path: root/doc/cha-cert-auth.texi

@node Certificate authentication
@section Certificate authentication
@cindex certificate authentication

The most known authentication method of @acronym{TLS} are certificates.
The PKIX @xcite{PKIX} public key infrastructure is daily used by anyone
using a browser today. @acronym{GnuTLS} provides a simple API to 
use the @acronym{X.509} certificates @xcite{PKIX}.

The key exchange algorithms supported by certificate authentication are
shown in @ref{tab:key-exchange}.

@float Table,tab:key-exchange
@multitable @columnfractions .2 .7

@headitem Key exchange @tab Description

@item RSA @tab
The RSA algorithm is used to encrypt a key and send it to the peer.
The certificate must allow the key to be used for encryption.

@item DHE_@-RSA @tab
The RSA algorithm is used to sign ephemeral Diffie-Hellman parameters
which are sent to the peer. The key in the certificate must allow the
key to be used for signing. Note that key exchange algorithms which
use ephemeral Diffie-Hellman parameters, offer perfect forward
secrecy. That means that even if the private key used for signing is
compromised, it cannot be used to reveal past session data.

@item ECDHE_@-RSA @tab
The RSA algorithm is used to sign ephemeral elliptic curve Diffie-Hellman 
parameters which are sent to the peer. The key in the certificate must allow 
the key to be used for signing. It also offers perfect forward
secrecy. That means that even if the private key used for signing is
compromised, it cannot be used to reveal past session data.

@item DHE_@-DSS @tab
The DSA algorithm is used to sign ephemeral Diffie-Hellman parameters
which are sent to the peer. The certificate must contain DSA
parameters to use this key exchange algorithm. DSA is the algorithm
of the Digital Signature Standard (DSS).

@item ECDHE_@-ECDSA @tab
The Elliptic curve DSA algorithm is used to sign ephemeral elliptic
curve Diffie-Hellman parameters which are sent to the peer. The 
certificate must contain ECDSA parameters (i.e., EC and marked for signing) 
to use this key exchange algorithm. 

@end multitable
@caption{Supported key exchange algorithms.}
@end float

@menu
* X.509 certificates::
* OpenPGP certificates::
* Advanced certificate verification::
* Digital signatures::
@end menu

@node X.509 certificates
@subsection @acronym{X.509} certificates
@cindex X.509 certificates

The @acronym{X.509} protocols rely on a hierarchical trust model. In
this trust model Certification Authorities (CAs) are used to certify
entities.  Usually more than one certification authorities exist, and
certification authorities may certify other authorities to issue
certificates as well, following a hierarchical model.

@float Figure,fig-x509
@image{gnutls-x509,7cm}
@caption{An example of the X.509 hierarchical trust model.}
@end float

One needs to trust one or more CAs for his secure communications. In
that case only the certificates issued by the trusted authorities are
acceptable.  The framework is illustrated on @ref{fig-x509}.

@menu
* X.509 certificate structure::
* Importing an X.509 certificate::
* X.509 certificate names::
* X.509 distinguished names::
* X.509 extensions::
* X.509 public and private keys::
* Verifying X.509 certificate paths::
* Verifying a certificate in the context of TLS session::
* Verification using PKCS11::
@end menu

@node X.509 certificate structure
@subsubsection @acronym{X.509} certificate structure

An @acronym{X.509} certificate usually contains information about the
certificate holder, the signer, a unique serial number, expiration
dates and some other fields @xcite{PKIX} as shown in @ref{tab:x509}.

@float Table,tab:x509
@multitable @columnfractions .2 .7

@headitem Field @tab Description

@item version @tab
The field that indicates the version of the certificate.

@item serialNumber @tab
This field holds a unique serial number per certificate.

@item signature @tab
The issuing authority's signature.

@item issuer @tab
Holds the issuer's distinguished name.

@item validity @tab
The activation and expiration dates.

@item subject @tab
The subject's distinguished name of the certificate.

@item extensions @tab
The extensions are fields only present in version 3 certificates.

@end multitable
@caption{X.509 certificate fields.}
@end float

The certificate's @emph{subject or issuer name} is not just a single
string.  It is a Distinguished name and in the @acronym{ASN.1}
notation is a sequence of several object identifiers with their corresponding
values. Some of available OIDs to be used in an @acronym{X.509}
distinguished name are defined in @file{gnutls/x509.h}.

The @emph{Version} field in a certificate has values either 1 or 3 for
version 3 certificates.  Version 1 certificates do not support the
extensions field so it is not possible to distinguish a CA from a
person, thus their usage should be avoided.

The @emph{validity} dates are there to indicate the date that the
specific certificate was activated and the date the certificate's key
would be considered invalid.


In @acronym{GnuTLS} the @acronym{X.509} certificate structures are
handled using the @code{gnutls_x509_crt_t} type and the corresponding
private keys with the @code{gnutls_x509_privkey_t} type.  All the
available functions for @acronym{X.509} certificate handling have
their prototypes in @file{gnutls/x509.h}. An example program to
demonstrate the @acronym{X.509} parsing capabilities can be found in
@ref{ex-x509-info}.

@node Importing an X.509 certificate
@subsubsection Importing an X.509 certificate

The certificate structure should be initialized using @funcref{gnutls_x509_crt_init}, and 
a certificate structure can be imported using @funcref{gnutls_x509_crt_import}. 

@showfuncC{gnutls_x509_crt_init,gnutls_x509_crt_import,gnutls_x509_crt_deinit}

In several functions an array of certificates is required. To assist in initialization
and import the following two functions are provided.

@showfuncB{gnutls_x509_crt_list_import,gnutls_x509_crt_list_import2}

In all cases after use a certificate must be deinitialized using @funcref{gnutls_x509_crt_deinit}.
Note that although the functions above apply to @code{gnutls_x509_crt_t} structure, similar functions
exist for the CRL structure @code{gnutls_x509_crl_t}.

@node X.509 certificate names
@subsubsection X.509 certificate names
@cindex X.509 certificate name

X.509 certificates allow for multiple names and types of names to be specified.
CA certificates often rely on X.509 distinguished names (see @ref{X.509 distinguished names})
for unique identification, while end-user and server certificates rely on the
'subject alternative names'. The subject alternative names provide a typed name, e.g.,
a DNS name, or an email address, which identifies the owner of the certificate.
The following functions provide access to that names.

@showfuncB{gnutls_x509_crt_get_subject_alt_name2,gnutls_x509_crt_set_subject_alt_name}
@showfuncC{gnutls_subject_alt_names_init,gnutls_subject_alt_names_get,gnutls_subject_alt_names_set}

Note however, that server certificates often used the Common Name (CN), part of the
certificate DistinguishedName to place a single DNS address. That practice is discouraged
(see @xcite{RFC6125}), because only a single address can be specified, and the CN field is
free-form making matching ambiguous.

@node X.509 distinguished names
@subsubsection X.509 distinguished names
@cindex X.509 distinguished name

The ``subject'' of an X.509 certificate is not described by
a single name, but rather with a distinguished name. This in
X.509 terminology is a list of strings each associated an object
identifier. To make things simple GnuTLS provides @funcref{gnutls_x509_crt_get_dn2}
which follows the rules in @xcite{RFC4514} and returns a single
string. Access to each string by individual object identifiers
can be accessed using @funcref{gnutls_x509_crt_get_dn_by_oid}.

@showfuncdesc{gnutls_x509_crt_get_dn2}
@showfuncC{gnutls_x509_crt_get_dn,gnutls_x509_crt_get_dn_by_oid,gnutls_x509_crt_get_dn_oid}

Similar functions exist to access the distinguished name
of the issuer of the certificate.

@showfuncE{gnutls_x509_crt_get_issuer_dn,gnutls_x509_crt_get_issuer_dn2,gnutls_x509_crt_get_issuer_dn_by_oid,gnutls_x509_crt_get_issuer_dn_oid,gnutls_x509_crt_get_issuer}

The more powerful @funcref{gnutls_x509_crt_get_subject} and 
@funcref{gnutls_x509_dn_get_rdn_ava} provide efficient but low-level access
to the contents of the distinguished name structure.

@showfuncB{gnutls_x509_crt_get_subject,gnutls_x509_crt_get_issuer}
@showfuncdesc{gnutls_x509_dn_get_rdn_ava}

@node X.509 extensions
@subsubsection X.509 extensions
@cindex X.509 extensions

X.509 version 3 certificates include a list of extensions that can
be used to obtain additional information on the subject or the issuer
of the certificate. Those may be e-mail addresses, flags that indicate whether the
belongs to a CA etc.  All the supported @acronym{X.509} version 3
extensions are shown in @ref{tab:x509-ext}.

The certificate extensions access is split into two parts. The first
requires to retrieve the extension, and the second is the parsing part.

To enumerate and retrieve the DER-encoded extension data available in a certificate the following
two functions are available.
@showfuncC{gnutls_x509_crt_get_extension_info,gnutls_x509_crt_get_extension_data2,gnutls_x509_crt_get_extension_by_oid2}

After a supported DER-encoded extension is retrieved it can be parsed using the APIs in @code{x509-ext.h}.
Complex extensions may require initializing an intermediate structure that holds the
parsed extension data. Examples of simple parsing functions are shown below.
@showfuncD{gnutls_x509_ext_import_basic_constraints,gnutls_x509_ext_export_basic_constraints,gnutls_x509_ext_import_key_usage,gnutls_x509_ext_export_key_usage}

More complex extensions, such as Name Constraints, require an intermediate structure, in that
case @code{gnutls_x509_name_constraints_t} to be initialized in order to store the parsed
extension data. 
@showfuncB{gnutls_x509_ext_import_name_constraints,gnutls_x509_ext_export_name_constraints}

After the name constraints are extracted in the structure, the following functions
can be used to access them.

@showfuncD{gnutls_x509_name_constraints_get_permitted,gnutls_x509_name_constraints_get_excluded,gnutls_x509_name_constraints_add_permitted,gnutls_x509_name_constraints_add_excluded}
@showfuncB{gnutls_x509_name_constraints_check,gnutls_x509_name_constraints_check_crt}

Other utility functions are listed below.
@showfuncB{gnutls_x509_name_constraints_init,gnutls_x509_name_constraints_deinit}

Similar functions exist for all of the other supported extensions, listed in @ref{tab:x509-ext}.

@float Table,tab:x509-ext
@multitable @columnfractions .3 .2 .4

@headitem Extension @tab OID @tab Description

@item Subject key id @tab 2.5.29.14 @tab
An identifier of the key of the subject.

@item Key usage @tab 2.5.29.15 @tab
Constraints the key's usage of the certificate.

@item Private key usage period @tab 2.5.29.16 @tab
Constraints the validity time of the private key.

@item Subject alternative name @tab 2.5.29.17 @tab
Alternative names to subject's distinguished name.

@item Issuer alternative name @tab 2.5.29.18 @tab
Alternative names to the issuer's distinguished name.

@item Basic constraints @tab 2.5.29.19 @tab
Indicates whether this is a CA certificate or not, and specify the
maximum path lengths of certificate chains.

@item Name constraints @tab 2.5.29.30 @tab
A field in CA certificates that restricts the scope of the name of
issued certificates.

@item CRL distribution points @tab 2.5.29.31 @tab
This extension is set by the CA, in order to inform about the
location of issued Certificate Revocation Lists.

@item Certificate policy @tab 2.5.29.32 @tab
This extension is set to indicate the certificate policy as object
identifier and may contain a descriptive string or URL.

@item Extended key usage @tab 2.5.29.54 @tab
Inhibit any policy extension. Constraints the any policy OID
(@code{GNUTLS_X509_OID_POLICY_ANY}) use in the policy extension.

@item Authority key identifier @tab 2.5.29.35 @tab
An identifier of the key of the issuer of the certificate. That is
used to distinguish between different keys of the same issuer.

@item Extended key usage @tab 2.5.29.37 @tab
Constraints the purpose of the certificate.

@item Authority information access @tab 1.3.6.1.5.5.7.1.1 @tab
Information on services by the issuer of the certificate.

@item Proxy Certification Information @tab 1.3.6.1.5.5.7.1.14 @tab
Proxy Certificates includes this extension that contains the OID of
the proxy policy language used, and can specify limits on the maximum
lengths of proxy chains.  Proxy Certificates are specified in
@xcite{RFC3820}.

@end multitable
@caption{Supported X.509 certificate extensions.}
@end float

Note, that there are also direct APIs to access extensions that may
be simpler to use for non-complex extensions. They are available
in @code{x509.h} and some examples are listed below.
@showfuncD{gnutls_x509_crt_get_basic_constraints,gnutls_x509_crt_set_basic_constraints,gnutls_x509_crt_get_key_usage,gnutls_x509_crt_set_key_usage}


@node X.509 public and private keys
@subsubsection Accessing public and private keys

Each X.509 certificate contains a public key that corresponds to a private key. To
get a unique identifier of the public key the @funcref{gnutls_x509_crt_get_key_id}
function is provided. To export the public key or its parameters you may need
to convert the X.509 structure to a @code{gnutls_pubkey_t}. See 
@ref{Abstract public keys} for more information.

@showfuncdesc{gnutls_x509_crt_get_key_id}

The private key parameters may be directly accessed by using one of the following functions.

@showfuncE{gnutls_x509_privkey_get_pk_algorithm2,gnutls_x509_privkey_export_rsa_raw2,gnutls_x509_privkey_export_ecc_raw,gnutls_x509_privkey_export_dsa_raw,gnutls_x509_privkey_get_key_id}

@node Verifying X.509 certificate paths
@subsubsection Verifying @acronym{X.509} certificate paths
@cindex verifying certificate paths

Verifying certificate paths is important in @acronym{X.509}
authentication. For this purpose the following functions are
provided.

@showfuncdesc{gnutls_x509_trust_list_add_cas}
@showfuncdesc{gnutls_x509_trust_list_add_named_crt}
@showfuncdesc{gnutls_x509_trust_list_add_crls}
@showfuncdesc{gnutls_x509_trust_list_verify_crt}
@showfuncdesc{gnutls_x509_trust_list_verify_crt2}
@showfuncdesc{gnutls_x509_trust_list_verify_named_crt}

@showfuncdesc{gnutls_x509_trust_list_add_trust_file}
@showfuncdesc{gnutls_x509_trust_list_add_trust_mem}
@showfuncdesc{gnutls_x509_trust_list_add_system_trust}

The verification function will verify a given certificate chain against a list of certificate
authorities and certificate revocation lists, and output
a bit-wise OR of elements of the @code{gnutls_@-certificate_@-status_t} 
enumeration shown in @ref{gnutls_certificate_status_t}. The @code{GNUTLS_@-CERT_@-INVALID} flag
is always set on a verification error and more detailed flags will also be set when appropriate.

@showenumdesc{gnutls_certificate_status_t,The @code{gnutls_@-certificate_@-status_t} enumeration.}

An example of certificate verification is shown in @ref{ex-verify2}.
It is also possible to have a set of certificates that
are trusted for a particular server but not to authorize other certificates.
This purpose is served by the functions @funcref{gnutls_x509_trust_list_add_named_crt} and @funcref{gnutls_x509_trust_list_verify_named_crt}.

@node Verifying a certificate in the context of TLS session
@subsubsection Verifying a certificate in the context of TLS session
@cindex verifying certificate paths
@tindex gnutls_certificate_verify_flags

When operating in the context of a TLS session, the trusted certificate
authority list may also be set using:
@showfuncD{gnutls_certificate_set_x509_trust_file,gnutls_certificate_set_x509_trust_dir,gnutls_certificate_set_x509_crl_file,gnutls_certificate_set_x509_system_trust}

These functions allow the specification of the trusted certificate authorities, either
via a file, a directory or use the system-specified certificate authorities. 
Unless the authorities are application specific, it is generally recommended
to use the system trust storage (see @funcref{gnutls_certificate_set_x509_system_trust}).

Unlike the previous section it is not required to setup a trusted list, and there
are two approaches to verify the peer's certificate and identity.
The recommended in GnuTLS 3.5.0 and later is via the @funcref{gnutls_session_set_verify_cert},
but for older GnuTLS versions you may use an explicit callback set via
@funcref{gnutls_certificate_set_verify_function} and then utilize
@funcref{gnutls_certificate_verify_peers3} for verification.
The reported verification status is identical to the verification functions described 
in the previous section.

Note that in certain cases it is required to check the marked purpose of
the end certificate (e.g. @code{GNUTLS_KP_TLS_WWW_SERVER}); in these cases
the more advanced @funcref{gnutls_session_set_verify_cert2} and
@funcref{gnutls_certificate_verify_peers} should be used instead.

There is also the possibility to pass some input to the verification
functions in the form of flags. For @funcref{gnutls_x509_trust_list_verify_crt2} the
flags are passed directly, but for
@funcref{gnutls_certificate_verify_peers3}, the flags are set using
@funcref{gnutls_certificate_set_verify_flags}.  All the available
flags are part of the enumeration
@code{gnutls_@-certificate_@-verify_@-flags} shown in @ref{gnutls_certificate_verify_flags}.

@showenumdesc{gnutls_certificate_verify_flags,The @code{gnutls_@-certificate_@-verify_@-flags} enumeration.}

@node Verification using PKCS11
@subsubsection Verifying a certificate using PKCS #11
@cindex verifying certificate with pkcs11

Some systems provide a system wide trusted certificate storage accessible using
the PKCS #11 API. That is, the trusted certificates are queried and accessed using the
PKCS #11 API, and trusted certificate properties, such as purpose, are marked using
attached extensions. One example is the p11-kit trust module@footnote{see @url{http://p11-glue.freedesktop.org/trust-module.html}.}.

These special PKCS #11 modules can be used for GnuTLS certificate verification if marked as trust 
policy modules, i.e., with @code{trust-policy: yes} in the p11-kit module file.
The way to use them is by specifying to the file verification function (e.g., @funcref{gnutls_certificate_set_x509_trust_file}),
a pkcs11 URL, or simply @code{pkcs11:} to use all the marked with trust policy modules.

The trust modules of p11-kit assign a purpose to trusted authorities using the extended
key usage object identifiers. The common purposes are shown in @ref{tab:purposes}. Note
that typically according to @xcite{RFC5280} the extended key usage object identifiers apply to end certificates. Their
application to CA certificates is an extension used by the trust modules.

@float Table,tab:purposes
@multitable @columnfractions .2 .2 .6

@headitem Purpose @tab OID @tab Description

@item GNUTLS_KP_TLS_WWW_SERVER @tab
1.3.6.1.5.5.7.3.1 @tab
The certificate is to be used for TLS WWW authentication. When in a CA certificate, it
indicates that the CA is allowed to sign certificates for TLS WWW authentication.

@item GNUTLS_KP_TLS_WWW_CLIENT @tab
1.3.6.1.5.5.7.3.2 @tab
The certificate is to be used for TLS WWW client authentication. When in a CA certificate, it
indicates that the CA is allowed to sign certificates for TLS WWW client authentication.

@item GNUTLS_KP_CODE_SIGNING @tab
1.3.6.1.5.5.7.3.3 @tab
The certificate is to be used for code signing. When in a CA certificate, it
indicates that the CA is allowed to sign certificates for code signing.

@item GNUTLS_KP_EMAIL_PROTECTION @tab
1.3.6.1.5.5.7.3.4 @tab
The certificate is to be used for email protection. When in a CA certificate, it
indicates that the CA is allowed to sign certificates for email users.

@item GNUTLS_KP_OCSP_SIGNING @tab
1.3.6.1.5.5.7.3.9 @tab
The certificate is to be used for signing OCSP responses. When in a CA certificate, it
indicates that the CA is allowed to sign certificates which sign OCSP reponses.

@item GNUTLS_KP_ANY @tab
2.5.29.37.0 @tab
The certificate is to be used for any purpose. When in a CA certificate, it
indicates that the CA is allowed to sign any kind of certificates.

@end multitable
@caption{Key purpose object identifiers.}
@end float

With such modules, it is recommended to use the verification functions @funcref{gnutls_x509_trust_list_verify_crt2},
or @funcref{gnutls_certificate_verify_peers}, which allow to explicitly specify the key purpose. The
other verification functions which do not allow setting a purpose, would operate as if
@code{GNUTLS_KP_TLS_WWW_SERVER} was requested from the trusted authorities.

@node OpenPGP certificates
@subsection @acronym{OpenPGP} certificates
@cindex OpenPGP certificates

Previous versions of GnuTLS supported limited @acronym{OpenPGP} key
authentication. That functionality has been deprecated and although
is still made available, it is no longer recommended to use.
The reason is that, supporting alternative authentication
methods, when X.509 and PKIX were new and not well established, seemed like a
good idea, in today's internet X.509 is unquestionably the main
container for certificates. As such supporting more options with no clear
use-cases, is a distraction that consumes considerable resources for
improving and testing. For that we have decided to mark this functionality
as deprecated, and will be dropped in one of the next GnuTLS major releases.


@node Advanced certificate verification
@subsection Advanced certificate verification
@cindex Certificate verification

The verification of X.509 certificates in the HTTPS and other Internet protocols is typically 
done by loading a trusted list of commercial Certificate Authorities
(see @funcref{gnutls_certificate_set_x509_system_trust}), and using them as trusted anchors.
However, there are several examples (eg. the Diginotar incident) where one of these
authorities was compromised. This risk can be mitigated by using in addition to CA certificate verification,
other verification methods. In this section we list the available in GnuTLS methods.

@menu
* Verifying a certificate using trust on first use authentication::
* Verifying a certificate using DANE::
@end menu

@node Verifying a certificate using trust on first use authentication
@subsubsection Verifying a certificate using trust on first use authentication
@cindex verifying certificate paths
@cindex SSH-style authentication
@cindex Trust on first use
@cindex Key pinning

It is possible to use a trust on first use (TOFU) authentication 
method in GnuTLS. That is the concept used by the SSH programs, where the 
public key of the peer is not verified, or verified in an out-of-bound way,
but subsequent connections to the same peer require the public key to 
remain the same.  Such a system in combination with the typical CA 
verification of a certificate, and OCSP revocation checks,
can help to provide multiple factor verification, where a single point of
failure is not enough to compromise the system. For example a server compromise
may be detected using OCSP, and a CA compromise can be detected using
the trust on first use method.
Such a hybrid system with X.509 and trust on first use authentication is 
shown in @ref{Simple client example with SSH-style certificate verification}.

See @ref{Certificate verification} on how to use the available functionality.

@node Verifying a certificate using DANE
@subsubsection Verifying a certificate using DANE (DNSSEC)
@cindex verifying certificate paths
@cindex DANE
@cindex DNSSEC

The DANE protocol is a protocol that can be used to verify TLS certificates
using the DNS (or better DNSSEC) protocols. The DNS security extensions (DNSSEC)
provide an alternative public key infrastructure to the commercial CAs that
are typically used to sign TLS certificates. The DANE protocol takes advantage
of the DNSSEC infrastructure to verify TLS certificates. This can be 
in addition to the verification by CA infrastructure or 
may even replace it where DNSSEC is fully deployed. Note however, that DNSSEC deployment is
fairly new and it would be better to use it as an additional verification
method rather than the only one.

The DANE functionality is provided by the @code{libgnutls-dane} library that is shipped
with GnuTLS and the function prototypes are in @code{gnutls/dane.h}. 
See @ref{Certificate verification} for information on how to use the library.

Note however, that the DANE RFC mandates the verification methods
one should use in addition to the validation via DNSSEC TLSA entries.
GnuTLS doesn't follow that RFC requirement, and the term DANE verification
in this manual refers to the TLSA entry verification. In GnuTLS any 
other verification methods can be used (e.g., PKIX or TOFU) on top of
DANE.

@node Digital signatures
@subsection Digital signatures
@cindex digital signatures

In this section we will provide some information about digital
signatures, how they work, and give the rationale for disabling some
of the algorithms used.

Digital signatures work by using somebody's secret key to sign some
arbitrary data.  Then anybody else could use the public key of that
person to verify the signature.  Since the data may be arbitrary it is
not suitable input to a cryptographic digital signature algorithm. For
this reason and also for performance cryptographic hash algorithms are
used to preprocess the input to the signature algorithm. This works as
long as it is difficult enough to generate two different messages with
the same hash algorithm output. In that case the same signature could
be used as a proof for both messages. Nobody wants to sign an innocent
message of donating 1 euro to Greenpeace and find out that they
donated 1.000.000 euros to Bad Inc.

For a hash algorithm to be called cryptographic the following three
requirements must hold:

@enumerate
@item Preimage resistance.
That means the algorithm must be one way and given the output of the
hash function @math{H(x)}, it is impossible to calculate @math{x}.

@item 2nd preimage resistance.
That means that given a pair @math{x,y} with @math{y=H(x)} it is
impossible to calculate an @math{x'} such that @math{y=H(x')}.

@item Collision resistance.
That means that it is impossible to calculate random @math{x} and
@math{x'} such @math{H(x')=H(x)}.
@end enumerate

The last two requirements in the list are the most important in
digital signatures. These protect against somebody who would like to
generate two messages with the same hash output. When an algorithm is
considered broken usually it means that the Collision resistance of
the algorithm is less than brute force. Using the birthday paradox the
brute force attack takes
@iftex
@math{2^{(\rm{hash\ size}) / 2}}
@end iftex
@ifnottex
@math{2^{((hash size) / 2)}}
@end ifnottex
operations. Today colliding certificates using the MD5 hash algorithm
have been generated as shown in @xcite{WEGER}.

There has been cryptographic results for the SHA-1 hash algorithms as
well, although they are not yet critical.  Before 2004, MD5 had a
presumed collision strength of @math{2^{64}}, but it has been showed
to have a collision strength well under @math{2^{50}}.  As of November
2005, it is believed that SHA-1's collision strength is around
@math{2^{63}}.  We consider this sufficiently hard so that we still
support SHA-1.  We anticipate that SHA-256/386/512 will be used in
publicly-distributed certificates in the future.  When @math{2^{63}}
can be considered too weak compared to the computer power available
sometime in the future, SHA-1 will be disabled as well.  The collision
attacks on SHA-1 may also get better, given the new interest in tools
for creating them.

@subsubsection Trading security for interoperability

If you connect to a server and use GnuTLS' functions to verify the
certificate chain, and get a @code{GNUTLS_CERT_INSECURE_ALGORITHM}
validation error (see @ref{Verifying X.509 certificate paths}), it means
that somewhere in the certificate chain there is a certificate signed
using @code{RSA-MD2} or @code{RSA-MD5}.  These two digital signature
algorithms are considered broken, so GnuTLS fails verifying
the certificate.  In some situations, it may be useful to be
able to verify the certificate chain anyway, assuming an attacker did
not utilize the fact that these signatures algorithms are broken.
This section will give help on how to achieve that.

It is important to know that you do not have to enable any of
the flags discussed here to be able to use trusted root CA
certificates self-signed using @code{RSA-MD2} or @code{RSA-MD5}. The
certificates in the trusted list are considered trusted irrespective
of the signature.

If you are using @funcref{gnutls_certificate_verify_peers3} to verify the
certificate chain, you can call
@funcref{gnutls_certificate_set_verify_flags} with the flags:
@itemize
@item @code{GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD2}
@item @code{GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5}
@item @code{GNUTLS_VERIFY_ALLOW_SIGN_WITH_SHA1}
@item @code{GNUTLS_VERIFY_ALLOW_BROKEN}
@end itemize
as in the following example:

@example
  gnutls_certificate_set_verify_flags (x509cred,
                                       GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5);
@end example

This will signal the verifier algorithm to enable @code{RSA-MD5} when
verifying the certificates.

If you are using @funcref{gnutls_x509_crt_verify} or
@funcref{gnutls_x509_crt_list_verify}, you can pass the
@code{GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5} parameter directly in the
@code{flags} parameter.

If you are using these flags, it may also be a good idea to warn the
user when verification failure occur for this reason.  The simplest is
to not use the flags by default, and only fall back to using them
after warning the user.  If you wish to inspect the certificate chain
yourself, you can use @funcref{gnutls_certificate_get_peers} to extract
the raw server's certificate chain, @funcref{gnutls_x509_crt_list_import} to parse each of the certificates, and
then @funcref{gnutls_x509_crt_get_signature_algorithm} to find out the
signing algorithm used for each certificate.  If any of the
intermediary certificates are using @code{GNUTLS_SIGN_RSA_MD2} or
@code{GNUTLS_SIGN_RSA_MD5}, you could present a warning.