1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
|
\section{Authentication using SRP\index{SRP authentication}}
Authentication using the SRP\footnote{SRP stands for Secure Remote Password and
is described in \cite{RFC2945}. The SRP key exchange is not a part of the \tlsI{} protocol}
protocol is actually password authentication. The two peers can be identified using a
single password, or there can be combinations where the client is
authenticated using SRP and the server using a certificate.
\par
The advantage of SRP authentication, over other proposed secure password
authentication schemas, is that 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 \emph{UNIX} ``passwd'' file, where the contents of this file did not cause
harm to the system security if they were revealed.
The SRP holds 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. See \cite{TOMSRP} for a detailed description
of the SRP protocol, and for the Stanford SRP libraries.
\par
The implementation in \gnutls{} is based on paper \cite{TLSSRP}.
The available key exchange methods are shown in \hyperref{figure}{figure }{}{fig:srp}.
\begin{figure}[hbtp]
\begin{tabular}{|l|p{9cm}|}
\hline
SRP & Authentication using the SRP protocol.
\\
\hline
SRP\_DSS & Client authentication using the SRP protocol. Server is
authenticated using a certificate with DSA parameters.
\\
\hline
SRP\_RSA & Client authentication using the SRP protocol. Server is
authenticated using a certificate with RSA parameters.
\\
\hline
\end{tabular}
\caption{Supported SRP key exchange algorithms}
\label{fig:srp}
\end{figure}
The default behaviour of \gnutls{} is to read the usernames and
SRP verifiers from password files. These password files are the ones used
by the \emph{srp libraries} and can be specified using the
\printfunc{gnutls_srp_set_server_credentials_file}{gnutls\_srp\_set\_server\_credentials\_file}.
If a different password file format is to be used, then the
function \printfunc{gnutls_srp_set_server_credentials_function}{gnutls\_srp\_set\_server\_credentials\_function},
should be called, in order to set an appropriate callback.
Some helper functions such as
\begin{itemize}
\item \printfunc{gnutls_srp_verifier}{gnutls\_srp\_verifier}
\item \printfunc{gnutls_srp_base64_encode}{gnutls\_srp\_base64\_encode}
\item \printfunc{gnutls_srp_base64_decode}{gnutls\_srp\_base64\_decode}
\end{itemize}
are included in \gnutls{}, and may be used to generate, and maintain
SRP verifiers, and password files.
A program to manipulate the required parameters
for SRP authentication is also included. See section \ref{srpcrypt} on
page \pageref{srpcrypt} for more information.
|
