Ubuntu Manpage: PGP::Sign - Create detached PGP signatures for data, securely

Provided by: libpgp-sign-perl_0.20-4_all

NAME

       PGP::Sign - 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
The path to the program to use to generate signatures. This is set at the time of
installation, but can be overridden.

$PGP::Sign::PGPV
The path to the program to use to verify signatures. This is set at the time of
installation, but can be overridden. There are two separate variables since PGP 5.0
uses two separate program names for signing and verifying; for PGP 2.6, 6.5.2, or
GnuPG, just set both this and $PGP::Sign::PGPS to the same value.

$PGP::Sign::PGPPATH
The path to a directory containing the PGP key rings that should be used. If this
isn't set, all versions of PGP will use the value of the environment variable PGPPATH
or $HOME/.pgp (the default). GnuPG will use the value of the environment variable
GNUPGHOME or $HOME/.gnupg. Note that PGP when signing may want to write randseed.bin
(or randseed.rnd) in this directory if there isn't already a random seed there, so if
you're encountering problems with signing, make sure the directory PGP is using is
writeable by the user doing the signing. Note also that if you're using GnuPG and the
Entropy Gathering Daemon (egd), the entropy socket or a link to it must be located in
this directory.

$PGP::Sign::PGPSTYLE
What style of command line arguments and responses to expect from PGP. The only three
valid values for this variable are "PGP2" for PGP 2.6 behavior, "PGP5" for PGP 5.0
behavior, "PGP6" for PGP 6.5 behavior, and "GPG" for GnuPG behavior. What command
line arguments PGP::Sign uses when running PGP are entirely determined by this
variable. It is set at the time of installation, but can be overridden.

$PGP::Sign::TMPDIR
The directory in which temporary files are created. Defaults to TMPDIR if set, and
/tmp if not.

$PGP::Sign::MUNGE
If this variable is set to a true value, PGP::Sign will automatically strip trailing
spaces when signing or verifying signatures. This will make the resulting signatures
and verification compatible with programs that generate attached signatures (since PGP
ignores trailing spaces when generating or checking attached signatures). See the
more extensive discussion of whitespace below.

ENVIRONMENT

       PGPPATH
           If $PGP::Sign::PGPPATH is set and $PGP::Sign::PGPSTYLE is something other than "GPG",
           PGP::Sign sets PGPPATH to tell PGP where to find its key rings.  (GnuPG uses a command
           line argument instead.)

       TMPDIR
           The directory in which to create temporary files.  Can be overridden by changing
           $PGP::Sign::TMPDIR.  If not set, defaults /tmp.

       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
           We couldn't fork off a PGP process for some reason, given (at least as the system
           reported it) after the colon.

       No signature from PGP (command not found?)
           We tried to generate a signature but the output from the command we tried to run
           didn't contain anything that looked like a signature.  One common explanation for this
           is that the path in $PGP::Sign::PGPS is invalid and that binary doesn't exist.

       %s returned exit status %d
           Some command that we ran, or tried to run, returned a non-zero exit status.  %s will
           contain the exact binary name that PGP::Sign was attempting to run.

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 <rra@stanford.edu>

COPYRIGHT AND LICENSE

       Copyright 1997, 1998, 1999, 2000, 2002, 2004 Russ Allbery <rra@stanford.edu>.

       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.