@node Shared-key and anonymous authentication
@section Shared-key and anonymous authentication

In addition to certificate authentication, the TLS protocol may be
used with password, shared-key and anonymous authentication methods.
The rest of this chapter discusses details of these methods.

@menu
* SRP authentication::
* PSK authentication::
* Anonymous authentication::
@end menu

@node SRP authentication
@subsection SRP authentication

@menu
* Authentication using SRP::
* srptool Invocation::             Invoking srptool
@end menu

@node Authentication using SRP
@subsubsection Authentication using @acronym{SRP}
@cindex SRP authentication

@acronym{GnuTLS} supports authentication via the Secure Remote Password 
or @acronym{SRP} protocol (see @xcite{RFC2945,TOMSRP} for a description).
The @acronym{SRP} key exchange is an extension to the
@acronym{TLS} protocol, and it provides an authenticated with a 
password key exchange. The peers can be identified using a single password, 
or there can be combinations where the client is authenticated using @acronym{SRP}
and the server using a certificate.

The advantage of @acronym{SRP} authentication, over other proposed
secure password authentication schemes, is that @acronym{SRP} is not
susceptible to off-line dictionary attacks.
Moreover, SRP does not require the server to hold the user's password.
This kind of protection is similar to the one used traditionally in the @acronym{UNIX}
@file{/etc/passwd} file, where the contents of this file did not cause
harm to the system security if they were revealed.  The @acronym{SRP}
needs instead of the plain password something called a verifier, which
is calculated using the user's password, and if stolen cannot be used
to impersonate the user. 
@c The Stanford @acronym{SRP} libraries, include a PAM module that synchronizes
@c the system's users passwords with the @acronym{SRP} password
@c files. That way @acronym{SRP} authentication could be used for all users
@c of a system.

Typical conventions in SRP are a password file, called @file{tpasswd} that 
holds the SRP verifiers (encoded passwords) and another file, @file{tpasswd.conf},
which holds the allowed SRP parameters.  The included in GnuTLS helper 
follow those conventions. The srptool program, discussed in the next section
is a tool to manipulate the SRP parameters.

The implementation in @acronym{GnuTLS} is based on @xcite{TLSSRP}. The
supported key exchange methods are shown below.

@table @code

@item SRP:
Authentication using the @acronym{SRP} protocol.

@item SRP_DSS:
Client authentication using the @acronym{SRP} protocol. Server is
authenticated using a certificate with DSA parameters.

@item SRP_RSA:
Client authentication using the @acronym{SRP} protocol. Server is
authenticated using a certificate with RSA parameters.

@end table


@showfuncdesc{gnutls_srp_verifier}

@showfuncB{gnutls_srp_base64_encode_alloc,gnutls_srp_base64_decode_alloc}

@include invoke-srptool.texi

@node PSK authentication
@subsection PSK authentication

@menu
* Authentication using PSK::
* psktool Invocation::             Invoking psktool
@end menu

@node Authentication using PSK
@subsubsection Authentication using @acronym{PSK}
@cindex PSK authentication

Authentication using Pre-shared keys is a method to authenticate using
usernames and binary keys. This protocol avoids making use of public
key infrastructure and expensive calculations, thus it is suitable for
constraint clients.

The implementation in @acronym{GnuTLS} is based on @xcite{TLSPSK}.  
The supported @acronym{PSK} key exchange methods are:

@table @code

@item PSK:
Authentication using the @acronym{PSK} protocol.

@item DHE-PSK:
Authentication using the @acronym{PSK} protocol and Diffie-Hellman key
exchange.  This method offers perfect forward secrecy.

@item ECDHE-PSK:
Authentication using the @acronym{PSK} protocol and Elliptic curve Diffie-Hellman key
exchange.  This method offers perfect forward secrecy.

@item RSA-PSK:
Authentication using the @acronym{PSK} protocol for the client and an RSA certificate
for the server.

@end table


Helper functions to generate and maintain @acronym{PSK} keys are also included
in @acronym{GnuTLS}.

@showfuncC{gnutls_key_generate,gnutls_hex_encode,gnutls_hex_decode}

@include invoke-psktool.texi

@node Anonymous authentication
@subsection Anonymous authentication
@cindex anonymous authentication

The anonymous key exchange offers encryption without any
indication of the peer's identity.  This kind of authentication
is vulnerable to a man in the middle attack, but can be
used even if there is no prior communication or shared trusted parties 
with the peer. It is useful to establish a session over which certificate
authentication will occur in order to hide the indentities of the participants
from passive eavesdroppers.

Unless in the above case, it is not recommended to use anonymous authentication. 
In the cases where there is no prior communication with the peers, 
an alternative with better properties, such as key continuity, is trust on first use
(see @ref{Verifying a certificate using trust on first use authentication}).

The available key exchange algorithms for anonymous authentication are
shown below, but note that few public servers support them, and they 
have to be explicitly enabled.

@table @code

@item ANON_DH:
This algorithm exchanges Diffie-Hellman parameters.

@item ANON_ECDH:
This algorithm exchanges elliptic curve Diffie-Hellman parameters. It is more 
efficient than ANON_DH on equivalent security levels.

@end table
