new() - create a new signature from parameters

  my $signature = Mail::DKIM::ARC::MessageSignature->new(
                      [ Algorithm => 'rsa-sha256', ]
                      [ Signature => $base64, ]
                      [ Method => 'relaxed', ]
                      [ Domain => 'example.org', ]
                      [ Instance => 1, ]
                      [ Headers => 'from:subject:date:message-id', ]
                      [ Query => 'dns', ]
                      [ Selector => 'alpha', ]
                      [ Timestamp => time(), ]
                      [ Expiration => time() + 86400, ]
                  );

The only differences between this module and Mail::DKIM::Signature are the header name, and that 'instance' is an integer rather than a base64 encoded value.

instance() - get or set the signing instance (i=) field

  my $i = $signature->instance;

Instances must be integers less than 1024 according to the spec.

NOTE: the i= field is "Identity" in DKIM and is a base64 value, but in ARC it is "Instance" and an integer. The parsing routine does not check that the i= value is a number.