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Diffstat (limited to 'doc/cha-cert-auth.texi')
| -rw-r--r-- | doc/cha-cert-auth.texi | 145 |
1 files changed, 12 insertions, 133 deletions
diff --git a/doc/cha-cert-auth.texi b/doc/cha-cert-auth.texi index 60faccb28b..6145fdf851 100644 --- a/doc/cha-cert-auth.texi +++ b/doc/cha-cert-auth.texi @@ -4,9 +4,8 @@ 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} supports both -@acronym{X.509} certificates @xcite{PKIX} and @acronym{OpenPGP} -certificates using a common API. +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}. @@ -465,137 +464,17 @@ other verification functions which do not allow setting a purpose, would operate @subsection @acronym{OpenPGP} certificates @cindex OpenPGP certificates -The @acronym{OpenPGP} key authentication relies on a distributed trust -model, called the ``web of trust''. The ``web of trust'' uses a -decentralized system of trusted introducers, which are the same as a -CA. @acronym{OpenPGP} allows anyone to sign anyone else's public -key. When Alice signs Bob's key, she is introducing Bob's key to -anyone who trusts Alice. If someone trusts Alice to introduce keys, -then Alice is a trusted introducer in the mind of that observer. -For example in @ref{fig-openpgp}, David trusts Alice to be an introducer and Alice -signed Bob's key thus Dave trusts Bob's key to be the real one. - -@float Figure,fig-openpgp -@image{gnutls-pgp,8cm} -@caption{An example of the OpenPGP trust model.} -@end float - -There are some key points that are important in that model. In the -example Alice has to sign Bob's key, only if she is sure that the key -belongs to Bob. Otherwise she may also make Dave falsely believe that -this is Bob's key. Dave has also the responsibility to know who to -trust. This model is similar to real life relations. - -Just see how Charlie behaves in the previous example. Although he has -signed Bob's key - because he knows, somehow, that it belongs to Bob - -he does not trust Bob to be an introducer. Charlie decided to trust -only Kevin, for some reason. A reason could be that Bob is lazy -enough, and signs other people's keys without being sure that they -belong to the actual owner. - -@float Table,tab:openpgp-certificate -@multitable @columnfractions .2 .7 - -@headitem Field @tab Description - -@item version @tab -The field that indicates the version of the OpenPGP structure. - -@item user ID @tab -An RFC 2822 string that identifies the owner of the key. There may be -multiple user identifiers in a key. - -@item public key @tab -The main public key of the certificate. - -@item expiration @tab -The expiration time of the main public key. - -@item public subkey @tab -An additional public key of the certificate. There may be multiple subkeys -in a certificate. - -@item public subkey expiration @tab -The expiration time of the subkey. - -@end multitable -@caption{OpenPGP certificate fields.} -@end float - - -@subsubsection @acronym{OpenPGP} certificate structure - -In @acronym{GnuTLS} the @acronym{OpenPGP} certificate structures -@xcite{RFC2440} are handled using the @code{gnutls_openpgp_crt_t} type. -A typical certificate contains the user ID, which is an RFC 2822 -mail and name address, a public key, possibly a number of additional -public keys (called subkeys), and a number of signatures. The various -fields are shown in @ref{tab:openpgp-certificate}. - -The additional subkeys may provide key for various different purposes, -e.g. one key to encrypt mail, and another to sign a TLS key exchange. -Each subkey is identified by a unique key ID. -The keys that are to be used in a TLS key exchange that requires -signatures are called authentication keys in the OpenPGP jargon. -The mapping of TLS key exchange methods to public keys is shown in -@ref{tab:openpgp-key-exchange}. - -@float Table,tab:openpgp-key-exchange -@multitable @columnfractions .2 .7 - -@headitem Key exchange @tab Public key requirements - -@item RSA @tab -An RSA public key that allows encryption. - -@item DHE_@-RSA @tab -An RSA public key that is marked for authentication. - -@item ECDHE_@-RSA @tab -An RSA public key that is marked for authentication. - -@item DHE_@-DSS @tab -A DSA public key that is marked for authentication. - -@end multitable -@caption{The types of (sub)keys required for the various TLS key exchange methods.} -@end float - -The corresponding private keys are stored in the -@code{gnutls_openpgp_privkey_t} type. All the prototypes for the key -handling functions can be found in @file{gnutls/openpgp.h}. - -@subsubsection Verifying an @acronym{OpenPGP} certificate - -The verification functions of @acronym{OpenPGP} keys, included in -@acronym{GnuTLS}, are simple ones, and do not use the features of the -``web of trust''. For that reason, if the verification needs are -complex, the assistance of external tools like @acronym{GnuPG} and -GPGME@footnote{@url{http://www.gnupg.org/related_software/gpgme/}} is -recommended. - -In GnuTLS there is a verification function for OpenPGP certificates, -the @funcref{gnutls_openpgp_crt_verify_ring}. This checks an -@acronym{OpenPGP} key against a given set of public keys (keyring) and -returns the key status. The key verification status is the same as in -@acronym{X.509} certificates, although the meaning and interpretation -are different. For example an @acronym{OpenPGP} key may be valid, if -the self signature is ok, even if no signers were found. The meaning -of verification status flags is the same as in the @acronym{X.509} certificates -(see @ref{gnutls_certificate_verify_flags}). - -@showfuncdesc{gnutls_openpgp_crt_verify_ring} - -@showfuncdesc{gnutls_openpgp_crt_verify_self} - -@subsubsection Verifying a certificate in the context of a TLS session - -Similarly with X.509 certificates, one needs to specify -the OpenPGP keyring file in the credentials structure. The certificates -in this file will be used by @funcref{gnutls_certificate_verify_peers3} -to verify the signatures in the certificate sent by the peer. +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. -@showfuncdesc{gnutls_certificate_set_openpgp_keyring_file} @node Advanced certificate verification @subsection Advanced certificate verification |