Provided by: libmail-dkim-perl_1.20230212-1ubuntu1_all 
NAME
Mail::DKIM::Verifier - verifies a DKIM-signed message
VERSION
version 1.20230212
SYNOPSIS
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;
# there might be multiple signatures, what is the result per signature?
foreach my $signature ($dkim->signatures)
{
print 'signature identity: ' . $signature->identity . "\n";
print 'verify result: ' . $signature->result_detail . "\n";
}
# the alleged author of the email may specify how to handle email
foreach my $policy ($dkim->policies)
{
die 'fraudulent message' if ($policy->apply($dkim) eq 'reject');
}
DESCRIPTION
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.
The results of the verification can be checked with "result()" or "signatures()".
Messages that do not verify may be checked against the alleged sender's published signing policy with
"policies()" and "apply()" in Mail::DKIM::Policy.
CONSTRUCTOR
new()
Constructs an object-oriented verifier.
my $dkim = Mail::DKIM::Verifier->new();
my $dkim = Mail::DKIM::Verifier->new(%options);
The only options supported at this time are:
Debug_Canonicalization
if specified, the canonicalized message for the first signature is written to the referenced string
or file handle.
Strict
If true, rejects sha1 hashes and signing keys shorter than 1024 bits.
METHODS
PRINT()
Feeds part of the message to the verifier.
$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.
Please note that although the PRINT() method expects you to use SMTP-style line termination characters,
you should NOT use the SMTP-style dot-stuffing technique described in RFC 2821 section 4.5.2. Nor should
you use a <CR><LF>.<CR><LF> sequence to terminate the message.
CLOSE()
Call this when finished feeding in the message.
$dkim->CLOSE;
This method finishes the canonicalization process, computes a hash, and verifies the signature.
fetch_author_domain_policies()
Retrieves ADSP records from DNS.
my @policies = $dkim->fetch_author_domain_policies;
foreach my $policy (@policies)
{
my $policy_result = $policy->apply($dkim);
}
This method will retrieve all applicable "author-domain-signing-practices" published in DNS for this
message. Author policies are keyed to the email address(es) in the From: header, i.e. the claimed author
of the message.
This method returns a *list* of policy records, since there is allowed to be zero or multiple email
addresses in the From: header.
The result of the apply() method is one of: "accept", "reject", "neutral".
See also: "policies()".
fetch_author_policy()
Retrieves a signing policy from DNS.
my $policy = $dkim->fetch_author_policy;
my $policy_result = $policy->apply($dkim);
This method retrieves the DKIM Sender Signing Practices record as described in Internet Draft
draft-ietf-dkim-ssp-00-01dc. This Internet Draft is now obsolete; this method is only kept for backward-
compatibility purposes.
Please use the "policies()" method instead.
fetch_sender_policy()
Retrieves a signing policy from DNS.
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".
The result of the apply() method is one of: "accept", "reject", "neutral".
See also: "policies()".
load()
Load the entire message from a file handle.
$dkim->load($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).
message_originator()
Access the "From" header.
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()".
message_sender()
Access the "From" or "Sender" header.
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.
To get just the email address part, do:
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.
policies()
Retrieves applicable signing policies from DNS.
my @policies = $dkim->policies;
foreach my $policy (@policies)
{
$policy_result = $policy->apply($dkim);
# $policy_result is one of "accept", "reject", "neutral"
}
This method searches for and returns any signing policies that would apply to this message. Signing
policies are selected based on the domain that the message *claims* to be from. So, for example, if a
message claims to be from security@bank, and forwarded by trusted@listserv, when in reality the message
came from foe@evilcorp, this method would check for signing policies for security@bank and
trusted@listserv. The signing policies might tell whether foe@evilcorp (the real sender) is allowed to
send mail claiming to be from your bank or your listserv.
I say "might tell", because in reality this is still really hard to specify with any accuracy. In
addition, most senders do not publish useful policies.
result()
Access the result of the verification.
my $result = $dkim->result;
Gives the result of the verification. The following values are possible:
pass
Returned if a valid DKIM-Signature header was found, and the signature contains a correct value for
the message.
fail
Returned if a valid DKIM-Signature header was found, but the signature does not contain a correct
value for the message.
invalid
Returned if a DKIM-Signature could not be checked because of a problem in the signature itself or the
public key record. I.e. the signature could not be processed.
temperror
Returned if a DKIM-Signature could not be checked due to some error which is likely transient in
nature, such as a temporary inability to retrieve a public key. A later attempt may produce a better
result.
none
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". To examine the results of individual signatures, use the
"signatures()" method to retrieve the signature objects. See "result()" in Mail::DKIM::Signature.
result_detail()
Access the result, plus details if available.
my $detail = $dkim->result_detail;
The detail is constructed by taking the result (e.g. "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
signature()
Access the message's DKIM signature.
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".
signatures()
Access all of this message's signatures.
my @all_signatures = $dkim->signatures;
Use $signature->result or $signature->result_detail to access the verification results of each signature.
AUTHORS
• Jason Long <jason@long.name>
• Marc Bradshaw <marc@marcbradshaw.net>
• Bron Gondwana <brong@fastmailteam.com> (ARC)
THANKS
Work on ensuring that this module passes the ARC test suite was generously sponsored by Valimail
(https://www.valimail.com/)
COPYRIGHT AND LICENSE
• Copyright (C) 2013 by Messiah College
• Copyright (C) 2010 by Jason Long
• Copyright (C) 2017 by Standcore LLC
• Copyright (C) 2020 by FastMail Pty Ltd
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.
perl v5.38.2 2024-02-27 Mail::DKIM::Verifier(3pm)