@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 verify the @acronym{X.509} certificates as in @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:: * Raw public-keys:: * 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{https://p11-glue.github.io/p11-glue/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 responses. @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 Raw public-keys @subsection Raw public-keys @cindex Raw public-keys There are situations in which a rather large certificate / certificate chain is undesirable or impractical. An example could be a resource constrained sensor network in which you do want to use authentication of and encryption between your devices but where your devices lack loads of memory or processing power. Furthermore, there are situations in which you don't want to or can't rely on a PKIX. TLS is, next to a PKIX environment, also commonly used with self-signed certificates in smaller deployments where the self-signed certificates are distributed to all involved protocol endpoints out-of-band. This practice does, however, still require the overhead of the certificate generation even though none of the information found in the certificate is actually used. With raw public-keys, only a subset of the information found in typical certificates is utilized: namely, the SubjectPublicKeyInfo structure (in ASN.1 format) of a PKIX certificate that carries the parameters necessary to describe the public-key. Other parameters found in PKIX certificates are omitted. By omitting various certificate-related structures, the resulting raw public-key is kept fairly small in comparison to the original certificate, and the code to process the keys can be simpler. It should be noted however, that the authenticity of these raw keys must be verified by an out-of-band mechanism or something like @acronym{TOFU}. @menu * Importing raw public-keys:: @end menu @node Importing raw public-keys @subsubsection Importing raw public-keys Raw public-keys and their private counterparts can best be handled by using the abstract types @code{gnutls_pubkey_t} and @code{gnutls_privkey_t} respectively. To learn how to use these see @ref{Abstract key types}. @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{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.