code: def issuer(self, value):
          
  
          is_oscrypto = isinstance(value, asymmetric.Certificate)
          if not isinstance(value, x509.Certificate) and not is_oscrypto:
              raise TypeError(_pretty_message(
                  '''
                  issuer must be an instance of asn1crypto.x509.Certificate or
                  oscrypto.asymmetric.Certificate, not %s
                  ''',
                  _type_name(value)
              ))
  
          if is_oscrypto:
              value = value.asn1
  
          self._issuer = value.subject
  
          self._key_identifier = self._subject_public_key.sha1
          self._authority_key_identifier = x509.AuthorityKeyIdentifier({
              'key_identifier': value.public_key.sha1
          })
docs: An asn1crypto.x509.Certificate object of the issuer. Used to populate
          both the issuer field, but also the authority key identifier extension.