bionic (3) Authen::Passphrase.3pm.gz
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.