorityKeyIdentifierFromCrl returns the Authority Key Identifier
  // for the supplied CRL. The authority key identifier can be used to identify
  // the public key corresponding to the private key which was used to sign the CRL.