# 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 Transport Layer Security (TLS) protocol. A X.509 certificate is represented by the class `X509_Certificate`. The data of an X.509 certificate is stored as a `shared_ptr` to a structure containing the decoded information. So copying `X509_Certificate` objects is quite cheap.

class `X509_Certificate`
  `X509_Certificate`(const std::string &filename)

    Load a certificate from a file. PEM or DER is accepted.

  `X509_Certificate`(const std::vector<uint8_t> &in)

    Load a certificate from a byte string.

  `X509_Certificate`(DataSource &source)

    Load a certificate from an abstract `DataSource`.

  X509_DN `subject_dn`() const

    Returns the distinguished name (DN) of the certificate’s subject. This is the primary place where information about the subject of the certificate is stored. However “modern” information that doesn’t fit in the X.500 framework, such as DNS name, email, IP address, or XMPP address, appears instead in the subject alternative name.

  X509_DN `issuer_dn`() const

    Returns the distinguished name (DN) of the certificate’s issuer, ie the CA that issued this certificate.

  const AlternativeName &`subject_alt_name`() const

    Return the subjects alternative name. This is used to store values like associated URIs, DNS addresses, and email addresses.

  const AlternativeName &`issuer_alt_name`() const

    Return alternative names for the issuer.

  std::unique_ptr<Public_Key> `load_subject_public_key`() const

    Deserialize the stored public key and return a new object. This might throw, if it happens that the public key object stored in the certificate is malformed in some way, or in the case that the public key algorithm used is not supported by the library.

    See Serializing Public Keys for more information about what to do with the returned object. It may be any type of key, in principle, though RSA and ECDSA are most common.

  std::vector<uint8_t> `subject_public_key_bits`() const

    Return the binary encoding of the subject public key. This value (or a hash of it) is used in various protocols, eg for public key pinning.

  AlgorithmIdentifier `subject_public_key_algo`() const

    Return an algorithm identifier that identifies the algorithm used in the subject’s public key.

  std::vector<uint8_t> `serial_number`() const

    Return the certificates serial number. The tuple of issuer DN and serial number should be unique.

  std::vector<uint8> `raw_subject_dn`() const

    Return the binary encoding of the subject DN.

  std::vector<uint8> `raw_issuer_dn`() const

    Return the binary encoding of the issuer DN.

  X509_Time `not_before`() const

    Returns the point in time the certificate becomes valid

  X509_Time `not_after`() const

    Returns the point in time the certificate expires

  const Extensions &`v3_extensions`() const

    Returns all extensions of this certificate. You can use this to examine any extension data associated with the certificate, including custom extensions the library doesn’t know about.

  std::vector<uint8_t> `authority_key_id`() const

    Return the authority key id, if set. This is an arbitrary string; in the issuing certificate this will be the subject key id.

  std::vector<uint8_t> `subject_key_id`() const

    Return the subject key id, if set.

  bool `allowed_extended_usage`(const OID &usage) const

    Return true if and only if the usage OID appears in the extended key usage extension. Also will return true if the extended key usage extension is not used in the current certificate.

  std::vector<OID> `extended_key_usage`() const

    Return the list of extended key usages. May be empty.

  std::string `fingerprint`(const std::string &hash_fn = "SHA-1") const

    Return a fingerprint for the certificate, which is basically just a hash of the binary contents. Normally SHA-1 or SHA-256 is used, but any hash function is allowed.

  Key_Constraints `constraints`() const

    Returns either an enumeration listing key constraints (what the associated key can be used for) or `NO_CONSTRAINTS` if the relevant extension was not included. Example values are `DIGITAL_SIGNATURE` and `KEY_CERT_SIGN`. More than one value might be specified.

  bool `matches_dns_name`(const std::string &name) const

    Check if the certificate’s subject alternative name DNS fields match `name`. This function also handles wildcard certificates.

  std::string `to_string`() const

    Returns a free-form human readable string describing the certificate.

## X.509 Distinguished Names

class `X509_DN`
  bool `has_field`(const std::string &attr) const

    Returns true if `get_attribute` or `get_first_attribute` will return a value.

  std::vector<std::string> `get_attribute`(const std::string &attr) const

    Return all attributes associated with a certain attribute type.

  std::string `get_first_attribute`(const std::string &attr) const

    Like `get_attribute` but returns just the first attribute, or empty if the DN has no attribute of the specified type.

  std::multimap<OID, std::string> `get_attributes`() const

    Get all attributes of the DN. The OID maps to a DN component such as 2.5.4.10 (“Organization”), and the strings are UTF-8 encoded.

  std::multimap<std::string, std::string> `contents`() const

    Similar to `get_attributes`, but the OIDs are decoded to strings.

  void `add_attribute`(const std::string &key, const std::string &val)

    Add an attribute to a DN.

  void `add_attribute`(const OID &oid, const std::string &val)

    Add an attribute to a DN using an OID instead of string-valued attribute type.

The `X509_DN` type also supports iostream extraction and insertion operators, for formatted input and output.

## 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.

class `Extensions`
  void `add`(Certificate_Extension *extn, bool critical = false)

    Adds a new extension to the extensions object. If an extension of the same type already exists, `extn` will replace it. If `critical` is true the extension will be marked as critical in the encoding.

  bool `add_new`(Certificate_Extension *extn, bool critical = false)

    Like `add` but an existing extension will not be replaced. Returns true if the extension was used, false if an extension of the same type was already in place.

  void `replace`(Certificate_Extension *extn, bool critical = false)

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

  std::unique_ptr<Certificate_Extension> `get`(const OID &oid) const

    Searches for an extension by OID and returns the result

  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.

  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.

  std::map<OID, std::pair<std::vector<uint8_t>, 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.

## Certificate 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

void `Certificate_Store`::```add_crl`(const X509_CRL &crl)

and all future verifications will take into account the provided CRL.

### 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<uint8_t>` 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 Transport Layer Security (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.

class `Certificate_Store_In_Memory`
  `Certificate_Store_In_Memory`(const std::string &dir)

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

  `Certificate_Store_In_Memory`(const X509_Certificate &cert)

    Adds given certificate to the store

  `Certificate_Store_In_Memory`()

    Create an empty store

  void `add_certificate`(const X509_Certificate &cert)

    Add a certificate to the store

  void `add_certificate`(std::shared_ptr<const X509_Certificate> cert)

    Add a certificate already in a shared_ptr to the store

  void `add_crl`(const X509_CRL &crl)

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

  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.

class `Certificate_Store_In_SQL`
  `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.

  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.

  `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.

  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

  std::vector<std::shared_ptr<const X509_Certificate>> `find_certs_for_key`(const Private_Key &key) const

    > Returns all certificates for private key `key`

  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.

  void `remove_key`(const Private_Key &key)

    Removes `key` from the store

  void `revoke_cert`(const X509_Certificate&, CRL_Code, const X509_Time &time = X509_Time())

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

  void `affirm_cert`(const X509_Certificate&)

    Reverses the revocation for `cert`

  std::vector<X509_CRL> `generate_crls`() const

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

The `Certificate_Store_In_SQL` class operates on an abstract `SQL_Database` object. If support for sqlite3 was enabled at build time, Botan includes an implementation of this interface for sqlite3, and a subclass of `Certificate_Store_In_SQL` which creates or opens a sqlite3 database.

class `Certificate_Store_In_SQLite`
  `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 certificate 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`:

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 = std::vector<std::shared_ptr<const OCSP::Response>>())

  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 completely. `ocsp_resp` allows adding additional OCSP responses retrieved from outside of the path validation. Note that OCSP online checks are done only as long as the http_util module was compiled in. Availability of online OCSP checks can be checked using the macro BOTAN_HAS_ONLINE_REVOCATION_CHECKS.

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

The result of the validation is returned as a class:

class `Path_Validation_Result`

  > Specifies the result of the validation

  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.

  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.

  const X509_Certificate &`trust_root`() const

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

  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.

  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 relatively innocuous and common error) when the signature isn’t even valid.

  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.

  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:

> `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.

> `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 corresponds 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.

### Creating New Certificates

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:

`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.

There is an alternative constructor that lets you set additional options, namely the padding scheme that will be used by the X509_CA object to sign certificates and certificate revocation lists. If the padding is not set explicitly, the CA will use the padding scheme that was used when signing the CA certificate.

`X509_CA`::```X509_CA`(const X509_Certificate &cert, const Private_Key &key, const std::map<std::string, std::string> &opts, const std::string &hash_fn, RandomNumberGenerator &rng)

The only option valid at this moment is “padding”. The supported padding schemes can be found in src/lib/pubkey/padding.cpp. Some alternative names for the padding schemes are understood, as well.

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:

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

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

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<uint8_t>` 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:

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:

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 `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.

PKCS10_Request `PKCS10_Request`::```create`(const Private_Key &key, const X509_DN &subject_dn, const Extensions &extensions, const std::string &hash_fn, RandomNumberGenerator &rng, const std::string &padding_scheme = "", const std::string &challenge = "")

  This function (added in 2.5) is similar to `create_cert_req` but allows specifying all the parameters directly. In fact `create_cert_req` just creates the DN and extensions from the options, then uses this call to actually create the `PKCS10_Request` object.

## 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`.

Additionally there are a small selection of `std::vector<std::string>` members, which allow space for repeating elements: `more_org_units` and `more_dns`.

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.

Moreover, you can specify the padding scheme to be used when digital signatures are computed by calling function `set_padding_scheme` with a string representing the padding scheme. This way, you can control the padding scheme for self-signed certificates and PKCS #10 requests. The padding scheme used by a CA when building a certificate or a certificate revocation list can be set in the `X509_CA` constructor. The supported padding schemes can be found in src/lib/pubkey/padding.cpp. Some alternative names for the padding schemes are understood, as well.

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, which is useful if you want to encode a custom extension, or encode an extension in a way differently from how Botan defaults.

### 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.

class `OCSP`::```Request`
  `OCSP`::```Request`(const X509_Certificate &issuer_cert, const BigInt &subject_serial)

    Create a new OCSP request

  `OCSP`::```Request`(const X509_Certificate &issuer_cert, const X509_Certificate &subject_cert)

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

  std::vector<uint8_t> `BER_encode`() const

    Encode the current OCSP request as a binary string.

  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.

class `OCSP`::```Response`
  `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.

  Certificate_Status_Code `check_signature`(const std::vector<Certificate_Store *> &trust_roots, const std::vector<std::shared_ptr<const X509_Certificate>> &cert_path = const std::vector<std::shared<const X509_Certificate>>()) 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.

  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.

  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).

  const X509_Time &`produced_at`() const

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

  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.

  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.

  const std::vector<uint8_t> &`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`.

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.

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`.