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<DRAFT!>
HOWTO certificates
1. Introduction
How you handle certificates depend a great deal on what your role is.
Your role can be one or several of:
- User of some client software
- User of some server software
- Certificate authority
This file is for users who wish to get a certificate of their own.
Certificate authorities should read ca.txt.
In all the cases shown below, the standard configuration file, as
compiled into openssl, will be used. You may find it in /etc/,
/usr/local/ssl/ or somewhere else. The name is openssl.cnf, and
is better described in another HOWTO <config.txt?>. If you want to
use a different configuration file, use the argument '-config {file}'
with the command shown below.
2. Relationship with keys
Certificates are related to public key cryptography by containing a
public key. To be useful, there must be a corresponding private key
somewhere. With OpenSSL, public keys are easily derived from private
keys, so before you create a certificate or a certificate request, you
need to create a private key.
Private keys are generated with 'openssl genrsa' if you want a RSA
private key, or 'openssl gendsa' if you want a DSA private key.
Further information on how to create private keys can be found in
another HOWTO <keys.txt?>. The rest of this text assumes you have
a private key in the file privkey.pem.
3. Creating a certificate request
To create a certificate, you need to start with a certificate
request (or, as some certificate authorities like to put
it, "certificate signing request", since that's exactly what they do,
they sign it and give you the result back, thus making it authentic
according to their policies). A certificate request can then be sent
to a certificate authority to get it signed into a certificate, or if
you have your own certificate authority, you may sign it yourself, or
if you need a self-signed certificate (because you just want a test
certificate or because you are setting up your own CA).
The certificate request is created like this:
openssl req -new -key privkey.pem -out cert.csr
Now, cert.csr can be sent to the certificate authority, if they can
handle files in PEM format. If not, use the extra argument '-outform'
followed by the keyword for the format to use (see another HOWTO
<formats.txt?>). In some cases, that isn't sufficient and you will
have to be more creative.
When the certificate authority has then done the checks the need to
do (and probably gotten payment from you), they will hand over your
new certificate to you.
Section 5 will tell you more on how to handle the certificate you
received.
4. Creating a self-signed test certificate
If you don't want to deal with another certificate authority, or just
want to create a test certificate for yourself. This is similar to
creating a certificate request, but creates a certificate instead of
a certificate request. This is NOT the recommended way to create a
CA certificate, see ca.txt.
openssl req -new -x509 -key privkey.pem -out cacert.pem -days 1095
5. What to do with the certificate
If you created everything yourself, or if the certificate authority
was kind enough, your certificate is a raw DER thing in PEM format.
Your key most definitely is if you have followed the examples above.
However, some (most?) certificate authorities will encode them with
things like PKCS7 or PKCS12, or something else. Depending on your
applications, this may be perfectly OK, it all depends on what they
know how to decode. If not, There are a number of OpenSSL tools to
convert between some (most?) formats.
So, depending on your application, you may have to convert your
certificate and your key to various formats, most often also putting
them together into one file. The ways to do this is described in
another HOWTO <formats.txt?>, I will just mention the simplest case.
In the case of a raw DER thing in PEM format, and assuming that's all
right for yor applications, simply concatenating the certificate and
the key into a new file and using that one should be enough. With
some applications, you don't even have to do that.
By now, you have your cetificate and your private key and can start
using the software that depend on it.
--
Richard Levitte

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<DRAFT!>
HOWTO keys
1. Introduction
Keys are the basis of public key algorithms and PKI. Keys usually
come in pairs, with one half being the public key and the other half
being the private key. With OpenSSL, the private key contains the
public key information as well, so a public key doesn't need to be
generated separately.
Public keys come in several flavors, using different cryptographic
algorithms. The most popular ones associated with certificates are
RSA and DSA, and this HOWTO will show how to generate each of them.
2. To generate a RSA key
A RSA key can be used both for encryption and for signing.
Generating a key for the RSA algorithm is quite easy, all you have to
do is the following:
openssl genrsa -des3 -out privkey.pem 2048
With this variant, you will be prompted for a protecting password. If
you don't want your key to be protected by a password, remove the flag
'-des3' from the command line above.
NOTE: if you intend to use the key together with a server
certificate, it may be a good thing to avoid protecting it
with a password, since that would mean someone would have to
type in the password every time the server needs to access
the key.
The number 2048 is the size of the key, in bits. Today, 2048 or
higher is recommended for RSA keys, as fewer amount of bits is
consider insecure or to be insecure pretty soon.
3. To generate a DSA key
A DSA key can be used for signing only. This is important to keep
in mind to know what kind of purposes a certificate request with a
DSA key can really be used for.
Generating a key for the DSA algorithm is a two-step process. First,
you have to generate parameters from which to generate the key:
openssl dsaparam -out dsaparam.pem 2048
The number 2048 is the size of the key, in bits. Today, 2048 or
higher is recommended for DSA keys, as fewer amount of bits is
consider insecure or to be insecure pretty soon.
When that is done, you can generate a key using the parameters in
question (actually, several keys can be generated from the same
parameters):
openssl gendsa -des3 -out privkey.pem dsaparam.pem
With this variant, you will be prompted for a protecting password. If
you don't want your key to be protected by a password, remove the flag
'-des3' from the command line above.
NOTE: if you intend to use the key together with a server
certificate, it may be a good thing to avoid protecting it
with a password, since that would mean someone would have to
type in the password every time the server needs to access
the key.
--
Richard Levitte

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<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.
1. Introduction
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.
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.
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 are, as far as has been imagined so far:
- 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.
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 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);
- in all other cases, proxy certificate validation can be enabled
before starting the application by setting the envirnoment variable
OPENSSL_ALLOW_PROXY with some non-empty value.
There are thoughts to allow proxy certificates with a line in the
default openssl.cnf, but that's still in the future.
3. How to create proxy cerificates
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:
[ v3_proxy ]
# A proxy certificate MUST NEVER be a CA certificate.
basicConstraints=CA:FALSE
# Usual authority key ID
authorityKeyIdentifier=keyid,issuer:always
# Now, for the extension that marks this certificate as a proxy one
proxyCertInfo=critical,language:id-ppl-anyLanguage,pathlen:1,policy:text:AB
It's also possible to give the proxy extension in a separate section:
proxyCertInfo=critical,@proxy_ext
[ proxy_ext ]
language=id-ppl-anyLanguage
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:
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's no need for it. Instead, use
the text: tag, like this:
policy=text:räksmörgås
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
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.
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.
The 'policy' setting can be split up in multiple lines like this:
0.policy=This is
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!
Now that you have created an extension section for your proxy
certificate, you can now easily create a proxy certificate like this:
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):
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 slightly complicated part is 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 structure of data that's 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.
This is some cookbook code for you to fill in:
/* In this example, I will use a view of granted rights as a bit
array, one bit for each possible right. */
typedef struct your_rights {
unsigned char rights[total_rights / 8];
} YOUR_RIGHTS;
/* The following procedure will create an index for the ex_data
store in the X509 validation context the first time it's called.
Subsequent calls will return the same index. */
static int get_proxy_auth_ex_data_idx(void)
{
static volatile int idx = -1;
if (idx < 0)
{
CRYPTO_w_lock(CRYPTO_LOCK_X509_STORE);
if (idx < 0)
{
idx = X509_STORE_CTX_get_ex_new_index(0,
"for verify callback",
NULL,NULL,NULL);
}
CRYPTO_w_unlock(CRYPTO_LOCK_X509_STORE);
}
return idx;
}
/* Callback to be given to the X509 validation procedure. */
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
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
CAs, followed by the EE certificate, followed by
the possible proxy certificates. */
{
X509 *xs = ctx->current_cert;
if (xs->ex_flags & EXFLAG_PROXY)
{
YOUR_RIGHTS *rights =
(YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
get_proxy_auth_ex_data_idx());
PROXY_CERT_INFO_EXTENSION *pci =
X509_get_ext_d2i(xs, NID_proxyCertInfo, NULL, NULL);
switch (OBJ_obj2nid(pci->proxyPolicy->policyLanguage))
{
case NID_Independent:
/* Do whatever you need to grant explicit rights to
this particular proxy certificate, usually by
pulling them from some database. If there are none
to be found, clear all rights (making this and any
subsequent proxy certificate void of any rights).
*/
memset(rights->rights, 0, sizeof(rights->rights));
break;
case NID_id_ppl_inheritAll:
/* This is basically a NOP, we simply let the current
rights stand as they are. */
break;
default:
/* This is usually the most complex section of code.
You really do whatever you want as long as you
follow RFC 3820. In the example we use here, the
simplest thing to do is to build another, temporary
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
array. */
{
int i;
YOUR_RIGHTS tmp_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
it and sets the bits in the YOUR_RIGHTS pointed
at by the third argument. */
process_rights((char *) pci->proxyPolicy->policy->data,
pci->proxyPolicy->policy->length,
&tmp_rights);
for(i = 0; i < total_rights / 8; i++)
rights->rights[i] &= tmp_rights.rights[i];
}
break;
}
PROXY_CERT_INFO_EXTENSION_free(pci);
}
else if (!(xs->ex_flags & EXFLAG_CA))
{
/* We have a EE certificate, let's use it to set default!
*/
YOUR_RIGHTS *rights =
(YOUR_RIGHTS *)X509_STORE_CTX_get_ex_data(ctx,
get_proxy_auth_ex_data_idx());
/* The following procedure finds out what rights the owner
of the current certificate has, and sets them in the
YOUR_RIGHTS structure pointed at by the second
argument. */
set_default_rights(xs, rights);
}
}
return ok;
}
static int my_X509_verify_cert(X509_STORE_CTX *ctx,
YOUR_RIGHTS *needed_rights)
{
int i;
int (*save_verify_cb)(int ok,X509_STORE_CTX *ctx) = ctx->verify_cb;
YOUR_RIGHTS rights;
X509_STORE_CTX_set_verify_cb(ctx, verify_callback);
X509_STORE_CTX_set_ex_data(ctx, get_proxy_auth_ex_data_idx(), &rights);
X509_STORE_CTX_set_flags(ctx, X509_V_FLAG_ALLOW_PROXY_CERTS);
ok = X509_verify_cert(ctx);
if (ok == 1)
{
ok = check_needed_rights(rights, needed_rights);
}
X509_STORE_CTX_set_verify_cb(ctx, save_verify_cb);
return ok;
}
If you use SSL or TLS, you can easily set up a callback to have the
certificates checked properly, using the code above:
SSL_CTX_set_cert_verify_callback(s_ctx, my_X509_verify_cert, &needed_rights);
--
Richard Levitte