@node Introduction to TLS
@chapter Introduction to @acronym{TLS} and @acronym{DTLS}

@acronym{TLS} stands for ``Transport Layer Security'' and is the
successor of SSL, the Secure Sockets Layer protocol @xcite{SSL3}
designed by Netscape.  @acronym{TLS} is an Internet protocol, defined
by @acronym{IETF}@footnote{IETF, or Internet Engineering Task Force,
is a large open international community of network designers,
operators, vendors, and researchers concerned with the evolution of
the Internet architecture and the smooth operation of the Internet.
It is open to any interested individual.}, described in @xcite{RFC5246}.  
The protocol provides
confidentiality, and authentication layers over any reliable transport
layer.  The description, below, refers to @acronym{TLS} 1.0 but also
applies to @acronym{TLS} 1.2 @xcite{RFC5246} and @acronym{SSL} 3.0,
since the differences of these protocols are not major.  

The @acronym{DTLS} protocol, or ``Datagram @acronym{TLS}'' @xcite{RFC4347} is a
protocol with identical goals as @acronym{TLS}, but can operate
under unreliable transport layers, such as @acronym{UDP}. The
discussions below apply to this protocol as well, except when
noted otherwise.

@menu
* TLS layers::
* The transport layer::
* The TLS record protocol::
* The TLS Alert Protocol::
* The TLS Handshake Protocol::
* TLS Extensions::
* Selecting cryptographic key sizes::
* How to use TLS in application protocols::
* On SSL 2 and older protocols::
@end menu

@node TLS layers
@section TLS layers
@cindex TLS layers

@acronym{TLS} is a layered protocol, and consists of the record
protocol, the handshake protocol and the alert protocol. The record
protocol is to serve all other protocols and is above the transport
layer.  The record protocol offers symmetric encryption, data
authenticity, and optionally compression.
The alert protocol offers some signaling to the other protocols. It
can help informing the peer for the cause of failures and other error
conditions.  @xref{The Alert Protocol}, for more information.  The
alert protocol is above the record protocol.

The handshake protocol is responsible for the security parameters'
negotiation, the initial key exchange and authentication.  
@xref{The Handshake Protocol}, for more information about the handshake
protocol.  The protocol layering in TLS is shown in @ref{fig:tls-layers}.

@float Figure,fig:tls-layers
@image{gnutls-layers,12cm}
@caption{The TLS protocol layers.}
@end float

@node The transport layer
@section The transport layer
@cindex transport protocol
@cindex transport layer

@acronym{TLS} is not limited to any transport layer and can be used
above any transport layer, as long as it is a reliable one.  @acronym{DTLS}
can be used over reliable and unreliable transport layers.
@acronym{GnuTLS} supports TCP and UDP layers transparently using
the Berkeley sockets API. However, any transport layer can be used
by providing callbacks for @acronym{GnuTLS} to access the transport layer 
(for details see @ref{TLS and DTLS sessions}).

@node The TLS record protocol
@section The TLS record protocol
@cindex record protocol

The record protocol is the secure communications provider. Its purpose
is to encrypt, authenticate and ---optionally--- compress packets.
The record layer functions can be called at any time after
the handshake process is finished, when there is need to receive
or send data. In @acronym{DTLS} however, due to re-transmission
timers used in the handshake out-of-order handshake data might
be received for some time (maximum 60 seconds) after the handshake
process is finished. For this reason programs using @acronym{DTLS}
should call @funcref{gnutls_record_recv} or @funcref{gnutls_record_recv_seq}
for every packet received by the peer, even if no data were
expected. 

As you may have already noticed, the functions which access the record
protocol, are quite limited, given the importance of this protocol in
@acronym{TLS}.  This is because the record protocol's parameters are
all set by the handshake protocol.
The record protocol initially starts with NULL parameters, which means
no encryption, and no MAC is used. Encryption and authentication begin
just after the handshake protocol has finished.

@showfuncC{gnutls_record_send,gnutls_record_recv,gnutls_record_recv_seq}
@showfuncB{gnutls_record_check_pending,gnutls_record_get_direction}

@menu
* Encryption algorithms used in the record layer::
* Compression algorithms used in the record layer::
* Weaknesses and countermeasures::
* On Record Padding::
@end menu

@node Encryption algorithms used in the record layer
@subsection Encryption algorithms used in the record layer
@cindex symmetric encryption algorithms

Confidentiality in the record layer is achieved by using symmetric
block encryption algorithms like @code{3DES}, @code{AES}
or stream algorithms like @code{ARCFOUR_128}.
 Ciphers are encryption algorithms that use a single, secret,
key to encrypt and decrypt data. Block algorithms in TLS also provide
protection against statistical analysis of the data.  Thus, if you're
using the @acronym{TLS} protocol, a random number of blocks will be
appended to data, to prevent eavesdroppers from guessing the actual
data size.

The supported in @acronym{GnuTLS} ciphers and MAC algorithms are shown in @ref{tab:ciphers} and
@ref{tab:macs}.

@float Table,tab:ciphers
@multitable @columnfractions .20 .70
@headitem Algorithm @tab Description
@item 3DES_CBC @tab
This is the DES block cipher algorithm used with triple
encryption (EDE). Has 64 bits block size and is used in CBC mode.

@item ARCFOUR_128 @tab
ARCFOUR_128 is a compatible algorithm with RSA's RC4 algorithm, which is considered to be a trade
secret. It is a fast cipher but considered weak today.

@item ARCFOUR_40 @tab
This is the ARCFOUR cipher fed with a 40 bit key,
which is considered weak.

@item AES_CBC @tab
AES or RIJNDAEL is the block cipher algorithm that replaces the old
DES algorithm.  Has 128 bits block size and is used in CBC mode.

@item AES_GCM @tab
This is the AES algorithm in the authenticated encryption GCM mode.
This mode combines message authentication and encryption and can
be extremely fast on CPUs that support hardware acceleration.

@item CAMELLIA_CBC @tab
This is an 128-bit block cipher developed by Mitsubishi and NTT. It
is one of the approved ciphers of the European NESSIE and Japanese
CRYPTREC projects.

@end multitable
@caption{Supported ciphers.}
@end float


@float Table,tab:macs
@multitable @columnfractions .20 .70
@headitem Algorithm @tab Description
@item MAC_MD5 @tab
This is a cryptographic hash algorithm designed by Ron Rivest. Outputs
128 bits of data.

@item MAC_SHA1 @tab
A cryptographic hash algorithm designed by NSA. Outputs 160
bits of data.

@item MAC_SHA256 @tab
A cryptographic hash algorithm designed by NSA. Outputs 256
bits of data.

@item MAC_AEAD @tab
This indicates that an authenticated encryption algorithm, such as
GCM, is in use.

@end multitable
@caption{Supported MAC algorithms.}
@end float


@node Compression algorithms used in the record layer
@subsection Compression algorithms used in the record layer
@cindex compression algorithms

The TLS record layer also supports compression.  The algorithms
implemented in @acronym{GnuTLS} can be found in the table below.
The included algorithms perform really good when text, or other
compressible data are to be transferred, but offer nothing on already
compressed data, such as compressed images, zipped archives etc.
These compression algorithms, may be useful in high bandwidth TLS
tunnels, and in cases where network usage has to be minimized. It
should be noted however that compression increases latency.

The record layer compression in @acronym{GnuTLS} is implemented based
on the proposal @xcite{RFC3749}. The supported algorithms are shown in
@ref{gnutls_compression_method_t}.

@showenumdesc{gnutls_compression_method_t,Supported compression algorithms}

@node Weaknesses and countermeasures
@subsection Weaknesses and countermeasures

Some weaknesses that may affect the security of the record layer have
been found in @acronym{TLS} 1.0 protocol. These weaknesses can be
exploited by active attackers, and exploit the facts that

@enumerate

@item
@acronym{TLS} has separate alerts for ``decryption_failed'' and
``bad_record_mac''

@item
The decryption failure reason can be detected by timing the response
time.

@item
The IV for CBC encrypted packets is the last block of the previous
encrypted packet.

@end enumerate

Those weaknesses were solved in @acronym{TLS} 1.1 @xcite{RFC4346}
which is implemented in @acronym{GnuTLS}. For a detailed discussion
see the archives of the TLS Working Group mailing list and @xcite{CBCATT}.

@node On Record Padding
@subsection On record padding
@cindex record padding
@cindex bad_record_mac

The TLS protocol allows for random padding of records, to prevent
statistical analysis based on the length of exchanged messages (see @xcite{RFC5246} section 6.2.3.2).  
GnuTLS appears to be one of few implementation that take advantage of this text, 
and pad records by a random length.

The TLS implementation in the Symbian operating system, frequently
used by Nokia and Sony-Ericsson mobile phones, cannot handle
non-minimal record padding.  What happens when one of these clients
handshake with a GnuTLS server is that the client will fail to compute
the correct MAC for the record.  The client sends a TLS alert
(@code{bad_record_mac}) and disconnects.  Typically this will result
in error messages such as 'A TLS fatal alert has been received', 'Bad
record MAC', or both, on the GnuTLS server side.

GnuTLS implements a work around for this problem.  However, it has to
be enabled specifically.  It can be enabled by using
@funcref{gnutls_record_disable_padding}, or @funcref{gnutls_priority_set} with
the @code{%COMPAT} priority string (see @ref{Priority Strings}).

If you implement an application that have a configuration file, we
recommend that you make it possible for users or administrators to
specify a GnuTLS protocol priority string, which is used by your
application via @funcref{gnutls_priority_set}.  To allow the best
flexibility, make it possible to have a different priority string for
different incoming IP addresses.


@node The TLS Alert Protocol
@section The TLS alert protocol
@anchor{The Alert Protocol}
@cindex alert protocol

The alert protocol is there to allow signals to be sent between peers.
These signals are mostly used to inform the peer about the cause of a
protocol failure. Some of these signals are used internally by the
protocol and the application protocol does not have to cope with them
(e.g. @code{GNUTLS_@-A_@-CLOSE_@-NOTIFY}), and others refer to the
application protocol solely (e.g. @code{GNUTLS_@-A_@-USER_@-CANCELLED}).  An
alert signal includes a level indication which may be either fatal or
warning. Fatal alerts always terminate the current connection, and
prevent future re-negotiations using the current session ID. All alert
messages are summarized in @ref{tab:alerts}.


The alert messages are protected by the record protocol, thus the
information that is included does not leak. You must take extreme care
for the alert information not to leak to a possible attacker, via
public log files etc. The available functions to control the alert
protocol are shown below.

@showfuncdesc{gnutls_alert_get}

@showfuncdesc{gnutls_alert_send}

@showfuncB{gnutls_error_to_alert,gnutls_alert_get_name}

@include alerts.texi

@node The TLS Handshake Protocol
@section The TLS handshake protocol
@anchor{The Handshake Protocol}
@cindex handshake protocol

The handshake protocol is responsible for the ciphersuite negotiation,
the initial key exchange, and the authentication of the two peers.
This is fully controlled by the application layer, thus your program
has to set up the required parameters. The main handshake function
is @funcref{gnutls_handshake}. In the next paragraphs we elaborate on 
the handshake protocol, i.e., the ciphersuite negotiation.


@menu
* TLS Cipher Suites::           TLS session parameters.
* Client Authentication::       Requesting a certificate from the client.
* Resuming Sessions::           Reusing previously established keys.
* Interoperability::            About interoperability with other implementations.
@end menu

@node TLS Cipher Suites
@subsection TLS ciphersuites

The handshake protocol of @acronym{TLS} negotiates cipher suites of
a special form illustrated by the @code{TLS_DHE_RSA_WITH_3DES_CBC_SHA} cipher suite name.  A typical cipher
suite contains these parameters:

@itemize

@item The key exchange algorithm.
@code{DHE_RSA} in the example.

@item The Symmetric encryption algorithm and mode
@code{3DES_CBC} in this example.

@item The MAC@footnote{MAC stands for Message Authentication Code. It can be described as a keyed hash algorithm. See RFC2104.} algorithm used for authentication.
@code{MAC_SHA} is used in the above example.

@end itemize

The cipher suite negotiated in the handshake protocol will affect the
record protocol, by enabling encryption and data authentication.  Note
that you should not over rely on @acronym{TLS} to negotiate the
strongest available cipher suite. Do not enable ciphers and algorithms
that you consider weak.

All the supported ciphersuites are listed in @ref{ciphersuites}.

@node Client Authentication
@subsection Client authentication
@cindex client certificate authentication

In the case of ciphersuites that use certificate authentication, the
authentication of the client is optional in @acronym{TLS}.  A server
may request a certificate from the client using the
@funcref{gnutls_certificate_server_set_request} function. If a certificate
is to be requested from the client during the handshake, the server
will send a certificate request message that contains a list of
acceptable certificate signers. In @acronym{GnuTLS} the certificate
signers list is constructed using the trusted Certificate Authorities
by the server. That is the ones set using the following functions.

@showfuncB{gnutls_certificate_set_x509_trust_file,gnutls_certificate_set_x509_trust_mem}

@showfuncdesc{gnutls_certificate_server_set_request}

In cases where the server supports a large number of certificate authorities
it makes sense not to advertise all of the names to save bandwidth. That can
be controlled using the function @funcref{gnutls_certificate_send_x509_rdn_sequence}. 
This however will have the side-effect of not restricting the client to certificates
signed by server's acceptable signers.

@showfuncdesc{gnutls_certificate_send_x509_rdn_sequence}

@node Resuming Sessions
@subsection Resuming sessions
@anchor{resume}
@cindex resuming sessions
@cindex session resuming

The @funcref{gnutls_handshake} function, is expensive since a lot of
calculations are performed. In order to support many fast connections
to the same server a client may use session resuming. Session
resuming is a feature of the @acronym{TLS} protocol which allows a
client to connect to a server, after a successful handshake, without
the expensive calculations.  This is achieved by re-using the previously
established keys. @acronym{GnuTLS} supports this feature, and the
example in @ref{ex:resume-client} illustrates a typical use of it.

Keep in mind that sessions might be expired after some time, 
thus it may be normal for a server not to resume a session
even if you requested that.  That is to prevent temporal session keys
from becoming long-term keys. Also note that as a client you must enable, using the
priority functions, at least the algorithms used in the last session.

The resuming capability, mostly in the server side, is one of the
problems of a thread-safe TLS implementations. The problem is that all
threads must share information in order to be able to resume
sessions. The gnutls approach is, in case of a client, to leave all
the burden of resuming to the client. That is, copy and keep the
necessary parameters. The relevant functions are listed below.

@showfuncdesc{gnutls_session_get_data}

@showfuncdesc{gnutls_session_get_id}

@showfuncdesc{gnutls_session_set_data}

Server side is different. A server needs to specify callback
functions which store, retrieve and delete session data. These can be
registered with the functions shown below.

@showfuncdesc{gnutls_db_set_retrieve_function}

@showfuncdesc{gnutls_db_set_store_function}

@showfuncB{gnutls_db_set_ptr,gnutls_db_set_remove_function}

It might also be useful to be able to check for expired sessions in
order to remove them, and save space. The function
@funcref{gnutls_db_check_entry} is provided for that reason.

@showfuncdesc{gnutls_db_check_entry}

@node Interoperability
@subsection Interoperability

The @acronym{TLS} handshake is a complex procedure that negotiates all
required parameters for a secure session. @acronym{GnuTLS} supports
several @acronym{TLS} extensions, as well as the latest @acronym{TLS} protocol 
version 1.2. However few implementations are not able to
properly interoperate once faced with extensions or version protocols
they do not support and understand. The @acronym{TLS} protocol allows for a
graceful downgrade to the commonly supported options, but practice shows 
it is not always implemented correctly. 

Because there is no way to achieve maximum interoperability with broken peers
without sacrificing security, @acronym{GnuTLS} ignores such peers by default. 
This might not be acceptable in cases where maximum compatibility
is required. Thus we allow enabling compatibility with broken peers using
priority strings (see @ref{Priority Strings}). An example priority string that
is known to provide wide compatibility even with broken peers
is shown below:
@verbatim
NORMAL:-VERS-TLS-ALL:+VERS-TLS1.0:+VERS-SSL3.0:%COMPAT
@end verbatim
This priority string will only enable SSL 3.0 and TLS 1.0 as protocols and
will disable, via the @code{%COMPAT} keyword, several @acronym{TLS} protocol
options that are known to cause compatibility problems. Note however that
there are known attacks against those protocol versions and
if mode is used security is traded for compatibility.

@node TLS Extensions
@section TLS extensions
@cindex TLS extensions

A number of extensions to the @acronym{TLS} protocol have been
proposed mainly in @xcite{TLSEXT}. The extensions supported
in @acronym{GnuTLS} are:

@itemize
@item Maximum fragment length negotiation
@item Server name indication
@item Session tickets
@item Safe Renegotiation
@end itemize

and they will be discussed in the subsections that follow.

@subsection Maximum fragment length negotiation
@cindex TLS extensions
@cindex maximum fragment length

This extension allows a @acronym{TLS} implementation to negotiate a
smaller value for record packet maximum length. This extension may be
useful to clients with constrained capabilities. The functions shown
below can be used to control this extension.

@showfuncdesc{gnutls_record_get_max_size}

@showfuncdesc{gnutls_record_set_max_size}

@subsection Server name indication
@anchor{serverind}
@cindex TLS extensions
@cindex server name indication

A common problem in @acronym{HTTPS} servers is the fact that the
@acronym{TLS} protocol is not aware of the hostname that a client
connects to, when the handshake procedure begins. For that reason the
@acronym{TLS} server has no way to know which certificate to send.

This extension solves that problem within the @acronym{TLS} protocol,
and allows a client to send the HTTP hostname before the handshake
begins within the first handshake packet.  The functions
@funcref{gnutls_server_name_set} and @funcref{gnutls_server_name_get} can be
used to enable this extension, or to retrieve the name sent by a
client.

@showfuncdesc{gnutls_server_name_set}

@showfuncdesc{gnutls_server_name_get}

@subsection Session tickets
@cindex TLS extensions
@cindex session tickets
@cindex tickets

To resume a TLS session the server normally store some state.  This
complicates deployment, and typical situations the client can cache
information and send it to the server instead.  The Session Ticket
extension implements this idea, and it is documented in
RFC 5077 @xcite{TLSTKT}.

Clients can enable support for TLS tickets with
@funcref{gnutls_session_ticket_enable_client} and servers use
@funcref{gnutls_session_ticket_key_generate} to generate a key and
@funcref{gnutls_session_ticket_enable_server} to enable the extension.
Clients resume sessions using the normal session resumption procedure (see @ref{resume}).

@showfuncdesc{gnutls_session_ticket_key_generate}

@showfuncdesc{gnutls_session_ticket_enable_server}

@showfuncdesc{gnutls_session_ticket_enable_client}

@subsection Safe renegotiation
@cindex renegotiation
@cindex safe renegotiation

TLS gives the option to two communicating parties to renegotiate
and update their security parameters. One useful example of this feature
was for a client to initially connect using anonymous negotiation to a
server, and the renegotiate using some authenticated ciphersuite. This occurred
to avoid having the client sending its credentials in the clear.

However this renegotiation, as initially designed would not ensure that
the party one is renegotiating is the same as the one in the initial negotiation.
For example one server could forward all renegotiation traffic to an other
server who will see this traffic as an initial negotiation attempt.

This might be seen as a valid design decision, but it seems it was
not widely known or understood, thus today some application protocols the TLS
renegotiation feature in a manner that enables a malicious server to insert
content of his choice in the beginning of a TLS session.

The most prominent vulnerability was with HTTPS. There servers request
a renegotiation to enforce an anonymous user to use a certificate in order
to access certain parts of a web site.  The
attack works by having the attacker simulate a client and connect to a
server, with server-only authentication, and send some data intended
to cause harm.  The server will then require renegotiation from him
in order to perform the request. 
When the proper client attempts to contact the server,
the attacker hijacks that connection and forwards traffic to
the initial server that requested renegotiation.  The
attacker will not be able to read the data exchanged between the
client and the server.  However, the server will (incorrectly) assume
that the initial request sent by the attacker was sent by the now authenticated
client.  The result is a prefix plain-text injection attack.

The above is just one example.  Other vulnerabilities exists that do
not rely on the TLS renegotiation to change the client's authenticated
status (either TLS or application layer).

While fixing these application protocols and implementations would be
one natural reaction, an extension to TLS has been designed that
cryptographically binds together any renegotiated handshakes with the
initial negotiation.  When the extension is used, the attack is
detected and the session can be terminated.  The extension is
specified in @xcite{RFC5746}.

GnuTLS supports the safe renegotiation extension.  The default
behavior is as follows.  Clients will attempt to negotiate the safe
renegotiation extension when talking to servers.  Servers will accept
the extension when presented by clients.  Clients and servers will
permit an initial handshake to complete even when the other side does
not support the safe renegotiation extension.  Clients and servers
will refuse renegotiation attempts when the extension has not been
negotiated.

Note that permitting clients to connect to servers when the safe
renegotiation extension is not enabled, is open up for attacks.
Changing this default behavior would prevent interoperability against
the majority of deployed servers out there.  We will reconsider this
default behavior in the future when more servers have been upgraded.
Note that it is easy to configure clients to always require the safe
renegotiation extension from servers.

To modify the default behavior, we have introduced some new priority
strings (see @ref{Priority Strings}).  
The @code{%UNSAFE_RENEGOTIATION} priority string permits
(re-)handshakes even when the safe renegotiation extension was not
negotiated. The default behavior is @code{%PARTIAL_RENEGOTIATION} that will
prevent renegotiation with clients and servers not supporting the
extension. This is secure for servers but leaves clients vulnerable
to some attacks, but this is a trade-off between security and compatibility
with old servers. The @code{%SAFE_RENEGOTIATION} priority string makes
clients and servers require the extension for every handshake. The latter
is the most secure option for clients, at the cost of not being able
to connect to legacy servers. Servers will also deny clients that
do not support the extension from connecting.

It is possible to disable use of the extension completely, in both
clients and servers, by using the @code{%DISABLE_SAFE_RENEGOTIATION}
priority string however we strongly recommend you to only do this for
debugging and test purposes.

The default values if the flags above are not specified are:
@table @code

@item Server:
%PARTIAL_RENEGOTIATION

@item Client:
%PARTIAL_RENEGOTIATION

@end table

For applications we have introduced a new API related to safe
renegotiation.  The @funcref{gnutls_safe_renegotiation_status} function is
used to check if the extension has been negotiated on a session, and
can be used both by clients and servers.

@node Selecting cryptographic key sizes
@section Selecting cryptographic key sizes
@cindex key sizes

Because many algorithms are involved in TLS, it is not easy to set
a consistent security level.  For this reason in @ref{tab:key-sizes} we
present some correspondence between key sizes of symmetric algorithms
and public key algorithms based on @xcite{ECRYPT}. 
Those can be used to generate certificates with
appropriate key sizes as well as select parameters for Diffie-Hellman and SRP
authentication.

@float Table,tab:key-sizes
@multitable @columnfractions .10 .12 .10 .20 .32

@headitem Security bits @tab RSA, DH and SRP parameter size @tab ECC key size @tab Security parameter @tab Description

@item 64
@tab 816
@tab 128
@tab @code{WEAK}
@tab Very short term protection against small organizations

@item 80
@tab 1248
@tab 160
@tab @code{LOW}
@tab Very short term protection against agencies

@item 112
@tab 2432
@tab 224
@tab @code{NORMAL}
@tab Medium-term protection

@item 128
@tab 3248
@tab 256
@tab @code{HIGH}
@tab Long term protection

@item 256
@tab 15424
@tab 512
@tab @code{ULTRA}
@tab Foreseeable future

@end multitable
@caption{Key sizes and security parameters.}
@end float

The first column  provides a security parameter in a number of bits. This
gives an indication of the number of combinations to be tried by an adversary
to brute force a key. For example to test all possible keys in a 112 bit security parameter
@math{2^{112}} combinations have to be tried. For today's technology this is infeasible.
The next two columns correlate the security
parameter with actual bit sizes of parameters for DH, RSA, SRP and ECC algorithms.
A mapping to @code{gnutls_sec_param_t} value is given for each security parameter, on
the next column, and finally a brief description of the level.

@c @showenumdesc{gnutls_sec_param_t,The @code{gnutls_sec_@-param_t} enumeration.}

Note, however, that the values suggested here are nothing more than an
educated guess that is valid today. There are no guarantees that an
algorithm will remain unbreakable or that these values will remain
constant in time. There could be scientific breakthroughs that cannot
be predicted or total failure of the current public key systems by
quantum computers. On the other hand though the cryptosystems used in
TLS are selected in a conservative way and such catastrophic
breakthroughs or failures are believed to be unlikely.
The NIST publication SP 800-57 @xcite{NISTSP80057} contains a similar
table.

When using @acronym{GnuTLS} and a decision on bit sizes for a public
key algorithm is required, use of the following functions is  
recommended:

@showfuncdesc{gnutls_sec_param_to_pk_bits}

@showfuncdesc{gnutls_pk_bits_to_sec_param}

Those functions will convert a human understandable security parameter
of @code{gnutls_sec_param_t} type, to a number of bits suitable for a public 
key algorithm.

@include sec-tls-app.texi

@node On SSL 2 and older protocols
@section On SSL 2 and older protocols
@cindex SSL 2

One of the initial decisions in the @acronym{GnuTLS} development was
to implement the known security protocols for the transport layer.
Initially @acronym{TLS} 1.0 was implemented since it was the latest at
that time, and was considered to be the most advanced in security
properties.  Later the @acronym{SSL} 3.0 protocol was implemented
since it is still the only protocol supported by several servers and
there are no serious security vulnerabilities known.

One question that may arise is why we didn't implement @acronym{SSL}
2.0 in the library.  There are several reasons, most important being
that it has serious security flaws, unacceptable for a modern security
library.  Other than that, this protocol is barely used by anyone
these days since it has been deprecated since 1996.  The security
problems in @acronym{SSL} 2.0 include:

@itemize

@item Message integrity compromised.
The @acronym{SSLv2} message authentication uses the MD5 function, and
is insecure.

@item Man-in-the-middle attack.
There is no protection of the handshake in @acronym{SSLv2}, which
permits a man-in-the-middle attack.

@item Truncation attack.
@acronym{SSLv2} relies on TCP FIN to close the session, so the
attacker can forge a TCP FIN, and the peer cannot tell if it was a
legitimate end of data or not.

@item Weak message integrity for export ciphers.
The cryptographic keys in @acronym{SSLv2} are used for both message
authentication and encryption, so if weak encryption schemes are
negotiated (say 40-bit keys) the message authentication code use the
same weak key, which isn't necessary.

@end itemize

@cindex PCT
Other protocols such as Microsoft's @acronym{PCT} 1 and @acronym{PCT}
2 were not implemented because they were also abandoned and deprecated
by @acronym{SSL} 3.0 and later @acronym{TLS} 1.0.