Mail::DKIM::Verifier - verifies a DKIM-signed message
use Mail::DKIM::Verifier; # create a verifier object my $dkim = Mail::DKIM::Verifier->new(); # read an email from a file handle $dkim->load(*STDIN); # or read an email and pass it into the verifier, incrementally while (<STDIN>) { # remove local line terminators chomp; s/\015$//; # use SMTP line terminators $dkim->PRINT("$_\015\012"); } $dkim->CLOSE; # what is the result of the verify? my $result = $dkim->result;
The verifier object allows an email message to be scanned for DKIM and DomainKeys signatures and those signatures to be verified. The verifier tracks the state of the message as it is read into memory. When the message has been completely read, the signatures are verified and the results of the verification can be accessed.
To use the verifier, first create the verifier object. Then start "feeding" it the email message to be verified. When all the headers have been read, the verifier:
1. checks whether any DomainKeys/DKIM signatures were found 2. queries for the public keys needed to verify the signatures 3. sets up the appropriate algorithms and canonicalization objects 4. canonicalizes the headers and computes the header hash
Then, when the body of the message has been completely fed into the verifier, the body hash is computed and the signatures are verified.
my $dkim = Mail::DKIM::Verifier->new(); my $dkim = Mail::DKIM::Verifier->new(%options);
The only option supported at this time is:
if specified, the canonicalized message for the first signature is written to the referenced string or file handle.
$dkim->PRINT("a line of the message\015\012"); $dkim->PRINT("more of"); $dkim->PRINT(" the message\015\012bye\015\012");
Feeds content of the message being verified into the verifier. The API is designed this way so that the entire message does NOT need to be read into memory at once.
$dkim->CLOSE;
This method finishes the canonicalization process, computes a hash, and verifies the signature.
my $policy = $dkim->fetch_author_policy; my $policy_result = $policy->apply($dkim);
The "author" policy, as I call it, is the DKIM Sender Signing Practices record as described in Internet Draft draft-ietf-dkim-ssp-00-01dc. I call it the "author" policy because it is keyed to the email address in the From: header, i.e. the author of the message.
The IETF is still actively working on this Internet Draft, so the exact mechanisms are subject to change.
If the email being verified has no From header at all (which violates email standards), then this method will die.
die
The result of the apply() method is one of: "accept", "reject", "neutral".
my $policy = $dkim->fetch_sender_policy; my $policy_result = $policy->apply($dkim);
The "sender" policy is the sender signing policy as described by the DomainKeys specification, now available in RFC4870(historical). I call it the "sender" policy because it is keyed to the email address in the Sender: header, or the From: header if there is no Sender header. This is the person whom the message claims as the "transmitter" of the message (not necessarily the author).
If the email being verified has no From or Sender header from which to get an email address (which violates email standards), then this method will die.
$dkim->load($file_handle);
Reads a complete message from the designated file handle, feeding it into the verifier. The message must use <CRLF> line terminators (same as the SMTP protocol).
my $address = $dkim->message_originator;
Returns the "originator address" found in the message, as a Mail::Address object. This is typically the (first) name and email address found in the From: header. If there is no From: header, then an empty Mail::Address object is returned.
To get just the email address part, do:
my $email = $dkim->message_originator->address;
See also "message_sender()".
my $address = $dkim->message_sender;
Returns the "sender" found in the message, as a Mail::Address object. This is typically the (first) name and email address found in the Sender: header. If there is no Sender: header, it is the first name and email address in the From: header. If neither header is present, then an empty Mail::Address object is returned.
my $email = $dkim->message_sender->address;
The "sender" is the mailbox of the agent responsible for the actual transmission of the message. For example, if a secretary were to send a message for another person, the "sender" would be the secretary and the "originator" would be the actual author.
my $result = $dkim->result;
Gives the result of the verification. The following values are possible:
Returned if a valid DKIM-Signature header was found, and the signature contains a correct value for the message.
Returned if a valid DKIM-Signature header was found, but the signature does not contain a correct value for the message.
Returned if a DKIM-Signature could not be checked because of a problem in the signature itself or the public key record.
Returned if no DKIM-Signature headers (valid or invalid) were found.
In case of multiple signatures, the "best" result will be returned. Best is defined as "pass", followed by "fail", "invalid", and "none".
my $detail = $dkim->result_detail;
The detail is constructed by taking the result (i.e. one of "pass", "fail", "invalid" or "none") and appending any details provided by the verification process in parenthesis.
The following are possible results from the result_detail() method:
pass fail (bad RSA signature) fail (OpenSSL error: ...) fail (message has been altered) fail (body has been altered) invalid (bad identity) invalid (invalid domain in d tag) invalid (missing q tag) invalid (missing d tag) invalid (missing s tag) invalid (unsupported version 0.1) invalid (unsupported algorithm ...) invalid (unsupported canonicalization ...) invalid (unsupported query protocol ...) invalid (signature is expired) invalid (public key: not available) invalid (public key: unknown query type ...) invalid (public key: syntax error) invalid (public key: unsupported version) invalid (public key: unsupported key type) invalid (public key: missing p= tag) invalid (public key: invalid data) invalid (public key: does not support email) invalid (public key: does not support hash algorithm 'sha1') invalid (public key: does not support signing subdomains) invalid (public key: revoked) invalid (public key: granularity mismatch) invalid (public key: granularity is empty) invalid (public key: OpenSSL error: ...) none
my $sig = $dkim->signature;
Accesses the signature found and verified in this message. The returned object is of type Mail::DKIM::Signature.
In case of multiple signatures, the signature with the "best" result will be returned. Best is defined as "pass", followed by "fail", "invalid", and "none".
my @all_signatures = $dkim->signatures;
Use $signature->result or $signature->result_detail to access the verification results of each signature.
Jason Long, <jlong@messiah.edu>
Copyright (C) 2006-2007 by Messiah College
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.6 or, at your option, any later version of Perl 5 you may have available.
To install Mail::DKIM, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Mail::DKIM
CPAN shell
perl -MCPAN -e shell install Mail::DKIM
For more information on module installation, please visit the detailed CPAN module installation guide.