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<DRAFT!>
HOWTO certificates
1. Introduction
How you handle certificates depends a great deal on what your role is.
Your role can be one or several of:
- User of some client application
- User of some server application
- Certificate authority
This file is for users who wish to get a certificate of their own.
Certificate authorities should read https://www.openssl.org/docs/apps/ca.html.
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. By default the file is named
openssl.cnf and is described at https://www.openssl.org/docs/apps/config.html.
You can specify a different configuration file using the
'-config {file}' argument with the commands 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 -out privkey.pem' if
you want a RSA private key, or if you want a DSA private key:
'openssl dsaparam -out dsaparam.pem 2048; openssl gendsa -out privkey.pem dsaparam.pem'.
The private keys created by these commands are not passphrase protected;
it might or might not be the desirable thing. Further information on how to
create private keys can be found at https://www.openssl.org/docs/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 is sent to a certificate authority
to get it signed into a certificate. You can also sign the certificate
yourself if you have your own certificate authority or create a
self-signed certificate (typically for testing purpose).
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, -outform does not let you output the
certificate request in the right format and you will have to use one
of the various other commands that are exposed by openssl (or get
creative and use a combination of tools).
The certificate authority performs various checks (according to their
policies) and usually waits for payment from you. Once that is
complete, they send you your new certificate.
Section 5 will tell you more on how to handle the certificate you
received.
4. Creating a self-signed test certificate
You can create a self-signed certificate if you don't want to deal
with a certificate authority, or if you 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
https://www.openssl.org/docs/apps/ca.html.
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 your 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 certificate and your private key and can start
using applications 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. It is important to
know what 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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HOWTO proxy certificates
0. WARNING
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 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
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.
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.
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.
For these reasons, OpenSSL requires that the use of proxy certificates be
explicitly allowed. Currently, this can be done using the following methods:
- if the application directly calls X509_verify_cert(), it can first call:
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().
- proxy certificate validation can be enabled before starting the application
by setting the environment variable OPENSSL_ALLOW_PROXY_CERTS.
In the future, it might be possible to enable proxy certificates by editing
openssl.cnf.
3. How to create proxy certificates
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.
basicConstraints=CA:FALSE
# Usual authority key ID
authorityKeyIdentifier=keyid,issuer:always
# 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 specify 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 following syntags are
recognised:
text indicates that the string is simply bytes, without any encoding:
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):
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 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 lines, e.g. XML documents).
The 'policy' setting can be split up in multiple lines like this:
0.policy=This is
1.policy= a multi-
2.policy=line policy.
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 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
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
4. How to have your application interpret the policy?
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 figuring out how to pass data between your
application and the certificate validation procedure.
You need the following ingredients:
- 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 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 is attached to an X509 validation context.
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. */
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 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
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