path: root/doc/x509.txt

.. _x509_certificates:

Certificate Handling
=================================

A certificate is a binding between some identifying information
(called a *subject*) and a public key. This binding is asserted by a
signature on the certificate, which is placed there by some authority
(the *issuer*) that at least claims that it knows the subject named in
the certificate really "owns" the private key corresponding to the
public key in the certificate.

The major certificate format in use today is X.509v3, designed by ISO
and further hacked on by dozens (hundreds?) of other organizations.

When working with certificates, the main class to remember is
``X509_Certificate``. You can read an object of this type, but you can't create
one on the fly; a CA object is necessary for making a new certificate. So for
the most part, you only have to worry about reading them in, verifying the
signatures, and getting the bits of data in them (most commonly the public key,
and the information about the user of that key). An X.509v3 certificate can
contain a literally infinite number of items related to all kinds of
things. Botan doesn't support a lot of them, because nobody uses them and
they're an impossible mess to work with. This section only documents the most
commonly used ones of the ones that are supported; for the rest, read
``x509cert.h`` and ``asn1_obj.h`` (which has the definitions of various common
ASN.1 constructs used in X.509).

So what's in an X.509 certificate?
-----------------------------------

Obviously, you want to be able to get the public key. This is achieved by
calling the member function ``subject_public_key``, which will return a
``Public_Key``\*. As to what to do with this, read about ``load_key`` in
:ref:`serializing_public_keys`. In the general case, this could be any kind of
public key, though 99% of the time it will be an RSA key. However,
Diffie-Hellman, DSA, and ECDSA keys are also supported, so be careful about how
you treat this. It is also a wise idea to examine the value returned by
``constraints``, to see what uses the public key is approved for.

The second major piece of information you'll want is the name/email/etc of the
person to whom this certificate is assigned. Here is where things get a little
nasty. X.509v3 has two (well, mostly just two...) different places where you
can stick information about the user: the *subject* field, and in an extension
called *subjectAlternativeName*. The *subject* field is supposed to only
included the following information: country, organization, an organizational
sub-unit name, and a so-called common name. The common name is usually the name
of the person, or it could be a title associated with a position of some sort
in the organization. It may also include fields for state/province and
locality. What a locality is, nobody knows, but it's usually given as a city
name.

Botan doesn't currently support any of the Unicode variants used in ASN.1
(UTF-8, UCS-2, and UCS-4), any of which could be used for the fields in the
DN. This could be problematic, particularly in Asia and other areas where
non-ASCII characters are needed for most names. The UTF-8 and UCS-2 string
types *are* accepted (in fact, UTF-8 is used when encoding much of the time),
but if any of the characters included in the string are not in ISO 8859-1 (ie 0
... 255), an exception will get thrown. Currently the ``ASN1_String`` type
holds its data as ISO 8859-1 internally (regardless of local character set);
this would have to be changed to hold UCS-2 or UCS-4 in order to support
Unicode (also, many interfaces in the X.509 code would have to accept or return
a ``std::wstring`` instead of a ``std::string``).

Like the distinguished names, subject alternative names can contain a lot of
things that Botan will flat out ignore (most of which you would likely never
want to use). However, there are three very useful pieces of information that
this extension might hold: an email address (mailbox@example.com), a DNS name
(somehost.example.com), or a URI (http://www.example.com).

So, how to get the information? Call ``subject_info`` with the name of the
piece of information you want, and it will return a ``std::string`` that is
either empty (signifying that the certificate doesn't have this information),
or has the information requested. There are several names for each possible
item, but the most easily readable ones are: "Name", "Country", "Organization",
"Organizational Unit", "Locality", "State", "RFC822", "URI", and "DNS". These
values are returned as a ``std::string``.

You can also get information about the issuer of the certificate in the same
way, using ``issuer_info``.

X.509v3 Extensions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

X.509v3 specifies a large number of possible extensions. Botan supports some,
but by no means all of them. This section lists which ones are supported, and
notes areas where there may be problems with the handling.

 - Key Usage and Extended Key Usage: No problems known.

 - Basic Constraints: No problems known. The default for a v1/v2 certificate is
   assume it's a CA if and only if the option "x509/default_to_ca" is set. A v3
   certificate is marked as a CA if (and only if) the basic constraints
   extension is present and set for a CA cert.

 - Subject Alternative Names: Only the "rfc822Name", "dNSName", and
   "uniformResourceIdentifier" fields will be stored; all others are
   ignored.

 - Issuer Alternative Names: Same restrictions as the Subject
   Alternative Names extension. New certificates generated by Botan
   never include the issuer alternative name.

 - Authority Key Identifier: Only the version using KeyIdentifier is
   supported. If the GeneralNames version is used and the extension is
   critical, an exception is thrown. If both the KeyIdentifier and GeneralNames
   versions are present, then the KeyIdentifier will be used, and the
   GeneralNames ignored.

 - Subject Key Identifier: No problems known.

Revocation Lists
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It will occasionally happen that a certificate must be revoked before its
expiration date. Examples of this happening include the private key being
compromised, or the user to which it has been assigned leaving an
organization. Certificate revocation lists are an answer to this problem
(though online certificate validation techniques are starting to become
somewhat more popular). Every once in a while the CA will release a new CRL,
listing all certificates that have been revoked. Also included is various
pieces of information like what time a particular certificate was revoked, and
for what reason. In most systems, it is wise to support some form of
certificate revocation, and CRLs handle this easily.

For most users, processing a CRL is quite easy. All you have to do is call the
constructor, which will take a filename (or a ``DataSource&``). The CRLs
can either be in raw BER/DER, or in PEM format; the constructor will figure out
which format without any extra information. For example::

   X509_CRL crl1("crl1.der");

   DataSource_Stream in("crl2.pem");
   X509_CRL crl2(in);

After that, pass the ``X509_CRL`` object to a ``X509_Store`` object
with

.. cpp:function:: X509_Code X509_Store::add_crl(const X509_CRL& crl)

and all future verifications will take into account the certificates
listed, assuming ``add_crl`` returns ``VERIFIED``. If it doesn't
return ``VERIFIED``, then the return value is an error code signifying
that the CRL could not be processed due to some problem (which could
be something like the issuing certificate could not being found, an
invalid signature, or the CRL having some format problem). For more
about the ``X509_Store`` API, read :ref:`x509_store`.

Reading Certificates
---------------------------------

``X509_Certificate`` has two constructors, each of which takes a source of
data; a filename to read, and a ``DataSource&``::

   X509_Certificate cert1("cert1.pem");

   /* This file contains two certificates, concatenated */
   DataSource_Stream in("certs2_and_3.pem");

   X509_Certificate cert2(in); // read the first cert
   X509_Certificate cert3(in); // read the second cert

.. _x509_store:

Storing and Using Certificates
---------------------------------

If you read a certificate, you probably want to verify the signature on
it. However, consider that to do so, we may have to verify the signature on the
certificate that we used to verify the first certificate, and on and on until
we hit the top of the certificate tree somewhere. It would be a might huge pain
to have to handle all of that manually in every application, so there is
something that does it for you: ``X509_Store``.

The basic operations are: put certificates and CRLs into it, search for
certificates, and attempt to verify certificates. That's about it. In the
future, there will be support for online retrieval of certificates and CRLs (eg
with the HTTP cert-store interface currently under consideration by PKIX).

Adding Certificates
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can add new certificates to a certificate store using any of these
functions:

.. cpp:function:: void X509_Store::add_cert(const X509_Certificate& cert, \
   bool trusted = false)

.. cpp:function:: void X509_Store::add_cert(DataSource& source)

.. cpp:function:: void X509_Store::add_trusted_certs(DataSource& source)

The versions that take a ``DataSource&`` will add all the certificates
that it can find in that source.

All of them add the cert(s) to the store. The "trusted" certificates are the
ones that you are willing to trust for certification purposes. For example, say
your application is working with certificates that are owned by employees of
some company, and all of their certificates are signed by the company CA, whose
certificate is in turned signed by a commercial root CA. What you would then do
is include the certificate of the commercial CA with your application, and read
it in as a trusted certificate. From there, you could verify the company CA's
certificate, and then use that to verify the end user's certificates. Only
self-signed certificates may be considered trusted.

Adding CRLs
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. cpp:function:: X509_Code X509_Store::add_crl(const X509_CRL& crl)

This will process the CRL and mark the revoked certificates. This will
also work if a revoked certificate is added to the store sometime
after the CRL is processed. The function can return an error code
(listed later), or will return ``VERIFIED`` if everything completed
successfully.

Storing Certificates
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can output a set of certificates by calling ``PEM_encode``, which
will return a ``std::string`` containing each of the certificates in
the store, PEM encoded and concatenated. This simple format can easily
be read by both Botan and other libraries/applications.

Certificate Stores
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

An object of type ``Certificate_Store`` is a generalized interface to an
external source for certificates (and CRLs). Examples of such a store would be
one that looked up the certificates in a SQL database, or by contacting a CGI
script running on a HTTP server. There are currently three mechanisms for
looking up a certificate, and one for retrieving CRLs. By default, most of
these mechanisms will return an empty ``std::vector`` of
``X509_Certificate``. This storage mechanism is *only* queried when doing
certificate validation: it allows you to distribute only the root key with an
application, and let some online method handle getting all the other
certificates that are needed to validate an end entity certificate. In
particular, the search routines will not attempt to access the external
database.

The three certificate lookup methods are ``by_SKID`` (Subject Key Identifier),
``by_name`` (the CommonName DN entry), and ``by_email`` (stored in either the
distinguished name, or in a subjectAlternativeName extension). The name and
email versions take a ``std::string``, while the SKID version takes a
``SecureVector<byte>`` containing the subject key identifier in raw binary. You
can choose not to implement ``by_name`` or ``by_email``, but ``by_SKID`` is
mandatory to implement, and, currently, is the only version that is used by
``X509_Store``.

Finally, there is a method for finding CRLs, called ``get_crls_for``, that
takes an ``X509_Certificate`` object, and returns a ``std::vector`` of
``X509_CRL``. While normally there will be only one CRL, the use of the vector
makes it easy to return no CRLs (eg, if the certificate store doesn't support
retrieving them), or return multiple ones (for example, if the certificate
store can't determine precisely which key was used to sign the
certificate). Implementing the function is optional, and by default will return
no CRLs. If it is available, it will be used by ``X509_CRL``.

As for using such a store, you have to tell ``X509_Store`` about it
with

.. cpp:function:: void X509_Store::add_new_certstore(Certificate_Store* new_store)

  The store object will be owned by (and deleted by) ``X509_Store``,
  so make sure to allocate it with ``new``.

Verifying Certificates
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Verifying a certificate requires that we build up a chain of trust, starting
from the root (usually a commercial CA), down through some number of
intermediate CAs, and finally reaching the actual certificate in
question. Thus, to verify, we actually have to have all those certificates on
hand (or at the very least, know where we can get the ones we need).

The class which handles both storing certificates, and verifying them,
is

.. cpp:class:: X509_Store

.. cpp:type:: Cert_Usage

    Can be any of:
       * ``ANY`` (any usage is OK)
       * ``CRL_SIGNING``
       * ``TLS_SERVER`` (for SSL/TLS server authentication)
       * ``TLS_CLIENT`` (for SSL/TLS client authentication)
       * ``CODE_SIGNING``
       * ``EMAIL_PROTECTION`` (usually this means S/MIME)
       * ``TIME_STAMPING`` (in theory any time stamp application,
         usually IETF PKIX's Time Stamp Protocol)

.. cpp:function:: X509_Code validate_cert(const X509_Certificate& cert, \
                                          Cert_Usage usage = ANY)

    Return ``VERIFIED`` if the certificate can safely be considered
    valid for the usage(s) described by *usage*, and an error code if
    it is not.

First, how does :cpp:class`X509_Store::validate_cert` know if a certificate is
valid? A certificate is valid if both of the following hold: a) the signature
in the certificate can be verified using the public key in the issuer's
certificate, and b) the issuer's certificate is a valid CA certificate. Note
that this definition is recursive. We get out of this by "bottoming out" when
we reach a certificate that we consider trusted. In general this will either be
a commercial root CA, or an organization or application specific CA.

There are a few other restrictions (validity periods, key usage restrictions,
etc), but the above summarizes the major points of the validation algorithm. In
theory, Botan implements the certificate path validation algorithm given in RFC
2459, but in practice it does not (yet), because we don't support the X.509v3
policy or name constraint extensions.

The default ``ANY`` does not mean valid for any use, it means "is valid for
some usage". This is usually what you want; requiring that a random certificate
support a particular usage will likely result in a lot of failures, unless your
application is very careful to always issue certificates with the proper
extensions, and you never use certificates generated by other apps.

Return values for ``validate_cert`` (and ``add_crl``) include:

* VERIFIED: The certificate is valid for the specified use.
* INVALID_USAGE: The certificate cannot be used for the specified use.

* CANNOT_ESTABLISH_TRUST: The root certificate was not marked as trusted.

* CERT_CHAIN_TOO_LONG: The certificate chain exceeded the length
  allowed by a basicConstraints extension.

* SIGNATURE_ERROR: An invalid signature was found

* POLICY_ERROR: Some problem with the certificate policies was found.

* CERT_FORMAT_ERROR: Some format problem was found in a certificate.
* CERT_ISSUER_NOT_FOUND: The issuer of a certificate could not be found.
* CERT_NOT_YET_VALID: The certificate is not yet valid.
* CERT_HAS_EXPIRED: The certificate has expired.
* CERT_IS_REVOKED: The certificate has been revoked.
* CRL_FORMAT_ERROR: Some format problem was found in a CRL.
* CRL_ISSUER_NOT_FOUND: The issuer of a CRL could not be found.
* CRL_NOT_YET_VALID: The CRL is not yet valid.
* CRL_HAS_EXPIRED: The CRL has expired.
* CA_CERT_CANNOT_SIGN: The CA certificate found does not have an
  contain a public key that allows signature verification.
* CA_CERT_NOT_FOR_CERT_ISSUER: The CA cert found is not allowed to
  issue certificates.
* CA_CERT_NOT_FOR_CRL_ISSUER: The CA cert found is not allowed to
  issue CRLs.
* UNKNOWN_X509_ERROR: Some other error occurred.

Certificate Authorities
---------------------------------

Setting up a CA for X.509 certificates is perhaps the easiest thing to
do related to X.509. A CA is represented by the type ``X509_CA``,
which can be found in ``x509_ca.h``. A CA always needs its own
certificate, which can either be a self-signed certificate (see below
on how to create one) or one issued by another CA (see the section on
PKCS #10 requests). Creating a CA object is done by the following
constructor:

.. cpp:function:: X509_CA::X509_CA(const X509_Certificate& cert, \
   const Private_Key& key)

The private key is the private key corresponding to the public key in the
CA's certificate.

Requests for new certificates are supplied to a CA in the form on PKCS
#10 certificate requests (called a ``PKCS10_Request`` object in
Botan). These are decoded in a similar manner to
certificates/CRLs/etc. A request is vetted by humans (who somehow
verify that the name in the request corresponds to the name of the
entity who requested it), and then signed by a CA key, generating a
new certificate:

.. cpp:function:: X509_Certificate \
   X509_CA::sign_request(const PKCS10_Request& req) const

Here's an example:

.. literalinclude examples/ca.cpp

Generating CRLs
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

As mentioned previously, the ability to process CRLs is highly important in
many PKI systems. In fact, according to strict X.509 rules, you must not
validate any certificate if the appropriate CRLs are not available (though
hardly any systems are that strict). In any case, a CA should have a valid CRL
available at all times.

Of course, you might be wondering what to do if no certificates have
been revoked. Never fear; empty CRLs, which revoke nothing at all, can
be issued. To generate a new, empty CRL, just call

.. cpp:function:: X509_CRL X509_CA::new_crl(u32bit seconds_to_expiration = 0)

  This function will return a new, empty CRL. The
  ``seconds_to_expiration`` parameter is the number of seconds before
  the CRL expires. If it is set to the (default) value of zero, then a
  reasonable default (currently 7 days) will be used.

On the other hand, you may have issued a CRL before. In that case, you will
want to issue a new CRL that contains all previously revoked
certificates, along with any new ones. This is done by calling

.. cpp:function:: X509_CRL X509_CA::update_crl(const X509_CRL& old_crl, \
   std::vector<CRL_Entry> new_revoked, size_t seconds_to_expiration = 0)

  Where ``X509_CRL`` is the last CRL this CA issued, and
  ``new_revoked`` is a list of any newly revoked certificates. The
  function returns a new ``X509_CRL`` to make available for
  clients.

The ``CRL_Entry`` type is a structure that contains, at a minimum, the serial
number of the revoked certificate. As serial numbers are never repeated, the
pairing of an issuer and a serial number (should) distinctly identify any
certificate. In this case, we represent the serial number as a
``SecureVector<byte>`` called ``serial``. There are two additional (optional)
values, an enumeration called ``CRL_Code`` that specifies the reason for
revocation (``reason``), and an object that represents the time that the
certificate became invalid (if this information is known).

If you wish to remove an old entry from the CRL, insert a new entry for the
same cert, with a ``reason`` code of ``DELETE_CRL_ENTRY``. For example, if a
revoked certificate has expired 'normally', there is no reason to continue to
explicitly revoke it, since clients will reject the cert as expired in any
case.

Self-Signed Certificates
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Generating a new self-signed certificate can often be useful, for example when
setting up a new root CA, or for use in email applications. The library
provides a utility function for this:

.. cpp:function:: X509_Certificate create_self_signed_cert( \
   const X509_Cert_Options& opts, const Private_Key& key)

   Where *key* is the private key you wish to use (the public key,
   used in the certificate itself, is extracted from the private key),
   and *opts* is an structure that has various bits of information
   that will be used in creating the certificate (this structure, and
   its use, is discussed below).

An example:

.. literalinclude:: examples/self_sig.cpp

Creating PKCS #10 Requests
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Also in ``x509self.h``, there is a function for generating new PKCS #10
certificate requests:

.. cpp:function:: PKCS10_Request create_cert_req( \
   const X509_Cert_Options& opts, const Private_Key& key)

This function acts quite similarly to
:cpp:func:`create_self_signed_cert`, except it instead returns a PKCS
#10 certificate request. After creating it, one would typically
transmit it to a CA, who signs it and returns a freshly minted X.509
certificate.

An example:

.. literalinclude:: examples/pkcs10.cpp

Certificate Options
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

What is this ``X509_Cert_Options`` thing we've been passing around?  It's a
class representing a bunch of information that will end up being stored into
the certificate. This information comes in 3 major flavors: information about
the subject (CA or end-user), the validity period of the certificate, and
restrictions on the usage of the certificate.

First and foremost is a number of ``std::string`` members, which contains
various bits of information about the user: ``common_name``, ``serial_number``,
``country``, ``organization``, ``org_unit``, ``locality``, ``state``,
``email``, ``dns_name``, and ``uri``. As many of these as possible should be
filled it (especially an email address), though the only required ones are
``common_name`` and ``country``.

There is another value that is only useful when creating a PKCS #10 request,
which is called ``challenge``. This is a challenge password, which you can
later use to request certificate revocation (*if* the CA supports doing
revocations in this manner).

Then there is the validity period; these are set with ``not_before`` and
``not_after``. Both of these functions also take a ``std::string``, which
specifies when the certificate should start being valid, and when it should
stop being valid. If you don't set the starting validity period, it will
automatically choose the current time. If you don't set the ending time, it
will choose the starting time plus a default time period. The arguments to
these functions specify the time in the following format: "2002/11/27
1:50:14". The time is in 24-hour format, and the date is encoded as
year/month/day. The date must be specified, but you can omit the time or
trailing parts of it, for example "2002/11/27 1:50" or "2002/11/27".

Lastly, you can set constraints on a key. The one you're mostly likely to want
to use is to create (or request) a CA certificate, which can be done by calling
the member function ``CA_key``. This should only be used when needed.

Other constraints can be set by calling the member functions
``add_constraints`` and ``add_ex_constraints``. The first takes a
``Key_Constraints`` value, and replaces any previously set value. If no value
is set, then the certificate key is marked as being valid for any usage.  You
can set it to any of the following (for more than one usage, OR them together):
``DIGITAL_SIGNATURE``, ``NON_REPUDIATION``, ``KEY_ENCIPHERMENT``,
``DATA_ENCIPHERMENT``, ``KEY_AGREEMENT``, ``KEY_CERT_SIGN``, ``CRL_SIGN``,
``ENCIPHER_ONLY``, ``DECIPHER_ONLY``. Many of these have quite special
semantics, so you should either consult the appropriate standards document
(such as RFC 3280), or just not call ``add_constraints``, in which case the
appropriate values will be chosen for you.

The second function, ``add_ex_constraints``, allows you to specify an OID that
has some meaning with regards to restricting the key to particular usages. You
can, if you wish, specify any OID you like, but there is a set of standard ones
that other applications will be able to understand. These are the ones
specified by the PKIX standard, and are named "PKIX.ServerAuth" (for TLS server
authentication), "PKIX.ClientAuth" (for TLS client authentication),
"PKIX.CodeSigning", "PKIX.EmailProtection" (most likely for use with S/MIME),
"PKIX.IPsecUser", "PKIX.IPsecTunnel", "PKIX.IPsecEndSystem", and
"PKIX.TimeStamping". You can call "add_ex_constraints" any number of times -
each new OID will be added to the list to include in the certificate.