man PGP::Sign (3): Create detached PGP signatures for data, securely

SYNOPSIS

use PGP::Sign;
($signature, $version) = pgp_sign ($keyid, $passphrase, @data);
$signer = pgp_verify ($signature, $version, @data);
@errors = PGP::Sign::pgp_error;

DESCRIPTION

This module is designed to do one and only one thing securely and well; namely, generate and check detached PGP signatures for some arbitrary data. It doesn't do encryption, it doesn't manage keyrings, it doesn't verify signatures, it just signs things. This is ideal for applications like PGPMoose or control message generation that just need a fast signing mechanism. It supports versions 2.6, 5.0, and 6.5.2 of PGP, as well as GnuPG, and therefore supports any of the signature types understood by those programs provided they are installed.

The interface is very simple; just call pgp_sign() with a key ID, a pass phrase, and some data, or call pgp_verify() with a signature (in the form generated by pgp_sign()), a version number (which can be undef if you don't want to give a version), and some data. The data can be specified in pretty much any form you can possibly consider data and a few you might not. Scalars and arrays are passed along to PGP; references to arrays are walked and passed one element at a time (to avoid making a copy of the array); file handles, globs, or references to globs are read a line at a time and passed to PGP; and references to code are even supported (see below). About the only thing that we don't handle are references to references (which are just printed to PGP, which probably isn't what you wanted) and hashes (which are treated like arrays, which doesn't make a lot of sense).

If you give either function a reference to a sub, it will repeatedly call that sub, sending the results to PGP to be signed, until the sub returns undef. What this lets you do is pass the function an anonymous sub that walks your internal data and performs some manipulations on it a line at a time, thus allowing you to sign a slightly modified form of your data (with initial dashes escaped, for example) without having to use up memory to make an internal copy of it.

In a scalar context, pgp_sign() returns the signature as an ASCII-armored block with embedded newlines (but no trailing newline). In a list context, it returns a two-element list consisting of the signature as above and the PGP version that signed it (if that information was present in the signature). Warning: It's expected that in the future this interface will change, and pgp_sign() will instead return a list consisting of the ASCII-armored block and all headers found in the armor.

If you're using GnuPG, pgp_sign() will pass it the option --force-v3-sigs so that it will generate PGP 5.0-compatible signatures.

pgp_sign() will return undef in the event of any sort of error.

pgp_verify() returns the signer of the message in the case of a good signature, the empty string in the case of a bad signature, and undef in the event of some error. It takes the same sort of data sources as pgp_sign().

pgp_error() (which isn't exported by default) returns the error encountered by the last pgp_sign() or pgp_verify(), or undef if there was no error. In a list context, a list of lines is returned; in a scalar context, a long string with embedded newlines is returned.

Six global variables can be modified (note that these may eventually be partially or completely replaced with an interface via a PGP::Sign::config() call):

$PGP::Sign::PGPS

ENVIRONMENT

PGPPATH

In addition, all environment variables that PGP normally honors will be passed along to PGP and will likely have their expected effects. This includes PGPPATH, unless it is overridden (see above).

DIAGNOSTICS

Mostly the contents of @PGP::Sign::ERROR (returned by pgp_error()) are just the output of PGP. The exceptions are:

Execution of %s failed: %s

BUGS

PGP::Sign does not currently work with binary data, as it unconditionally forces text mode in PGP by using the "-t" option. This is a high priority to fix, but I'd like to implement some sort of generic way of setting PGP options rather than just adding more entry points.

There's no way of generating version four DSS signatures. This will be fixed by the same improvement that will fix the previous bug.

PGP, all versions that I have available, behaves differently when generating attached signatures than when generating detached signatures. For attached signatures, trailing whitespace in lines of the data is not significant, but for detached signatures it is. This, combined with the fact that there's no way that I can see to get PGP to verify a detached signature without using files on disk, means that in order to maintain the intended default semantics of this module (manipulating detached signatures), I had to use temporary files in the implementation of pgp_verify(). PGP::Sign sets its umask before creating those temporary files and opens them with O_EXCL, but files may be left behind in the event that an application using pgp_verify() crashes unexpectedly. Setting $PGP::Sign::TMPDIR is recommended.

Also, because of this incompatibility, you need to be aware of what the process checking the signatures you generate is expecting. If that process is going to turn your signature into an attached signature for verification (as pgpverify does for Usenet control messages), then you need to pre-munge your data to remove trailing whitespace at the ends of lines before feeding it to PGP. PGP::Sign will do that for you if you set $PGP::Sign::MUNGE to a true value.

To add even more confusion to the mix, earlier versions of GnuPG followed an interpretation of RFC 2440 that specified text-mode signatures are performed against data with canonicalized line endings and with trailing whitespace removed (see section 5.2.1). There is no difference specified there between attached and detached signatures, and GnuPG treated them both the same. Versions of GnuPG at least after 1.0 appear to have changed to follow the PGP behavior instead.

When verification of a signature fails, currently not very much information about what failed is available (since an invalid signature isn't considered an error in the pgp_error() sense).

CAVEATS

This module is fairly good at what it does, but it doesn't do very much. At one point, I had plans to provide more options and more configurability in the future, particularly the ability to handle binary data, that would probably mean API changes. I'm not sure at this point whether I'll get to that, or just replace this module with one that only uses GnuPG as I see no reason to use any other PGP implementation at this point and GnuPG has a much nicer programmatic interface.

However, just in case, the interface to this module should not be considered stable yet; you may have to change your application when you upgrade to a newer version of this module. The README will list API changes.

RESTRICTIONS

PGP::Sign passes pass phrases to PGP via an open one-ended pipe, since this is the only secure method (both command line switches and environment variables can potentially be read by other users on the same machine using ps). This should be supported by any recent version of PGP; I have tested it against 2.6.2, 2.6.3i, 5.0, 6.5.2, GnuPG 0.9.2, and GnuPG 1.0.1. Implicit in this mechanism, though, is the requirement that the operating system on which you're running this module supports passing an open pipe to an exec()ed subprocess. This may cause portability problems to certain substandard operating systems.

AUTHOR

Russ Allbery <[email protected]>

COPYRIGHT AND LICENSE

This program is free software; you may redistribute it and/or modify it under the same terms as Perl itself.

HISTORY

Based heavily on work by Andrew Gierth and benefitting greatly from input, comments, suggestions, and help from him, this module came about in the process of implementing PGPMoose signatures and control message signatures for Usenet. PGPMoose is the idea of Greg Rose, and signcontrol and pgpverify are the idea of David Lawrence.

Support for PGPPATH, the test suite, some bug fixes, and the impetus to get another version released came from Andrew Ford. Thank you.

Original support for GnuPG from Todd Underwood and Monte Mitzelfelt. Code for using --status-fd based on code by Marco d'Itri.