Provided by: libauthen-passphrase-perl_0.008-3_all 
NAME
Authen::Passphrase - hashed passwords/passphrases as objects
SYNOPSIS
use Authen::Passphrase;
$ppr = Authen::Passphrase->from_crypt($passwd);
$ppr = Authen::Passphrase->from_rfc2307($userPassword);
if($ppr->match($passphrase)) { ...
$passphrase = $ppr->passphrase;
$crypt = $ppr->as_crypt;
$userPassword = $ppr->as_rfc2307;
DESCRIPTION
This is the base class for a system of objects that encapsulate passphrases. An object of
this type is a passphrase recogniser: its job is to recognise whether an offered
passphrase is the right one. For security, such passphrase recognisers usually do not
themselves know the passphrase they are looking for; they can merely recognise it when
they see it. There are many schemes in use to achieve this effect, and the intent of this
class is to provide a consistent interface to them all, hiding the details.
The CPAN package Authen-Passphrase contains implementations of several specific passphrase
schemes in addition to the base class. See the specific classes for notes on the security
properties of each scheme. In new systems, if there is a choice of which passphrase
algorithm to use, it is recommended to use Authen::Passphrase::SaltedDigest or
Authen::Passphrase::BlowfishCrypt. Most other schemes are too weak for new applications,
and should be used only for backward compatibility.
Side-channel cryptanalysis
Both the Authen-Passphrase framework and most of the underlying cryptographic algorithm
implementations are vulnerable to side-channel cryptanalytic attacks. However, the impact
of this is quite limited.
Unlike the case of symmetric encryption, where a side-channel attack can extract the
plaintext directly, the cryptographic operations involved in passphrase recognition don't
directly process the correct passphrase. A sophisticated side-channel attack, applied
when offering incorrect passphrases for checking, could potentially extract salt (from the
operation of the hashing algorithm) and the target hash value (from the comparison of hash
values). This would enable a cryptanalytic or brute-force attack on the passphrase
recogniser to be performed offline. This is not a disaster; the very intent of storing
only a hash of the correct passphrase is that leakage of these stored values doesn't
compromise the passphrase.
In a typical usage scenario for this framework, the side-channel attacks that can be
mounted are very restricted. If authenticating network users, typically an attacker has
no access at all to power consumption, electromagnetic emanation, and other such side
channels. The only side channel that is readily available is timing, and the precision of
timing measurements is significantly blunted by the normal processes of network
communication. For example, it would not normally be feasible to mount a timing attack
against hash value comparison (to see how far through the values the first mismatch was).
Perl as a whole has not been built as a platform for side-channel-resistant cryptography,
so hardening Authen-Passphrase and its underlying algorithms is not feasible. In any
serious use of Perl for cryptography, including for authentication using Authen-
Passphrase, an analysis should be made of the exposure to side-channel attacks, and if
necessary efforts made to further blunt the timing channel.
One timing attack that is very obviously feasible over the network is to determine which
of several passphrase hashing algorithms is being used. This can potentially distinguish
between classes of user accounts, or distinguish between existing and non-existing user
accounts when an attacker is guessing usernames. To obscure this information requires an
extreme restriction of the timing channel, most likely by explicitly pausing to
standardise the amount of time spent on authentication. This defence also rules out
essentially all other timing attacks.
PASSPHRASE ENCODINGS
Because hashed passphrases frequently need to be stored, various encodings of them have
been devised. This class has constructors and methods to support these.
crypt encoding
The Unix crypt() function, which performs passphrase hashing, returns hashes in a textual
format intended to be stored in a text file. In particular, such hashes are stored in
/etc/passwd (and now /etc/shadow) to control access to Unix user accounts. The same
textual format has been adopted and extended by other passphrase-handling software such as
password crackers.
For historical reasons, there are several different syntaxes used in this format. The
original DES-based password scheme represents its hashes simply as a string of thirteen
base 64 digits. An extended variant of this scheme uses nineteen base 64 digits, preceded
by an "_" marker. A more general syntax was developed later, which starts the string with
"$", an alphanumeric scheme identifier, and another "$".
In addition to actual passphrase hashes, the crypt format can also represent a couple of
special cases. The empty string indicates that there is no access control; it is possible
to login without giving a passphrase. Finally, any string that is not a possible output
of crypt() may be used to prevent login completely; "*" is the usual choice, but other
strings are used too.
crypt strings are intended to be used in text files that use colon and newline characters
as delimiters. This module treats the crypt string syntax as being limited to ASCII
graphic characters excluding colon.
RFC 2307 encoding
RFC 2307 describes an encoding system for passphrase hashes, to be used in the
"userPassword" attribute in LDAP databases. It encodes hashes as ASCII text, and supports
several passphrase schemes in an extensible way by starting the encoding with an
alphanumeric scheme identifier enclosed in braces. There are several standard scheme
identifiers. The "{CRYPT}" scheme allows the use of any crypt encoding.
This module treats the RFC 2307 string syntax as being limited to ASCII graphic
characters.
The RFC 2307 encoding is a good one, and is recommended for storage and exchange of
passphrase hashes.
CONSTRUCTORS
Authen::Passphrase->from_crypt(PASSWD)
Returns a passphrase recogniser object matching the supplied crypt encoding. This
constructor may only be called on the base class, not any subclass.
The specific passphrase recogniser class is loaded at runtime, so successfully loading
"Authen::Passphrase" does not guarantee that it will be possible to use a specific
type of passphrase recogniser. If necessary, check separately for presence and
loadability of the recogniser class.
Known scheme identifiers:
$1$ A baroque passphrase scheme based on MD5, designed by Poul-Henning Kamp and
originally implemented in FreeBSD. See Authen::Passphrase::MD5Crypt.
$2$
$2a$
Two versions of a passphrase scheme based on Blowfish, designed by Niels Provos
and David Mazieres for OpenBSD. See Authen::Passphrase::BlowfishCrypt.
$3$ The NT-Hash scheme, which stores the MD4 hash of the passphrase expressed in
Unicode. See Authen::Passphrase::NTHash.
$IPB2$
Invision Power Board 2.x salted MD5
$K4$
Kerberos AFS DES
$LM$
Half of the Microsoft LAN Manager hash scheme. The two halves of a LAN Manager
hash can be separated and manipulated independently; this represents such an
isolated half. See Authen::Passphrase::LANManagerHalf.
$NT$
The NT-Hash scheme, which stores the MD4 hash of the passphrase expressed in
Unicode. See Authen::Passphrase::NTHash.
The $3$ identifier refers to the same hash algorithm, but has a slightly different
textual format (an extra "$").
$P$ Portable PHP password hash (phpass), based on MD5. See
Authen::Passphrase::PHPass.
$VMS1$
$VMS2$
$VMS3$
Three variants of the Purdy polynomial system used in VMS. See
Authen::Passphrase::VMSPurdy.
$af$
Kerberos v4 TGT
$apr1$
A variant of the $1$ scheme, used by Apache.
$krb5$
Kerberos v5 TGT
The historical formats supported are:
"bbbbbbbbbbbbb"
("b" represents a base 64 digit.) The original DES-based Unix password hash
scheme. See Authen::Passphrase::DESCrypt.
"_bbbbbbbbbbbbbbbbbbb"
("b" represents a base 64 digit.) Extended DES-based passphrase hash scheme from
BSDi. See Authen::Passphrase::DESCrypt.
"" Accept any passphrase. See Authen::Passphrase::AcceptAll.
"*" To handle historical practice, anything non-empty but shorter than 13 characters
and not starting with "$" is treated as deliberately rejecting all passphrases.
(See Authen::Passphrase::RejectAll.) Anything 13 characters or longer, or
starting with "$", that is not recognised as a hash is treated as an error.
There are also two different passphrase schemes that use a crypt string consisting of
24 base 64 digits. One is named "bigcrypt" and appears in HP-UX, Digital Unix, and
OSF/1 (see Authen::Passphrase::BigCrypt). The other is named "crypt16" and appears in
Ultrix and Tru64 (see Authen::Passphrase::Crypt16). These schemes conflict. Neither
of them is accepted as a crypt string by this constructor; such strings are regarded
as invalid encodings.
Authen::Passphrase->from_rfc2307(USERPASSWORD)
Returns a passphrase recogniser object matching the supplied RFC 2307 encoding. This
constructor may only be called on the base class, not any subclass.
The specific passphrase recogniser class is loaded at runtime. See the note about
this for the "from_crypt" constructor above.
Known scheme identifiers:
{CLEARTEXT}
Passphrase stored in cleartext. See Authen::Passphrase::Clear.
{CRYPT}
The scheme identifier is followed by a crypt string.
{CRYPT16}
Used ambiguously by Exim, to refer to either crypt16 (see
Authen::Passphrase::Crypt16) or bigcrypt (see Authen::Passphrase::BigCrypt)
depending on compilation options. This is a bug, resulting from a confusion
between the two algorithms. This module does not support any meaning for this
scheme identifier.
{K5KEY}
Not a real passphrase scheme, but a placeholder to indicate that a Kerberos key
stored separately should be checked against. No data follows the scheme
identifier.
{KERBEROS}
Not a real passphrase scheme, but a placeholder to indicate that Kerberos should
be invoked to check against a user's passphrase. The scheme identifier is
followed by the user's username, in the form "name@realm".
{LANM}
Synonym for {LANMAN}, used by CommuniGate Pro.
{LANMAN}
The Microsoft LAN Manager hash scheme. See Authen::Passphrase::LANManager.
{MD4}
The MD4 digest of the passphrase is stored. See Authen::Passphrase::SaltedDigest.
{MD5}
The MD5 digest of the passphrase is stored. See Authen::Passphrase::SaltedDigest.
{MSNT}
The NT-Hash scheme, which stores the MD4 hash of the passphrase expressed in
Unicode. See Authen::Passphrase::NTHash.
{NS-MTA-MD5}
An MD5-based scheme used by Netscape Mail Server. See
Authen::Passphrase::NetscapeMail.
{RMD160}
The RIPEMD-160 digest of the passphrase is stored. See
Authen::Passphrase::SaltedDigest.
{SASL}
Not a real passphrase scheme, but a placeholder to indicate that SASL should be
invoked to check against a user's passphrase. The scheme identifier is followed
by the user's username.
{SHA}
The SHA-1 digest of the passphrase is stored. See
Authen::Passphrase::SaltedDigest.
{SMD5}
The MD5 digest of the passphrase plus a salt is stored. See
Authen::Passphrase::SaltedDigest.
{SSHA}
The SHA-1 digest of the passphrase plus a salt is stored. See
Authen::Passphrase::SaltedDigest.
{UNIX}
Not a real passphrase scheme, but a placeholder to indicate that Unix mechanisms
should be used to check against a Unix user's login passphrase. The scheme
identifier is followed by the user's username.
{WM-CRY}
Synonym for {CRYPT}, used by CommuniGate Pro.
METHODS
$ppr->match(PASSPHRASE)
Checks whether the supplied passphrase is correct. Returns a truth value.
$ppr->passphrase
If a matching passphrase can be easily determined by the passphrase recogniser then
this method will return it. This is only feasible for very weak passphrase schemes.
The method "die"s if it is infeasible.
$ppr->as_crypt
Encodes the passphrase recogniser in crypt format and returns the encoded result.
"die"s if the passphrase recogniser cannot be represented in this form.
$ppr->as_rfc2307
Encodes the passphrase recogniser in RFC 2307 format and returns the encoded result.
"die"s if the passphrase recogniser cannot be represented in this form.
SUBCLASSING
This class is designed to be subclassed, and cannot be instantiated alone. Any subclass
must implement the "match" method. That is the minimum required.
Subclasses should implement the "as_crypt" and "as_rfc2307" methods and the "from_crypt"
and "from_rfc2307" constructors wherever appropriate, with the following exception. If a
passphrase scheme has a crypt encoding but no native RFC 2307 encoding, so it can be RFC
2307 encoded only by using the "{CRYPT}" scheme, then "as_rfc2307" and "from_rfc2307"
should not be implemented by the class. There is a default implementation of the
"as_rfc2307" method that uses "{CRYPT}" and "as_crypt", and a default implementation of
the "from_rfc2307" method that recognises "{CRYPT}" and passes the embedded crypt string
to the "from_crypt" constructor.
Implementation of the "passphrase" method is entirely optional. It should be attempted
only for schemes that are so ludicrously weak as to allow passphrases to be cracked
reliably in a short time. Dictionary attacks are not appropriate implementations.
SEE ALSO
MooseX::Types::Authen::Passphrase, crypt(3), RFC 2307
AUTHOR
Andrew Main (Zefram) <zefram@fysh.org>
COPYRIGHT
Copyright (C) 2006, 2007, 2009, 2010, 2012 Andrew Main (Zefram) <zefram@fysh.org>
LICENSE
This module is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.