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SSL_CTX_set_verify(3)
Contents
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SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth,
SSL_set_verify_depth - Set peer certificate
verification parameters
#include <openssl/ssl.h>
void SSL_CTX_set_verify(
SSL_CTX *ctx,
int mode,
int (*verify_callback) (int, X509_STORE_CTX *) );
void SSL_set_verify(
SSL *s,
int mode,
int (*verify_callback) (int, X509_STORE_CTX *) );
void SSL_CTX_set_verify_depth(
SSL_CTX *ctx,
int depth ); void SSL_set_verify_depth(
SSL *s,
int depth ); int verify_callback(
int preverify_ok,
X509_STORE_CTX *x509_ctx );
The SSL_CTX_set_verify() function sets the verification
flags for ctx to be mode and specifies the verify_callback()
function to be used. If no callback function is
specified, the NULL pointer can be used for verify_callback().
The SSL_set_verify() function sets the verification flags
for ssl to be mode and specifies the verify_callback()
function to be used. If no callback function is specified,
the NULL pointer can be used for verify_callback(). In
this last case verify_callback set specifically for this
ssl remains. If no special callback was set, the default
callback for the underlying ctx that was valid at the the
time ssl was created with the SSL_new() function is used.
The SSL_CTX_set_verify_depth() function sets the maximum
depth for the certificate chain verification that will be
allowed for ctx.
The SSL_set_verify_depth() function sets the maximum depth
for the certificate chain verification that will be
allowed for ssl.
The verification of certificates can be controlled by a
set of logically or'ed mode flags: Server mode: the server
will not send a client certificate request to the client,
so the client will not send a certificate.
Client mode: if not using an anonymous cipher (by
default disabled), the server will send a certificate
which will be checked. The result of the certificate
verification process can be checked after
the TLS/SSL handshake using the SSL_get_verify_result()
function. The handshake will be continued
regardless of the verification result.
Server mode: the server sends a client certificate
request to the client. The certificate returned (if
any) is checked. If the verification process
fails, the TLS/SSL handshake is immediately
terminated with an alert message containing the
reason for the verification failure. The
behavior can be controlled by the additional
SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE
flags.
Client mode: the server certificate is verified. If
the verification process fails, the TLS/SSL handshake
is immediately terminated with an
alert message containing the reason for the verification
failure. If no server certificate is
sent, because an anonymous cipher is used, SSL_VERIFY_PEER
is ignored Server mode: if the client did
not return a certificate, the TLS/SSL handshake is
immediately terminated with a handshake failure
alert. This flag must be used together with
SSL_VERIFY_PEER.
Client mode: ignored Server mode: only request a
client certificate on the initial TLS/SSL handshake.
Do not ask for a client certificate again in
case of a renegotiation. This flag must be used
together with SSL_VERIFY_PEER.
Client mode: ignored
Either SSL_VERIFY_NONE or SSL_VERIFY_PEER must be set at
any time.
The actual verification procedure is performed either
using the built-in verification procedure or using another
application provided verification function set with the
SSL_CTX_set_cert_verify_callback() function. The following
descriptions apply in the case of the built-in procedure.
An application provided procedure also has access to the
verify depth information and the verify_callback() function,
but the way this information is used may be different.
The SSL_CTX_set_verify_depth() and SSL_set_verify_depth()
functions set the limit up to which depth certificates in
a chain are used during the verification procedure. If the
certificate chain is longer than allowed, the certificates
above the limit are ignored. Error messages are generated
as if these certificates would not be present. Most likely
a X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY will be
issued. The depth count is level 0:peer certificate, level
1: CA certificate, level 2: higher level CA certificate,
and so on. Setting the maximum depth to 2 allows the levels
0, 1, and 2. The default depth limit is 9, allowing
for the peer certificate and additional 9 CA certificates.
The verify_callback() function is used to control the
behavior when the SSL_VERIFY_PEER flag is set. It must be
supplied by the application and receives two arguments:
preverify_ok indicates, whether the verification of the
certificate in question was passed (preverify_ok=1) or not
(preverify_ok=0). The x509_ctx is a pointer to the complete
context used for the certificate chain verification.
The certificate chain is checked starting with the deepest
nesting level (the root CA certificate) and worked upward
to the peer's certificate. At each level signatures and
issuer attributes are checked. Whenever a verification
error is found, the error number is stored in x509_ctx.
The verify_callback() function is called with preverify_ok=0.
By applying X509_CTX_store_* functions, verify_callback
can locate the certificate in question and
perform additional steps. If no error is found for a certificate,
verify_callback() is called with preverify_ok=1
before advancing to the next level.
The return value of verify_callback() controls the strategy
of the further verification process. If verify_callback()
returns 0, the verification process stops with verification
failed state. If SSL_VERIFY_PEER is set, a verification
failure alert is sent to the peer and the TLS/SSL
handshake terminates. If verify_callback() returns 1, the
verification process is continued. If verify_callback()
always returns 1, the TLS/SSL handshake will never be terminated
because of this application experiencing a verification
failure. The calling process can, however,
retrieve the error code of the last verification error
using SSL_get_verify_result() or by maintaining its own
error storage managed by verify_callback().
If no verify_callback() is specified, the default callback
will be used. Its return value is identical to preverify_ok,
so that any verification failure will lead to a
termination of the TLS/SSL handshake with an alert message,
if SSL_VERIFY_PEER is set.
In client mode, it is not checked whether the SSL_VERIFY_PEER
flag is set, but whether SSL_VERIFY_NONE is not
set. This can lead to unexpected behavior, if the SSL_VERIFY_PEER
and SSL_VERIFY_NONE are not used as required (one
or the other must be set at any time).
The certificate verification depth set with the
SSL[_CTX]_verify_depth() function stops the verification
at a certain depth. The error message produced will be
that of an incomplete certificate chain and not
X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected.
The SSL*_set_verify*() functions do not provide diagnostic
information.
The following code sequence is an example of the verify_callback()
function that will always continue the
TLS/SSL handshake regardless of verification failure. The
callback realizes a verification depth limit with more
informational output.
All verification errors are printed. Information about the
certificate chain are printed on request. The example is
realized for a server that allows, but not require, client
certificates.
The example makes use of the ex_data technique to store
application data into or retrieve application data from
the SSL structure (see SSL_get_ex_new_index(3),
SSL_get_ex_data_X509(3), _STORE_CTX_idx(3)).
...
typedef struct {
int verbose_mode;
int verify_depth;
int always_continue;
} mydata_t;
int mydata_index;
...
static int verify_callback(int preverify_ok,
X509_STORE_CTX *ctx)
{
char buf[256];
X509 *err_cert;
int err, depth;
SSL *ssl;
mydata_t *mydata;
err_cert = X509_STORE_CTX_get_current_cert(ctx);
err = X509_STORE_CTX_get_error(ctx);
depth = X509_STORE_CTX_get_error_depth(ctx);
/*
* Retrieve the pointer to the SSL of the connection
currently treated
* and the application specific data stored into the
SSL object.
*/
ssl = X509_STORE_CTX_get_ex_data(ctx,
SSL_get_ex_data_X509_STORE_CTX_idx());
mydata = SSL_get_ex_data(ssl, mydata_index);
X509_NAME_oneline(X509_get_subject_name(err_cert),
buf, 256);
/*
* Catch a too long certificate chain. The depth limit
set using
* SSL_CTX_set_verify_depth() is by purpose set to
"limit+1" so
* that whenever the "depth>verify_depth" condition is
met, we
* have violated the limit and want to log this error
condition.
* We must do it here, because the CHAIN_TOO_LONG
error would not
* be found explicitly; only errors introduced by cutting
off the
* additional certificates would be logged.
*/
if (depth > mydata->verify_depth) {
preverify_ok = 0;
err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
X509_STORE_CTX_set_error(ctx, err);
}
if (!preverify_ok) {
printf("verify error:num=%d:%s:depth=%d:%s\n",
err,
X509_verify_cert_error_string(err),
depth, buf);
}
else if (mydata->verbose_mode)
{
printf("depth=%d:%s\n", depth, buf);
}
/*
* At this point, err contains the last verification
error. We can use
* it for something special
*/
if (!preverify_ok && (err ==
X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)
{
X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert),
buf, 256);
printf("issuer= %s\n", buf);
}
if (mydata->always_continue)
return 1;
else
return preverify_ok;
}
...
mydata_t mydata;
...
mydata_index = SSL_get_ex_new_index(0, "mydata index",
NULL, NULL, NULL);
...
SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE,
verify_callback);
/*
* Let the verify_callback catch the verify_depth error
so that we get
* an appropriate error in the logfile.
*/
SSL_CTX_set_verify_depth(verify_depth + 1);
/*
* Set up the SSL specific data into "mydata" and store
it into th SSL
* structure.
*/
mydata.verify_depth = verify_depth; ...
SSL_set_ex_data(ssl, mydata_index, &mydata);
...
SSL_accept(ssl); /* check of success left out for clarity
*/
if (peer = SSL_get_peer_certificate(ssl))
{
if (SSL_get_verify_result(ssl) == X509_V_OK)
{
/* The client sent a certificate which verified OK */
}
}
Functions: ssl(3), SSL_new(3), SSL_CTX_get_verify_mode(3),
SSL_get_verify_result(3), SSL_CTX_load_verify_locations(3), SSL_get_peer_certificate(3),
SSL_CTX_set_cert_verify_cb(3), SSL_get_ex_data_X509(3)
_STORE_CTX_idx(3), SSL_get_ex_new_index(3)
SSL_CTX_set_verify(3)
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