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<title>OpenDKIM Library (libopendkim)</title>
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<h1>OpenDKIM Library (libopendkim)</h1>
<h2>Introduction</h2>
<b>DomainKeys Identified Mail</b> ("DKIM") is a specification for signing
messages at the domain level using simple cryptographic methods to indicate
that the signing domain accepts some responsibility for the message. While
the most obvious application of this is to defend against forged mail,
other applications can make use of this capability. <p>
DKIM is an amalgamation of DomainKeys, created by Yahoo!, Inc., and
Internet Identified Mail (IIM) created by Cisco, Inc. Both can be found
as historical RFCs documents via the IETF web sites. More information
about DomainKeys can be found
<a href="http://antispam.yahoo.com/domainkeys">here</a>. <p>
This API (<tt>libopendkim</tt>) allows an application to sign or verify
messages according to the <b>DKIM</b> proposed standard. It also includes
a number of extensions to support other protocols that are not part of
DKIM itself. Support is included for ADSP, a mechanism for determining
whether or not the purported author domain claims all of its mail is signed.
Also provided is an implementation of a filter, using Sendmail's <b>milter</b>
package, that uses <tt>libopendkim</tt> to implement this facility. <p>
<h2>Data Types</h2>
<table columns="2" border="1">
<tr>
<td> <b> Data Type </b> </td>
<td> <b> Description </b> </td>
</tr>
<tr>
<td> <a href="dkim.html"> <tt>DKIM</tt> </a> </td>
<td> A signing/verifying context for a message. </td>
</tr>
<tr>
<td> <a href="dkim_alg_t.html"> <tt>dkim_alg_t</tt> </a> </td>
<td> A signature generation/verification method. </td>
</tr>
<tr>
<td> <a href="dkim_atps_t.html"> <tt>dkim_atps_t</tt> </a> </td>
<td> An Authorized Third-Party Signer test result. </td>
</tr>
<tr>
<td> <a href="dkim_canon_t.html"> <tt>dkim_canon_t</tt> </a> </td>
<td> A canonicalization method. </td>
</tr>
<tr>
<td> <a href="dkim_cbstat.html"> <tt>DKIM_CBSTAT</tt> </a> </td>
<td> Return value/status from user-provided callbacks. </td>
</tr>
<tr>
<td> <a href="dkim_dnssec.html"> <tt>DKIM_DNSSEC</tt> </a> </td>
<td> Key and policy record security evaluation codes. </td>
</tr>
<tr>
<td> <a href="dkim_lib.html"> <tt>DKIM_LIB</tt> </a> </td>
<td> An instance of the <tt>libopendkim</tt> service. </td>
</tr>
<tr>
<td> <a href="dkim_param_t.html"> <tt>dkim_param_t</tt> </a> </td>
<td> A signature parameter. </td>
</tr>
<tr>
<td> <a href="dkim_policy_t.html"> <tt>dkim_policy_t</tt> </a> </td>
<td> A sender signing policy. </td>
</tr>
<tr>
<td> <a href="dkim_presult.html"> <tt>DKIM_PRESULT</tt> </a> </td>
<td> A policy evaluation result. </td>
</tr>
<tr>
<td> <a href="dkim_pstate.html"> <tt>DKIM_PSTATE</tt> </a> </td>
<td> Policy lookup state information. </td>
</tr>
<tr>
<td> <a href="dkim_query_t.html"> <tt>dkim_query_t</tt> </a> </td>
<td> A key query method. </td>
</tr>
<tr>
<td> <a href="dkim_queryinfo.html"> <tt>DKIM_QUERYINFO</tt> </a> </td>
<td> A handle describing a required DNS query. </td>
</tr>
<tr>
<td> <a href="dkim_sigerror.html"> <tt>DKIM_SIGERROR</tt> </a> </td>
<td> Signature evaluation error codes. </td>
</tr>
<tr>
<td> <a href="dkim_siginfo.html"> <tt>DKIM_SIGINFO</tt> </a> </td>
<td> Private handle referencing information about a particular signature
on a signed message. </td>
</tr>
<tr>
<td> <a href="dkim_sigkey_t.html"> <tt>dkim_sigkey_t</tt> </a> </td>
<td> Private key data. </td>
</tr>
<tr>
<td> <a href="dkim_stat.html"> <tt>DKIM_STAT</tt> </a> </td>
<td> Return value/status. </td>
</tr>
</table>
<h2>Functions</h2>
<table columns="2" border="1">
<tr>
<td> <b> Function </b> </td>
<td> <b> Description </b> </td>
</tr>
<tr>
<td colspan="2"> <b>Administration</b>
</tr>
<tr>
<td> <a href="dkim_init.html"> <tt>dkim_init()</tt> </a> </td>
<td> Initialize an instance of the <b>DKIM</b> service. </td>
</tr>
<tr>
<td> <a href="dkim_flush_cache.html"> <tt>dkim_flush_cache()</tt> </a> </td>
<td> Flush the key/policy cache. </td>
</tr>
<tr>
<td> <a href="dkim_getcachestats.html"> <tt>dkim_getcachestats()</tt> </a> </td>
<td> Retrieve caching statistics. </td>
</tr>
<tr>
<td> <a href="dkim_geterror.html"> <tt>dkim_geterror()</tt> </a> </td>
<td> Retrieve the most recent internal error message associated with a
DKIM handle. </td>
</tr>
<tr>
<td> <a href="dkim_getmode.html"> <tt>dkim_getmode()</tt> </a> </td>
<td> Return the mode (signing or verifying) of a DKIM handle. </td>
</tr>
<tr>
<td> <a href="dkim_get_signer.html"> <tt>dkim_get_signer()</tt> </a> </td>
<td> Retrieve the current message signer (if any). </td>
</tr>
<tr>
<td> <a href="dkim_get_user_context.html"> <tt>dkim_get_user_context()</tt> </a> </td>
<td> Retrieve a specific user context pointer for a sign or verify operation
previously set by a call to <tt>dkim_set_user_context()</tt>. </td>
</tr>
<tr>
<td> <a href="dkim_libfeature.html"> <tt>dkim_libfeature()</tt> </a> </td>
<td> Test for availability of a particular feature in the library. </td>
</tr>
<tr>
<td> <a href="dkim_libversion.html"> <tt>dkim_libversion()</tt> </a> </td>
<td> Retrieve the version of libopendkim against which the application
is linked. </td>
</tr>
<tr>
<td> <a href="dkim_set_dns_callback.html"> <tt>dkim_set_dns_callback()</tt> </a> </td>
<td> Request a call back into the main program from time to time while
waiting for DNS results. </td>
</tr>
<tr>
<td> <a href="dkim_set_final.html"> <tt>dkim_set_final()</tt> </a> </td>
<td> Provide a function to perform final signature analysis and/or
re-ordering during verifications. </td>
</tr>
<tr>
<td> <a href="dkim_set_key_lookup.html"> <tt>dkim_set_key_lookup()</tt> </a> </td>
<td> Provide a function to perform key lookups, replacing the internal
implementation. </td>
</tr>
<tr>
<td> <a href="dkim_set_policy_lookup.html"> <tt>dkim_set_policy_lookup()</tt> </a> </td>
<td> Provide a function to perform policy lookups, replacing the internal
implementation. </td>
</tr>
<tr>
<td> <a href="dkim_set_prescreen.html"> <tt>dkim_set_prescreen()</tt> </a> </td>
<td> Provide a function to perform signature prescreening and/or
re-ordering during verifications. </td>
</tr>
<tr>
<td> <a href="dkim_set_signature_handle.html"> <tt>dkim_set_signature_handle()</tt> </a> </td>
<td> Provide a function to allocate a user-side signature description
structure and return a pointer to it. </td>
</tr>
<tr>
<td> <a href="dkim_set_signature_handle_free.html"> <tt>dkim_set_signature_handle_free()</tt> </a> </td>
<td> Provide a function to deallocate a user-side signature description
structure. </td>
</tr>
<tr>
<td> <a href="dkim_set_signature_tagvalues.html"> <tt>dkim_set_signature_tagvalues()</tt> </a> </td>
<td> Provide a function to receive signature-specific tags and values
for user-side analysis. </td>
</tr>
<tr>
<td> <a href="dkim_set_user_context.html"> <tt>dkim_set_user_context()</tt> </a> </td>
<td> Set a specific user context pointer for a sign or verify operation
which will be passed to user callbacks. </td>
</tr>
<tr>
<td> <a href="dkim_ssl_version.html"> <tt>dkim_ssl_version()</tt> </a> </td>
<td> Retrieve the OpenSSL version used when the library was compiled. </td>
</tr>
<tr>
<td> <a href="dkim_close.html"> <tt>dkim_close()</tt> </a> </td>
<td> Terminate an instance of the <b>DKIM</b> service. </td>
</tr>
<tr>
<td colspan="2"> <b>Signing</b>
</tr>
<tr>
<td> <a href="dkim_sign.html"> <tt>dkim_sign()</tt> </a> </td>
<td> Allocate a new <b>DKIM</b> handle for signing a message. </td>
</tr>
<tr>
<td> <a href="dkim_add_xtag.html"> <tt>dkim_add_xtag()</tt> </a> </td>
<td> Add an extension tag and corresponding value. </td>
</tr>
<tr>
<td> <a href="dkim_getpartial.html"> <tt>dkim_getpartial()</tt> </a> </td>
<td> Check partial signature request flag. </td>
</tr>
<tr>
<td> <a href="dkim_getsighdr.html"> <tt>dkim_getsighdr()</tt> </a> </td>
<td> Generate and return a signature header into a fixed-size buffer. </td>
</tr>
<tr>
<td> <a href="dkim_getsighdr_d.html"> <tt>dkim_getsighdr_d()</tt> </a> </td>
<td> Generate and return a signature header in a dynamically-allocated
buffer. </td>
</tr>
<tr>
<td> <a href="dkim_privkey_load.html"> <tt>dkim_privkey_load()</tt> </a> </td>
<td> Attempt to parse and load a signing key. </t>
</tr>
<tr>
<td> <a href="dkim_set_margin.html"> <tt>dkim_set_margin()</tt> </a> </td>
<td> Set the wrapping margin to use for signature header generation. </td>
</tr>
<tr>
<td> <a href="dkim_set_signer.html"> <tt>dkim_set_signer()</tt> </a> </td>
<td> Set the message signer. </td>
</tr>
<tr>
<td> <a href="dkim_setpartial.html"> <tt>dkim_setpartial()</tt> </a> </td>
<td> Request "l=" tag on a signature. </td>
</tr>
<tr>
<td colspan="2"> <b>Verifying</b>
</tr>
<tr>
<td> <a href="dkim_verify.html"> <tt>dkim_verify()</tt> </a> </td>
<td> Allocate a new <b>DKIM</b> handle for verifying a message. </td>
</tr>
<tr>
<td> <a href="dkim_atps_check.html"> <tt>dkim_atps_check()</tt> </a> </td>
<td> Perform an Authorized Third-Party Signer check. </td>
</tr>
<tr>
<td> <a href="dkim_diffheaders.html"> <tt>dkim_diffheaders()</tt> </a> </td>
<td> Compare original headers to received headers and look for approximate
matches to identify header munging in order to explain verification
failures. </td>
</tr>
<tr>
<td> <a href="dkim_get_reputation.html"> <tt>dkim_get_reputation()</tt> </a> </td>
<td> Query a DKIM reputation service. </td>
</tr>
<tr>
<td> <a href="dkim_getdomain.html"> <tt>dkim_getdomain()</tt> </a> </td>
<td> Return the sending domain from a message represented by a DKIM
handle. </td>
</tr>
<tr>
<td> <a href="dkim_getpolicystr.html"> <tt>dkim_getpolicystr()</tt> </a> </td>
<td> Translate a policy code into a user-friendly string. </td>
</tr>
<tr>
<td> <a href="dkim_getpresult.html"> <tt>dkim_getpresult()</tt> </a> </td>
<td> Retrieve detail about the sender's policy evaluation with respect
to the message associated with a DKIM handle. </td>
</tr>
<tr>
<td> <a href="dkim_getpresultstr.html"> <tt>dkim_getpresultstr()</tt> </a> </td>
<td> Translate a policy result code into a user-friendly string. </td>
</tr>
<tr>
<td> <a href="dkim_getresultstr.html"> <tt>dkim_getresultstr()</tt> </a> </td>
<td> Translate a DKIM_STAT constant into a string. </td>
</tr>
<tr>
<td> <a href="dkim_getsiglist.html"> <tt>dkim_getsiglist()</tt> </a> </td>
<td> Retrieve the array of signature handles associated with a message. </td>
</tr>
<tr>
<td> <a href="dkim_getsignature.html"> <tt>dkim_getsignature()</tt> </a> </td>
<td> Retrieve the signature handle to be used for final message
disposition. </td>
</tr>
<tr>
<td> <a href="dkim_getuser.html"> <tt>dkim_getuser()</tt> </a> </td>
<td> Return the sending user from a message represented by a DKIM
handle. </td>
</tr>
<tr>
<td> <a href="dkim_minbody.html"> <tt>dkim_minbody()</tt> </a> </td>
<td> Return number of bytes required to satisfy all active canonicalizations
referenced by a DKIM handle. </td>
</tr>
<tr>
<td> <a href="dkim_ohdrs.html"> <tt>dkim_ohdrs()</tt> </a> </td>
<td> Retrieve the original header set from a signature if such were
present. </td>
</tr>
<tr>
<td> <a href="dkim_policy.html"> <tt>dkim_policy()</tt> </a> </td>
<td> Evaluate sender signing policy information. </td>
</tr>
<tr>
<td> <a href="dkim_policy_getqueries.html">
<tt>dkim_policy_getqueries()</tt> </a> </td>
<td> Get the set of DNS queries needed to do a policy evaluaton. </td>
</tr>
<tr>
<td> <a href="dkim_policy_state_free.html">
<tt>dkim_policy_state_free()</tt> </a> </td>
<td> Free a policy state handle. </td>
</tr>
<tr>
<td> <a href="dkim_policy_state_new.html">
<tt>dkim_policy_state_new()</tt> </a> </td>
<td> Initialize a policy state handle. </td>
</tr>
<tr>
<td> <a href="dkim_policy_getdnssec.html"> <tt>dkim_policy_getdnssec()</tt> </a> </td>
<td> Retrieve DNSSEC evaluation of a sending domain's policy record. </td>
</tr>
<tr>
<td> <a href="dkim_policy_getreportinfo.html"> <tt>dkim_policy_getreportinfo()</tt> </a> </td>
<td> Retrieve information required to generate a policy failure
report. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getbh.html"> <tt>dkim_sig_getbh()</tt> </a> </td>
<td> Retrieve body hash test result from a signature handle. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getcanonlen.html"> <tt>dkim_sig_getcanonlen()</tt> </a> </td>
<td> Retrieve information regarding total canonicalized body length, and
the size of what was actually signed. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getcanons.html"> <tt>dkim_sig_getcanons()</tt> </a> </td>
<td> Retrieve the canonicalization modes used to generate a signature. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getcontext.html"> <tt>dkim_sig_getcontext()</tt> </a> </td>
<td> Retrieve user-side context specific to a signature. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getdnssec.html"> <tt>dkim_sig_getdnssec()</tt> </a> </td>
<td> Retrieve DNSSEC evaluation of a signature's key record. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getdomain.html"> <tt>dkim_sig_getdomain()</tt> </a> </td>
<td> Retrieve the domain name found in the signature on a message. </td>
</tr>
<tr>
<td> <a href="dkim_sig_geterror.html"> <tt>dkim_sig_geterror()</tt> </a> </td>
<td> Retrieve the error code associated with a rejected/disqualified
signature. </td>
</tr>
<tr>
<td> <a href="dkim_sig_geterrorstr.html"> <tt>dkim_sig_geterrorstr()</tt> </a> </td>
<td> Retrieve the text version of a signature error code. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getflags.html"> <tt>dkim_sig_getflags()</tt> </a> </td>
<td> Retrieve processing flags from a signature handle. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getidentity.html"> <tt>dkim_sig_getidentity()</tt> </a> </td>
<td> Retrieve the identity of the signing agent from a signature or
message. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getkeysize.html"> <tt>dkim_sig_getkeysize()</tt> </a> </td>
<td> Retrieve the size in bits of the key used to verify a message. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getqueries.html">
<tt>dkim_sig_getqueries()</tt> </a> </td>
<td> Get the set of DNS queries needed to complete signature
validation. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getreportinfo.html"> <tt>dkim_sig_getreportinfo()</tt> </a> </td>
<td> Retrieve information required to generate a verification failure
report. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getselector.html"> <tt>dkim_sig_getselector()</tt> </a> </td>
<td> Retrieve the selector found in a signature on a message. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getsignalg.html"> <tt>dkim_sig_getsignalg()</tt> </a> </td>
<td> Retrieve the signature algorithm used to sign a message. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getsignedhdrs.html"> <tt>dkim_sig_getsignedhdrs()</tt> </a> </td>
<td> Retrieve signed header data. </td>
</tr>
<tr>
<td> <a href="dkim_sig_getsigntime.html"> <tt>dkim_sig_getsigntime()</tt> </a> </td>
<td> Retrieve the timestamp on the signature of a message. </td>
</tr>
<tr>
<td> <a href="dkim_sig_hdrsigned.html"> <tt>dkim_sig_hdrsigned()</tt> </a> </td>
<td> Determine whether or not a particular header was signed. </td>
</tr>
<tr>
<td> <a href="dkim_sig_ignore.html"> <tt>dkim_sig_ignore()</tt> </a> </td>
<td> Flag a signature to be ignored when verifying. </td>
</tr>
<tr>
<td> <a href="dkim_sig_process.html"> <tt>dkim_sig_process()</tt> </a> </td>
<td> Process a signature for validity. </td>
</tr>
<tr>
<td colspan="2"> <b>Processing</b>
</tr>
<tr>
<td> <a href="dkim_header.html"> <tt>dkim_header()</tt> </a> </td>
<td> Process a header. </td>
</tr>
<tr>
<td> <a href="dkim_eoh.html"> <tt>dkim_eoh()</tt> </a> </td>
<td> Identify end of headers. </td>
</tr>
<tr>
<td> <a href="dkim_body.html"> <tt>dkim_body()</tt> </a> </td>
<td> Process a body chunk. </td>
</tr>
<tr>
<td> <a href="dkim_eom.html"> <tt>dkim_eom()</tt> </a> </td>
<td> Identify end of message. </td>
</tr>
<tr>
<td> <a href="dkim_chunk.html"> <tt>dkim_chunk()</tt> </a> </td>
<td> Process a message chunk. </td>
</tr>
<tr>
<td colspan="2"> <b>Utility</b>
</tr>
<tr>
<td> <a href="dkim_getid.html"> <tt>dkim_getid()</tt> </a> </td>
<td> Retrieve "id" string from handle. </td>
</tr>
<tr>
<td> <a href="dkim_get_msgdate.html"> <tt>dkim_get_msgdate()</tt> </a> </td>
<td> Attempt to parse the Date: header field of a message and return its
UNIX <tt>time_t</tt> conversion as a 64-bit unsigned integer. </td>
</tr>
<tr>
<td> <a href="dkim_get_sigsubstring.html"> <tt>dkim_get_sigsubstring()</tt> </a> </td>
<td> Retrieve a minimal signature substring for matching results to
signatures. </td>
</tr>
<tr>
<td> <a href="dkim_key_syntax.html"> <tt>dkim_key_syntax()</tt> </a> </td>
<td> Check the syntax of a key record. </td>
</tr>
<tr>
<td> <a href="dkim_mail_parse.html"> <tt>dkim_mail_parse()</tt> </a> </td>
<td> Parse an message header field, e.g. <tt>From:</tt>, to get user and
domain. </td>
</tr>
<tr>
<td> <a href="dkim_options.html"> <tt>dkim_options()</tt> </a> </td>
<td> Get or set library options. </td>
</tr>
<tr>
<td> <a href="dkim_policy_syntax.html"> <tt>dkim_policy_syntax()</tt> </a> </td>
<td> Check the syntax of a policy record. </td>
</tr>
<tr>
<td> <a href="dkim_qi_getname.html"> <tt>dkim_qi_getname()</tt> </a> </td>
<td> Retrieve the DNS name from a <tt>DKIM_QUERYINFO</tt> handle. </td>
</tr>
<tr>
<td> <a href="dkim_qi_gettype.html"> <tt>dkim_qi_gettype()</tt> </a> </td>
<td> Retrieve the DNS resource record type from a <tt>DKIM_QUERYINFO</tt>
handle. </td>
</tr>
<tr>
<td> <a href="dkim_sig_gethashes.html"> <tt>dkim_sig_gethashes()</tt> </a> </td>
<td> Retrieve computed hashes related to a signature. </td>
</tr>
<tr>
<td> <a href="dkim_sig_gettagvalue.html"> <tt>dkim_sig_gettagvalue()</tt> </a> </td>
<td> Retrieve arbitrary tags and values from signatures and keys. </td>
</tr>
<tr>
<td> <a href="dkim_sig_syntax.html"> <tt>dkim_sig_syntax()</tt> </a> </td>
<td> Check the syntax of a signature. </td>
</tr>
<tr>
<td colspan="2"> <b>DNS Operations</b>
</tr>
<tr>
<td> <a href="dkim_dns_set_query_cancel.html"> <tt>dkim_dns_set_query_cancel()</tt> </a> </td>
<td> Set the function to be used by the library to cancel a pending DNS
query whose result is no longer needed. </td>
</tr>
<tr>
<td> <a href="dkim_dns_set_query_service.html"> <tt>dkim_dns_set_query_service()</tt> </a> </td>
<td> Set the DNS query service handle to be used by the library. </td>
</tr>
<tr>
<td> <a href="dkim_dns_set_query_start.html"> <tt>dkim_dns_set_query_start()</tt> </a> </td>
<td> Set the DNS query start function to be used by the library. </td>
</tr>
<tr>
<td> <a href="dkim_dns_set_query_waitreply.html"> <tt>dkim_dns_set_query_waitreply()</tt> </a> </td>
<td> Set the function to be used by the library to wait for a
reply to a pending DNS query. </td>
</tr>
<tr>
<td colspan="2"> <b>Cleanup</b>
</tr>
<tr>
<td> <a href="dkim_free.html"> <tt>dkim_free()</tt> </a> </td>
<td> Destroy a per-message handle of the <b>DKIM</b> service. </td>
</tr>
</table> <br>
An overview of the general use of this API is available
<a href="overview.html">here</a>. An overview of the DNS resolver portion
of the API is available <a href="dns.html">here</a>.
<hr size="1">
<font size="-1">
Copyright (c) 2005-2008 Sendmail, Inc. and its suppliers.
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