@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 is no longer

made available. The reason is that, supporting alternative authentication

methods, when X.509 and PKIX were new on the Internet 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 the library. For that we have decided to drop

this functionality completely in 3.6.0.

@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.