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
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.
Load a certificate from an abstract
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.
Return the certificates serial number. The tuple of issuer DN and serial number should be unique.
Returns the distinguished name (DN) of the certificate’s subject.
Returns the distinguished name (DN) of the certificate’s issuer
Returns the point in time the certificate becomes valid
Returns the point in time the certificate expires
const Extensions &
- 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.
const AlternativeName &
Return the subjects alternative name. This is used to store values like associated URIs, DNS addresses, and email addresses.
const AlternativeName &
Return alternative names for the issuer.
fingerprint(const std::string &hash_name = "SHA-1") const¶
Return a fingerprint for the certificate.
Returns either an enumeration listing key constraints (what the associated key can be used for) or
NO_CONSTRAINTSif the relevent extension was not included. Example values are
KEY_CERT_SIGN. More than one value might be specified.
allowed_extended_usage(const OID &usage) const¶
Returns true if the OID is included in the extended usage extension.
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.
Returns a free-form human readable string describing the certificate.
- const AlternativeName &
X509_Certificate class has several other functions not described here.
See the header
x509cert.h for details.
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
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
Public_Key*. As to what to do with this, read about
load_key in 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.
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 (email@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
You can also get information about the issuer of the certificate in the same
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.
Validation of custom extensions during path validation is currently not supported.
add(Certificate_Extension *extn, bool critical = false)¶
Adds a new extension to the list.
criticalspecifies whether the extension should be marked as critical.
replace(Certificate_Extension *extn, bool critical = false)¶
Adds an extension to the list or replaces it, if the same extension was already added
get(const OID &oid) const¶
Searches for an extension by OID and returns the result
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.
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>>
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
add_crl(const X509_CRL &crl)¶
and all future verifications will take into account the provided CRL.
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
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
while the SKID parameter takes a
the subject key identifier in raw binary. Both lookup methods are
mandatory to implement.
Finally, there is a method for finding a CRL, called
that takes an
X509_Certificate object, and returns a
type makes it easy to return no CRLs by returning
(eg, if the certificate store doesn’t support retrieving CRLs).
Implementing the function is optional, and by default will return
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.
Certificate_Store_In_Memory(const std::string &dir)¶
Attempt to parse all files in
dir(including subdirectories) as certificates. Ignores errors.
Create an empty store
Add a certificate already in a shared_ptr to the store
add_crl(const X509_CRL &crl)¶
Add a certificate revocation list (CRL) to the store.
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.
- Create or open an existing certificate store from an SQL database. The password in
passwdwill be used to encrypt private keys.
insert_cert(const X509_Certificate &cert)¶
certinto the store. Returns false if the certificate is already known and true if insertion was successful.
remove_cert(const X509_Certificate &cert)¶
certfrom the store. Returns false if the certificate could not be found and true if removal was successful.
find_key(const X509_Certificate&) const¶
Returns the private key for “cert” or an empty shared_ptr if none was found
find_certs_for_key(const Private_Key &key) const¶
- Returns all certificates for private key
insert_key(const X509_Certificate &cert, const Private_Key &key)¶
certinto the store, returns false if the key is already known and true if insertion was successful.
remove_key(const Private_Key &key)¶
keyfrom the store
revoke_cert(const X509_Certificate&, CRL_Code, const X509_Time &time = X509_Time())¶
certas revoked starting from
Generates CRLs for all certificates marked as revoked. A CRL is returned for each unique issuer DN.
Certificate_Store_In_SQL class operates on an abstract
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.
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
passwdwill be used to encrypt private keys.
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
The last five parameters are optional.
hostnamespecifies a hostname which is matched against the subject DN in
end_certaccording to RFC 6125. An empty hostname disables hostname validation.
usagespecifies key usage restrictions that are compared to the key usage fields in end_cert according to RFC 5280, if not set to
validation_timeallows 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_timeoutsets the timeout for OCSP requests. The default of 0 disables OCSP checks alltogether.
ocsp_respallows 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 flavours of
The result of the validation is returned as a class:
- Specifies the result of the validation
Returns true if a certificate path from end_cert to a trusted root was found and all path validation checks passed.
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 &
- If the validation was successful, returns the certificate which is acting as the trust root for end_cert.
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.
const std::vector<std::set<Certificate_Status_Code>> &
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.
Returns the set of all cryptographic hash functions which are implicitly trusted for this validation to be correct.
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 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.
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(const X509_Certificate &cert, const Private_Key &key, const std::string &hash_fn, RandomNumberGenerator &rng)¶
key is the private key corresponding to the public key in the
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(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
sign_request(const PKCS10_Request &req, RandomNumberGenerator &rng, const X509_Time ¬_before, const X509_Time ¬_after)¶
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
new_crl(RandomNumberGenerator &rng, uint32_t next_update = 0)¶
This function will return a new, empty CRL. The
next_updateparameter 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
update_crl(const X509_CRL &last_crl, std::vector<CRL_Entry> new_entries, RandomNumberGenerator &rng, size_t next_update = 0)¶
last_crlis the last CRL this CA issued, and
new_entriesis a list of any newly revoked certificates. The function returns a new
X509_CRLto make available for clients.
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
serial. There are two additional (optional)
values, an enumeration called
CRL_Code that specifies the reason for
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
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:
create_self_signed_cert(const X509_Cert_Options &opts, const Private_Key &key, const std::string &hash_fn, RandomNumberGenerator &rng)¶
keyis the private key you wish to use (the public key, used in the certificate itself is extracted from the private key), and
optsis 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¶
x509self.h, there is a function for generating new PKCS #10
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
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
First and foremost is a number of
std::string members, which
contains various bits of information about the user:
uri. As many
of these as possible should be filled it (especially an email
address), though the only required ones are
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_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
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
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_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):
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
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.
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
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.
Request(const X509_Certificate &issuer_cert, const BigInt &subject_serial)¶
Create a new OCSP request
Request(const X509_Certificate &issuer_cert, const X509_Certificate &subject_cert)¶
Variant of the above, using serial number from
Encode the current OCSP request as a binary string.
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.
Response(const uint8_t response_bits, size_t response_bits_len)¶
Attempts to parse
response_bitsas 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.
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_pathargument: 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.
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.
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.
ref_timeis 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 &
Return the time this OCSP response was (claimed to be) produced at.
const X509_DN &
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> &
Return the SHA-1 hash of the public key of the signer. This is used to help find the issuing certificate. The
find_cert_by_pubkey_sha1can search on this value.
This field is optional in OCSP responses, and may not be set.
const std::vector<uint8_t> &
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
was compiled in; check by testing for the macro
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_serialand attempt to request it to responder at URI
ocsp_responderover 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.