Certificate Handling — Botan

# Botan

# Certificate Handling

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

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

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

## So what’s in an X.509 certificate?

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

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

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

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

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

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

### X.509v3 Extensions

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

> - Key Usage and Extended Key Usage: No problems known.
> - Basic Constraints: No problems known. The default for a v1/v2 certificate is assume it’s a CA if and only if the option “x509/default_to_ca” is set. A v3 certificate is marked as a CA if (and only if) the basic constraints extension is present and set for a CA cert.
> - Subject Alternative Names: Only the “rfc822Name”, “dNSName”, and “uniformResourceIdentifier” fields will be stored; all others are ignored.
> - Issuer Alternative Names: Same restrictions as the Subject Alternative Names extension. New certificates generated by Botan never include the issuer alternative name.
> - Authority Key Identifier: Only the version using KeyIdentifier is supported. If the GeneralNames version is used and the extension is critical, an exception is thrown. If both the KeyIdentifier and GeneralNames versions are present, then the KeyIdentifier will be used, and the GeneralNames ignored.
> - Subject Key Identifier: No problems known.

### Revocation Lists

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

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

    X509_CRL crl1("crl1.der");

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

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

X509_Code `X509_Store::``add_crl`(const X509_CRL &crl)

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

## 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

## Storing and Using Certificates

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

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

### Adding Certificates

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

void `X509_Store::``add_cert`(const X509_Certificate &cert, bool trusted = false)

void `X509_Store::``add_cert`(DataSource &source)

void `X509_Store::``add_trusted_certs`(DataSource &source)

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

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

### Adding CRLs

X509_Code `X509_Store::``add_crl`(const X509_CRL &crl)

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

### Storing Certificates

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

### Certificate Stores

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

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

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

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

void `X509_Store::``add_new_certstore`(Certificate_Store *new_store)

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

### Verifying Certificates

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

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

class ```X509_Store`

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

X509_Code ```validate_cert`(const X509_Certificate &cert, Cert_Usage usage = ANY)

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

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

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

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

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

- VERIFIED: The certificate is valid for the specified use.
- INVALID_USAGE: The certificate cannot be used for the specified use.
- CANNOT_ESTABLISH_TRUST: The root certificate was not marked as trusted.
- CERT_CHAIN_TOO_LONG: The certificate chain exceeded the length allowed by a basicConstraints extension.
- SIGNATURE_ERROR: An invalid signature was found
- POLICY_ERROR: Some problem with the certificate policies was found.
- CERT_FORMAT_ERROR: Some format problem was found in a certificate.
- CERT_ISSUER_NOT_FOUND: The issuer of a certificate could not be found.
- CERT_NOT_YET_VALID: The certificate is not yet valid.
- CERT_HAS_EXPIRED: The certificate has expired.
- CERT_IS_REVOKED: The certificate has been revoked.
- CRL_FORMAT_ERROR: Some format problem was found in a CRL.
- CRL_ISSUER_NOT_FOUND: The issuer of a CRL could not be found.
- CRL_NOT_YET_VALID: The CRL is not yet valid.
- CRL_HAS_EXPIRED: The CRL has expired.
- CA_CERT_CANNOT_SIGN: The CA certificate found does not have an contain a public key that allows signature verification.
- CA_CERT_NOT_FOR_CERT_ISSUER: The CA cert found is not allowed to issue certificates.
- CA_CERT_NOT_FOR_CRL_ISSUER: The CA cert found is not allowed to issue CRLs.
- UNKNOWN_X509_ERROR: Some other error occurred.

## Certificate Authorities

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

`X509_CA::``X509_CA`(const X509_Certificate &cert, const Private_Key &key)

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

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

X509_Certificate `X509_CA::``sign_request`(const PKCS10_Request &req) const

Here’s an example:

### 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`(u32bit seconds_to_expiration = 0)

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

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

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

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

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

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

### Self-Signed Certificates

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

X509_Certificate ```create_self_signed_cert`(const X509_Cert_Options &opts, const Private_Key &key)

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

An example:

    #include <botan/botan.h>
    #include <botan/x509self.h>
    #include <botan/rsa.h>
    #include <botan/dsa.h>
    using namespace Botan;

    #include <iostream>
    #include <fstream>
    #include <memory>

    int main(int argc, char* argv[])
       {
       if(argc != 7)
          {
          std::cout << "Usage: " << argv[0]
                    << " passphrase [CA|user] name country_code organization email"
                    << std::endl;
          return 1;
          }

       Botan::LibraryInitializer init;

       std::string CA_flag = argv[2];
       bool do_CA = false;

       if(CA_flag == "CA") do_CA = true;
       else if(CA_flag == "user") do_CA = false;
       else
          {
          std::cout << "Bad flag for CA/user switch: " << CA_flag << std::endl;
          return 1;
          }

       try
          {
          AutoSeeded_RNG rng;

          RSA_PrivateKey key(rng, 2048);

          std::ofstream priv_key("private.pem");
          priv_key << PKCS8::PEM_encode(key, rng, argv[1]);

          X509_Cert_Options opts;

          opts.common_name = argv[3];
          opts.country = argv[4];
          opts.organization = argv[5];
          opts.email = argv[6];
          /* Fill in other values of opts here */

          //opts.xmpp = "lloyd@randombit.net";

          if(do_CA)
             opts.CA_key();

          X509_Certificate cert =
             X509::create_self_signed_cert(opts, key, "SHA-256", rng);

          std::ofstream cert_file("cert.pem");
          cert_file << cert.PEM_encode();
       }
       catch(std::exception& e)
          {
          std::cout << "Exception: " << e.what() << std::endl;
          return 1;
          }

       return 0;
       }

### 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)

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.

An example:

    #include <botan/init.h>
    #include <botan/auto_rng.h>
    #include <botan/x509self.h>
    #include <botan/rsa.h>
    #include <botan/dsa.h>
    using namespace Botan;

    #include <iostream>
    #include <fstream>
    #include <memory>

    int main(int argc, char* argv[])
       {
       if(argc != 6)
          {
          std::cout << "Usage: " << argv[0] <<
             " passphrase name country_code organization email" << std::endl;
          return 1;
          }

       Botan::LibraryInitializer init;

       try
          {
          AutoSeeded_RNG rng;

          RSA_PrivateKey priv_key(rng, 1024);

          std::ofstream key_file("private.pem");
          key_file << PKCS8::PEM_encode(priv_key, rng, argv[1]);

          X509_Cert_Options opts;

          opts.common_name = argv[2];
          opts.country = argv[3];
          opts.organization = argv[4];
          opts.email = argv[5];

          PKCS10_Request req = X509::create_cert_req(opts, priv_key,
                                                     "SHA-256", rng);

          std::ofstream req_file("req.pem");
          req_file << req.PEM_encode();
       }
       catch(std::exception& e)
          {
          std::cout << e.what() << std::endl;
          return 1;
          }
       return 0;
       }

### Certificate Options

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

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

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

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

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

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

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