-<DRAFT!>
HOWTO proxy certificates
0. WARNING
-NONE OF THE CODE PRESENTED HERE HAVE BEEN CHECKED! They are just an
-example to show you how things can be done. There may be typos or
-type conflicts, and you will have to resolve them.
+NONE OF THE CODE PRESENTED HERE HAS BEEN CHECKED! The code is just examples to
+show you how things could be done. There might be typos or type conflicts, and
+you will have to resolve them.
1. Introduction
-Proxy certificates are defined in RFC 3820. They are really usual
-certificates with the mandatory extension proxyCertInfo.
+Proxy certificates are defined in RFC 3820. They are really usual certificates
+with the mandatory extension proxyCertInfo.
-Proxy certificates are issued by an End Entity (typically a user),
-either directly with the EE certificate as issuing certificate, or by
-extension through an already issued proxy certificate.. They are used
-to extend rights to some other entity (a computer process, typically,
-or sometimes to the user itself), so it can perform operations in the
-name of the owner of the EE certificate.
+Proxy certificates are issued by an End Entity (typically a user), either
+directly with the EE certificate as issuing certificate, or by extension through
+an already issued proxy certificate. Proxy certificates are used to extend
+rights to some other entity (a computer process, typically, or sometimes to the
+user itself). This allows the entity to perform operations on behalf of the
+owner of the EE certificate.
See http://www.ietf.org/rfc/rfc3820.txt for more information.
2. A warning about proxy certificates
-Noone seems to have tested proxy certificates with security in mind.
-Basically, to this date, it seems that proxy certificates have only
-been used in a world that's highly aware of them. What would happen
-if an unsuspecting application is to validate a chain of certificates
-that contains proxy certificates? It would usually consider the leaf
-to be the certificate to check for authorisation data, and since proxy
-certificates are controlled by the EE certificate owner alone, it's
-would be normal to consider what the EE certificate owner could do
-with them.
+No one seems to have tested proxy certificates with security in mind. To this
+date, it seems that proxy certificates have only been used in a context highly
+aware of them.
-subjectAltName and issuerAltName are forbidden in proxy certificates,
-and this is enforced in OpenSSL. The subject must be the same as the
-issuer, with one commonName added on.
+Existing applications might misbehave when trying to validate a chain of
+certificates which use a proxy certificate. They might incorrectly consider the
+leaf to be the certificate to check for authorisation data, which is controlled
+by the EE certificate owner.
-Possible threats are, as far as has been imagined so far:
+subjectAltName and issuerAltName are forbidden in proxy certificates, and this
+is enforced in OpenSSL. The subject must be the same as the issuer, with one
+commonName added on.
+
+Possible threats we can think of at this time include:
- impersonation through commonName (think server certificates).
- - use of additional extensions, possibly non-standard ones used in
- certain environments, that would grant extra or different
- authorisation rights.
+ - use of additional extensions, possibly non-standard ones used in certain
+ environments, that would grant extra or different authorisation rights.
+
+For these reasons, OpenSSL requires that the use of proxy certificates be
+explicitly allowed. Currently, this can be done using the following methods:
-For this reason, OpenSSL requires that the use of proxy certificates
-be explicitely allowed. Currently, this can be done using the
-following methods:
+ - if the application directly calls X509_verify_cert(), it can first call:
- - if the application calls X509_verify_cert() itself, it can do the
- following prior to that call (ctx is the pointer passed in the call
- to X509_verify_cert()):
+ X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
- X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
+ Where ctx is the pointer which then gets passed to X509_verify_cert().
- - in all other cases, proxy certificate validation can be enabled
- before starting the application by setting the envirnoment variable
- OPENSSL_ALLOW_PROXY_CERTS with some non-empty value.
+ - proxy certificate validation can be enabled before starting the application
+ by setting the environment variable OPENSSL_ALLOW_PROXY_CERTS.
-There are thoughts to allow proxy certificates with a line in the
-default openssl.cnf, but that's still in the future.
+In the future, it might be possible to enable proxy certificates by editing
+openssl.cnf.
-3. How to create proxy cerificates
+3. How to create proxy certificates
-It's quite easy to create proxy certificates, by taking advantage of
-the lack of checks of the 'openssl x509' application (*ahem*). But
-first, you need to create a configuration section that contains a
-definition of the proxyCertInfo extension, a little like this:
+Creating proxy certificates is quite easy, by taking advantage of a lack of
+checks in the 'openssl x509' application (*ahem*). You must first create a
+configuration section that contains a definition of the proxyCertInfo extension,
+for example:
[ v3_proxy ]
# A proxy certificate MUST NEVER be a CA certificate.
# Usual authority key ID
authorityKeyIdentifier=keyid,issuer:always
- # Now, for the extension that marks this certificate as a proxy one
+ # The extension which marks this certificate as a proxy
proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
-It's also possible to give the proxy extension in a separate section:
+It's also possible to specify the proxy extension in a separate section:
proxyCertInfo=critical,@proxy_ext
pathlen=0
policy=text:BC
-The policy value has a specific syntax, {syntag}:{string}, where the
-syntag determines what will be done with the string. The recognised
-syntags are as follows:
-
- text indicates that the string is simply the bytes, not
- encoded in any kind of way:
+The policy value has a specific syntax, {syntag}:{string}, where the syntag
+determines what will be done with the string. The following syntags are
+recognised:
- policy=text:räksmörgås
+ text indicates that the string is simply bytes, without any encoding:
- Previous versions of this design had a specific tag
- for UTF-8 text. However, since the bytes are copied
- as-is anyway, there's no need for it. Instead, use
- the text: tag, like this:
+ policy=text:räksmörgås
- policy=text:räksmörgås
+ Previous versions of this design had a specific tag for UTF-8 text.
+ However, since the bytes are copied as-is anyway, there is no need for
+ such a specific tag.
- hex indicates the string is encoded in hex, with colons
- between each byte (every second hex digit):
+ hex indicates the string is encoded in hex, with colons between each byte
+ (every second hex digit):
- policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
+ policy=hex:72:E4:6B:73:6D:F6:72:67:E5:73
- Previous versions of this design had a tag to insert a
- complete DER blob. However, the only legal use for
- this would be to surround the bytes that would go with
- the hex: tag with what's needed to construct a correct
- OCTET STRING. Since hex: does that, the DER tag felt
- superfluous, and was therefore removed.
+ Previous versions of this design had a tag to insert a complete DER
+ blob. However, the only legal use for this would be to surround the
+ bytes that would go with the hex: tag with whatever is needed to
+ construct a correct OCTET STRING. The DER tag therefore felt
+ superfluous, and was removed.
- file indicates that the text of the policy should really be
- taken from a file. The string is then really a file
- name. This is useful for policies that are large
- (more than a few of lines) XML documents, for example.
+ file indicates that the text of the policy should really be taken from a
+ file. The string is then really a file name. This is useful for
+ policies that are large (more than a few lines, e.g. XML documents).
The 'policy' setting can be split up in multiple lines like this:
1.polisy= a multi-
2.policy=line policy.
-NOTE: the proxy policy value is the part that determines the rights
-granted to the process using the proxy certificate. The value is
-completely dependent on the application reading and interpretting it!
+NOTE: the proxy policy value is the part which determines the rights granted to
+the process using the proxy certificate. The value is completely dependent on
+the application reading and interpreting it!
-Now that you have created an extension section for your proxy
-certificate, you can now easily create a proxy certificate like this:
+Now that you have created an extension section for your proxy certificate, you
+can easily create a proxy certificate by doing:
- openssl req -new -config openssl.cnf \
- -out proxy.req -keyout proxy.key
- openssl x509 -req -CAcreateserial -in proxy.req -days 7 \
- -out proxy.crt -CA user.crt -CAkey user.key \
- -extfile openssl.cnf -extensions v3_proxy
+ openssl req -new -config openssl.cnf -out proxy.req -keyout proxy.key
+ openssl x509 -req -CAcreateserial -in proxy.req -days 7 -out proxy.crt \
+ -CA user.crt -CAkey user.key -extfile openssl.cnf -extensions v3_proxy
-It's just as easy to create a proxy certificate using another proxy
-certificate as issuer (note that I'm using a different configuration
-section for it):
+You can also create a proxy certificate using another proxy certificate as
+issuer (note: I'm using a different configuration section for it):
- openssl req -new -config openssl.cnf \
- -out proxy2.req -keyout proxy2.key
- openssl x509 -req -CAcreateserial -in proxy2.req -days 7 \
- -out proxy2.crt -CA proxy.crt -CAkey proxy.key \
- -extfile openssl.cnf -extensions v3_proxy2
+ openssl req -new -config openssl.cnf -out proxy2.req -keyout proxy2.key
+ openssl x509 -req -CAcreateserial -in proxy2.req -days 7 -out proxy2.crt \
+ -CA proxy.crt -CAkey proxy.key -extfile openssl.cnf -extensions v3_proxy2
4. How to have your application interpret the policy?
-The basic way to interpret proxy policies is to prepare some default
-rights, then do a check of the proxy certificate against the a chain
-of proxy certificates, user certificate and CA certificates, and see
-what rights came out by the end. Sounds easy, huh? It almost is.
+The basic way to interpret proxy policies is to start with some default rights,
+then compute the resulting rights by checking the proxy certificate against
+the chain of proxy certificates, user certificate and CA certificates. You then
+use the final computed rights. Sounds easy, huh? It almost is.
-The slightly complicated part is how to pass data between your
+The slightly complicated part is figuring out how to pass data between your
application and the certificate validation procedure.
You need the following ingredients:
- - a callback routing that will be called for every certificate that's
- validated. It will be called several times for each certificates,
- so you must be attentive to when it's a good time to do the proxy
- policy interpretation and check, as well as to fill in the defaults
- when the EE certificate is checked.
+ - a callback function that will be called for every certificate being
+ validated. The callback be called several times for each certificate,
+ so you must be careful to do the proxy policy interpretation at the right
+ time. You also need to fill in the defaults when the EE certificate is
+ checked.
- - a structure of data that's shared between your application code and
- the callback.
+ - a data structure that is shared between your application code and the
+ callback.
- a wrapper function that sets it all up.
- - an ex_data index function that creates an index into the generic
- ex_data store that's attached to an X509 validation context.
+ - an ex_data index function that creates an index into the generic ex_data
+ store that is attached to an X509 validation context.
-This is some cookbook code for you to fill in:
+Here is some skeleton code you can fill in:
/* In this example, I will use a view of granted rights as a bit
array, one bit for each possible right. */
static int verify_callback(int ok, X509_STORE_CTX *ctx)
{
if (ok == 1) /* It's REALLY important you keep the proxy policy
- check within this secion. It's important to know
+ check within this section. It's important to know
that when ok is 1, the certificates are checked
from top to bottom. You get the CA root first,
followed by the possible chain of intermediate
if (xs->ex_flags & EXFLAG_PROXY)
{
- YOUR_RIGHTS *rights =
+ YOUR_RIGHTS *rights =
(YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
get_proxy_auth_ex_data_idx());
PROXY_CERT_INFO_EXTENSION *pci =
bit array and fill it with the rights granted by
the current proxy certificate, then use it as a
mask on the accumulated rights bit array, and
- voilà, you now have a new accumulated rights bit
+ voilà, you now have a new accumulated rights bit
array. */
{
int i;
YOUR_RIGHTS tmp_rights;
- memset(tmp_rights.rights, 0, sizeof(tmp_rights.rights));
+ memset(tmp_rights.rights, 0, sizeof(tmp_rights.rights));
/* process_rights() is supposed to be a procedure
that takes a string and it's length, interprets
{
/* We have a EE certificate, let's use it to set default!
*/
- YOUR_RIGHTS *rights =
+ YOUR_RIGHTS *rights =
(YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
get_proxy_auth_ex_data_idx());
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