path: root/doc/manual/x509.rst

.. _x509_certificates:

X.509 Certificates and CRLs
=================================

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, used for instance
in the :doc:`tls` protocol. A X.509 certificate is represented by

.. cpp:class:: X509_Certificate

 .. cpp:function:: Public_Key* subject_public_key() const

     Returns the public key of the subject

 .. cpp:function:: X509_DN issuer_dn() const

     Returns the distinguished name (DN) of the certificate's issuer

 .. cpp:function:: X509_DN subject_dn() const

     Returns the distinguished name (DN) of the certificate's subject

 .. cpp:function:: std::string start_time() const

     Returns the point in time the certificate becomes valid

 .. cpp:function:: std::string end_time() const

     Returns the point in time the certificate expires

 .. cpp:function:: Extensions v3_extensions() const

     Returns all extensions of this certificate

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.
The following listing lists which X.509v3 extensions 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. A self-signed v1 certificate
   is assumed to be a CA, while 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" and raw IPv4 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.

 - Name Constraints: No problems known (though encoding is not supported).

Any unknown critical extension in a certificate will lead to an
exception during path validation.

Extensions are handled by a special class taking care of encoding
and decoding. It also supports encoding and decoding of custom extensions.
To do this, it internally keeps two lists of extensions. Different lookup
functions are provided to search them.

.. note::

  Validation of custom extensions during path validation
  is currently not supported.

.. cpp:class:: Extensions

 .. cpp:function:: void add(Certificate_Extension* extn, bool critical = false)

     Adds a new extension to the list. ``critical`` specifies whether the extension
     should be marked as critical.

 .. cpp:function:: replace(Certificate_Extension* extn, bool critical = false)

     Adds an extension to the list or replaces it, if the same
     extension was already added

 .. cpp:function:: std::unique_ptr<Certificate_Extension> get(const OID& oid) const

     Searches for an extension by OID and returns the result

 .. cpp:function:: template<typename T> \
      std::unique_ptr<T> get_raw(const OID& oid)

     Searches for an extension by OID and returns the result.
     Only the unknown extensions, that is, extensions types that are not
     listed above, are searched for by this function.

 .. cpp:function:: std::vector<std::pair<std::unique_ptr<Certificate_Extension>, bool>> extensions() const

     Returns the list of extensions together with the corresponding
     criticality flag. Only contains the supported extension types
     listed above.

 .. cpp:function:: std::map<OID, std::pair<std::vector<byte>, bool>> extensions_raw() const

     Returns the list of extensions as raw, encoded bytes
     together with the corresponding criticality flag.
     Contains all extensions, known as well as unknown extensions.

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 ``Certificate_Store`` object
with

.. cpp:function:: void Certificate_Store::add_crl(const X509_CRL& crl)

and all future verifications will take into account the certificates
listed.

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

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::shared_ptr`` 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 certificate lookup methods are ``find_cert`` (by Subject
Distinguished Name and optional Subject Key Identifier) and
``find_cert_by_pubkey_sha1`` (by SHA-1 hash of the certificate's
public key). The Subject Distinguished Name is given as a ``X509_DN``,
while the SKID parameter takes a ``std::vector<byte>`` containing
the subject key identifier in raw binary. Both lookup methods are
mandatory to implement.

Finally, there is a method for finding a CRL, called ``find_crl_for``,
that takes an ``X509_Certificate`` object, and returns a
``std::shared_ptr`` of ``X509_CRL``. The ``std::shared_ptr`` return
type makes it easy to return no CRLs by returning ``nullptr``
(eg, if the certificate store doesn't support retrieving CRLs).
Implementing the function is optional, and by default will return
``nullptr``.

Certificate stores are used in the :doc:`tls` module to store a
list of trusted certificate authorities.

In Memory Certificate Store
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The in memory certificate store keeps all objects in memory only.
Certificates can be loaded from disk initially, but also added
later.

.. cpp:class:: Certificate_Store_In_Memory

 .. cpp:function:: Certificate_Store_In_Memory(const std::string& dir)

     Attempt to parse all files in ``dir`` (including subdirectories)
     as certificates. Ignores errors.

 .. cpp:function:: Certificate_Store_In_Memory(const X509_Certificate& cert)

     Adds given certificate to the store

 .. cpp:function:: Certificate_Store_In_Memory()

     Create an empty store

 .. cpp:function:: void add_certificate(const X509_Certificate& cert)

     Add a certificate to the store

 .. cpp:function:: void add_certificate(std::shared_ptr<const X509_Certificate> cert)

     Add a certificate already in a shared_ptr to the store

 .. cpp:function:: void add_crl(const X509_CRL& crl)

     Add a certificate revocation list (CRL) to the store.

 .. cpp:function:: void add_crl(std::shared_ptr<const X509_CRL> crl)

     Add a certificate revocation list (CRL) to the store as a shared_ptr

SQL-backed Certificate Stores
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The SQL-backed certificate stores store all objects in an SQL
database. They also additionally provide private key storage
and revocation of individual certificates.

.. cpp:class:: Certificate_Store_In_SQL

 .. cpp:function:: Certificate_Store_In_SQL(const std::shared_ptr<SQL_Database> db, \
    const std::string& passwd, RandomNumberGenerator& rng, const std::string& table_prefix = "")

     Create or open an existing certificate store from an SQL database.
     The password in ``passwd`` will be used to encrypt private keys.

 .. cpp:function:: bool insert_cert(const X509_Certificate& cert)

     Inserts ``cert`` into the store. Returns `false` if the certificate is
     already known and `true` if insertion was successful.

 .. cpp:function:: remove_cert(const X509_Certificate& cert)

     Removes ``cert`` from the store. Returns `false` if the certificate could not
     be found and `true` if removal was successful.


 .. cpp:function:: std::shared_ptr<const Private_Key> find_key(const X509_Certificate&) const

     Returns the private key for "cert" or an empty shared_ptr if none was found

 .. cpp:function:: std::vector<std::shared_ptr<const X509_Certificate>> \
    find_certs_for_key(const Private_Key& key) const

     Returns all certificates for private key ``key``

 .. cpp:function:: bool insert_key(const X509_Certificate& cert, const Private_Key& key)

     Inserts ``key`` for ``cert`` into the store, returns `false` if the key is
     already known and `true` if insertion was successful.

 .. cpp:function:: void remove_key(const Private_Key& key)

     Removes ``key`` from the store

 .. cpp:function:: void revoke_cert(const X509_Certificate&, CRL_Code, \
    const X509_Time& time = X509_Time())

     Marks ``cert`` as revoked starting from ``time``

 .. cpp:function:: void affirm_cert(const X509_Certificate&)

     Reverses the revokation for ``cert``

 .. cpp:function:: std::vector<X509_CRL> generate_crls() const

     Generates CRLs for all certificates marked as revoked.
     A CRL is returned for each unique issuer DN.

Botan currently only provides one SQL-backed certificate store using
sqlite.

.. cpp:class:: Certificate_Store_In_SQLite

 .. cpp:function:: Certificate_Store_In_SQLite(const std::string& db_path, \
    const std::string& passwd, RandomNumberGenerator& rng, const std::string& table_prefix = "")

     Create or open an existing certificate store from an sqlite database file.
     The password in ``passwd`` will be used to encrypt private keys. 

Path Validation
----------------------------------------

The process of validating a certfificate chain up to a trusted root is
called `path validation`, and in botan that operation is handled by a
set of functions in ``x509path.h`` named ``x509_path_validate``:

.. cpp:function:: Path_Validation_Result \
   x509_path_validate(const X509_Certificate& end_cert, \
   const Path_Validation_Restrictions& restrictions, \
   const Certificate_Store& store, const std::string& hostname = "", \
   Usage_Type usage = Usage_Type::UNSPECIFIED, \
   std::chrono::system_clock::time_point validation_time = std::chrono::system_clock::now(), \
   std::chrono::milliseconds ocsp_timeout = std::chrono::milliseconds(0), \
   const std::vector<std::shared_ptr<const OCSP::Response>>& ocsp_resp = {})

   The last five parameters are optional. ``hostname`` specifies a hostname which is
   matched against the subject DN in ``end_cert`` according to RFC 6125.
   An empty hostname disables hostname validation.
   ``usage`` specifies key usage restrictions that are compared
   to the key usage fields in `end_cert` according to RFC 5280, if not set to
   ``UNSPECIFIED``. ``validation_time`` allows setting the time point at which all certificates
   are validated. This is really only useful for testing. The default is the
   current system clock's current time. ``ocsp_timeout`` sets the timeout for
   OCSP requests. The default of 0 disables OCSP checks alltogether.
   ``ocsp_resp`` allows adding additional OCSP responses retrieved from outside
   of the path validation.

   For the different flavours of ``x509_path_validate``, check ``x509path.h``.

The result of the validation is returned as a class:

.. cpp:class:: Path_Validation_Result

   Specifies the result of the validation

  .. cpp:function:: bool successful_validation() const

     Returns true if a certificate path from *end_cert* to a trusted
     root was found and all path validation checks passed.

  .. cpp:function:: std::string result_string() const

     Returns a descriptive string of the validation status (for
     instance "Verified", "Certificate is not yet valid", or
     "Signature error"). This is the string value of
     the `result` function below.

  .. cpp:function:: const X509_Certificate& trust_root() const

     If the validation was successful, returns the certificate which
     is acting as the trust root for *end_cert*.

   .. cpp:function:: const std::vector<X509_Certificate>& cert_path() const

     Returns the full certificate path starting with the end entity
     certificate and ending in the trust root.

  .. cpp:function:: Certificate_Status_Code result() const

     Returns the 'worst' error that occurred during validation. For
     instance, we do not want an expired certificate with an invalid
     signature to be reported to the user as being simply expired (a
     relativly innocuous and common error) when the signature isn't
     even valid.

  .. cpp:function:: const std::vector<std::set<Certificate_Status_Code>>& all_statuses() const

     For each certificate in the chain, returns a set of status which
     indicate all errors which occurred during validation. This is
     primarily useful for diagnostic purposes.

  .. cpp:function:: std::set<std::string> trusted_hashes() const

     Returns the set of all cryptographic hash functions which are
     implicitly trusted for this validation to be correct.


A ``Path_Validation_Restrictions`` is passed to the path
validator and specifies restrictions and options for the validation
step. The two constructors are:

  .. cpp:function:: Path_Validation_Restrictions(bool require_rev, \
                                                 size_t minimum_key_strength, \
                                                 bool ocsp_all_intermediates, \
                                                 const std::set<std::string>& trusted_hashes)

    If `require_rev` is true, then any path without revocation
    information (CRL or OCSP check) is rejected with the code
    `NO_REVOCATION_DATA`. The `minimum_key_strength` parameter
    specifies the minimum strength of public key signature we will
    accept is. The set of hash names `trusted_hashes` indicates which
    hash functions we'll accept for cryptographic signatures.  Any
    untrusted hash will cause the error case `UNTRUSTED_HASH`.

  .. cpp:function:: Path_Validation_Restrictions(bool require_rev = false, \
                                                 size_t minimum_key_strength = 80, \
                                                 bool ocsp_all_intermediates = false)

    A variant of the above with some convenient defaults. The current
    default `minimum_key_strength` of 80 roughly cooresponds to 1024
    bit RSA. The set of trusted hashes is set to all SHA-2 variants,
    and, if `minimum_key_strength` is less than or equal to 80, then
    SHA-1 signatures will also be accepted.

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

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, \
                                   const std::string& hash_fn, \
                                   RandomNumberGenerator& rng)

The private ``key`` is the private key corresponding to the public key in the
CA's certificate. ``hash_fn`` is the name of the hash function to use
for signing, e.g., `SHA-256`. ``rng`` is queried for random during signing.

Requests for new certificates are supplied to a CA in the form of 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, \
                         RandomNumberGenerator& rng, \
                         const X509_Time& not_before, \
                         const X509_Time& not_after)

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(RandomNumberGenerator& rng, \
                                            uint32_t next_update = 0)

  This function will return a new, empty CRL. The
  ``next_update`` 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& last_crl, \
   std::vector<CRL_Entry> new_entries, RandomNumberGenerator& rng, \
   size_t next_update = 0)

  Where ``last_crl`` is the last CRL this CA issued, and
  ``new_entries`` 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
``secure_vector<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 ``REMOVE_FROM_CRL``. 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 specialized
protocols. 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, \
   const std::string& hash_fn, RandomNumberGenerator& rng)

   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).

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, \
   const std::string& hash_fn, RandomNumberGenerator& rng)

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.

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. For special cases, you can also add custom X.509v3
extensions.

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".

Third, 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
5280), 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.

Lastly, you can add any X.509v3 extensions in the `extensions`
member. This is really only useful if you want to encode custom
extensions in the certificate. Most users probably won't need this.
Note that extensions added this way will be overwritten by an
``X509_CA`` if also added by the ``X509_CA`` itself. This currently
includes the Basic Constraints, Key Usage, Authority Key ID, Subject
Key ID, Subject Alternative Name and Extended Key Usage extension.

OCSP Requests
----------------------------------------

A client makes an OCSP request to what is termed an 'OCSP responder'. This
responder returns a signed response attesting that the certificate in question
has not been revoked. The most recent OCSP specification is as of this
writing :rfc:`6960`.

Normally OCSP validation happens automatically as part of X.509 certificate
validation, as long as OCSP is enabled (by setting a non-zero ``ocsp_timeout``
in the call to ``x509_path_validate``, or for TLS by implementing the related
``tls_verify_cert_chain_ocsp_timeout`` callback and returning a non-zero value
from that). So most applications should not need to directly manipulate OCSP
request and response objects.

For those that do, the primary ocsp interface is in ``ocsp.h``. First a request
must be formed, using information contained in the subject certificate and in
the subject's issuing certificate.

.. cpp:class:: OCSP::Request

 .. cpp:function:: OCSP::Request(const X509_Certificate& issuer_cert, \
                                 const BigInt& subject_serial)

      Create a new OCSP request

 .. cpp:function:: OCSP::Request(const X509_Certificate& issuer_cert, \
                                 const X509_Certificate& subject_cert)

      Variant of the above, using serial number from ``subject_cert``.

 .. cpp:function:: std::vector<byte> BER_encode() const

      Encode the current OCSP request as a binary string.

 .. cpp:function:: std::string base64_encode() const

      Encode the current OCSP request as a base64 string.

Then the response is parsed and validated, and if valid, can be consulted
for certificate status information.

.. cpp:class:: OCSP::Response

  .. cpp:function:: OCSP::Response(const uint8_t response_bits[], size_t response_bits_len)

     Attempts to parse ``response_bits`` as an OCSP response. Throws an
     exception if parsing fails. Note that this does not verify that the OCSP
     response is valid (ie that the signature is correct), merely that the
     ASN.1 structure matches an OCSP response.

  .. cpp:function:: Certificate_Status_Code check_signature( \
                    const std::vector<Certificate_Store*>& trust_roots, \
                    const std::vector<std::shared_ptr<const X509_Certificate>>& cert_path = {}) const

     Find the issuing certificate of the OCSP response, and check the signature.

     If possible, pass the full certificate path being validated in
     the optional ``cert_path`` argument: this additional information
     helps locate the OCSP signer's certificate in some cases. If this
     does not return ``Certificate_Status_Code::OCSP_SIGNATURE_OK``,
     then the request must not be be used further.

  .. cpp:function:: Certificate_Status_Code verify_signature(const X509_Certificate& issuing_cert) const

     If the certificate that issued the OCSP response is already known (eg,
     because in some specific application all the OCSP responses will always
     be signed by a single trusted issuer whose cert is baked into the code)
     this provides an alternate version of `check_signature`.

  .. cpp:function:: Certificate_Status_Code status_for(const X509_Certificate& issuer, \
                        const X509_Certificate& subject, \
                        std::chrono::system_clock::time_point ref_time = std::chrono::system_clock::now()) const

     Assuming the signature is valid, returns the status for the subject certificate.
     Make sure to get the ordering of the issuer and subject certificates correct.

     The ``ref_time`` is normally just the system clock, but can be used if
     validation against some other reference time is desired (such as for
     testing, to verify an old previously valid OCSP response, or to use an
     alternate time source such as the Roughtime protocol instead of the local
     client system clock).

  .. cpp:function:: const X509_Time& produced_at() const

     Return the time this OCSP response was (claimed to be) produced at.

  .. cpp:function::  const X509_DN& signer_name() const

     Return the distinguished name of the signer. This is used to help
     find the issuing certificate.

     This field is optional in OCSP responses, and may not be set.

  .. cpp:function:: const std::vector<uint8_t>& signer_key_hash() const

     Return the SHA-1 hash of the public key of the signer. This is used to
     help find the issuing certificate. The ``Certificate_Store`` API
     ``find_cert_by_pubkey_sha1`` can search on this value.

     This field is optional in OCSP responses, and may not be set.

  .. cpp:function:: const std::vector<byte>& raw_bits() const

     Return the entire raw ASN.1 blob (for debugging or specialized decoding needs)

One common way of making OCSP requests is via HTTP, see :rfc:`2560`
Appendix A for details. A basic implementation of this is the function
``online_check``, which is available as long as the ``http_util`` module
was compiled in; check by testing for the macro ``BOTAN_HAS_HTTP_UTIL``.

.. cpp:function:: OCSP::Response online_check(const X509_Certificate& issuer, \
                                              const BigInt& subject_serial, \
                                              const std::string& ocsp_responder, \
                                              const Certificate_Store* trusted_roots)

   Assemble a OCSP request for serial number ``subject_serial`` and attempt to request
   it to responder at URI ``ocsp_responder`` over a new HTTP socket, parses and returns
   the response. If trusted_roots is not null, then the response is additionally
   validated using OCSP response API ``check_signature``. Otherwise, this call must be
   performed later by the application.

.. cpp:function:: OCSP::Response online_check(const X509_Certificate& issuer, \
                                              const X509_Certificate& subject, \
                                              const Certificate_Store* trusted_roots)

   Variant of the above but uses serial number and OCSP responder URI from ``subject``.