SSL_CTX_set_verify
SSL_CTX_set_verify(3) OpenSSL SSL_CTX_set_verify(3)
NAME
SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth,
SSL_set_verify_depth - set peer certificate verification parameters
SYNOPSIS
#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);
DESCRIPTION
SSL_CTX_set_verify() sets the verification flags for ctx to be mode
and specifies the verify_callback function to be used. If no callback
function shall be specified, the NULL pointer can be used for ver-
ify_callback.
SSL_set_verify() sets the verification flags for ssl to be mode and
specifies the verify_callback function to be used. If no callback
function shall be specified, the NULL pointer can be used for ver-
ify_callback. In this case last verify_callback set specifically for
this ssl remains. If no special callback was set before, the default
callback for the underlying ctx is used, that was valid at the the
time ssl was created with SSL_new(3).
SSL_CTX_set_verify_depth() sets the maximum depth for the certificate
chain verification that shall be allowed for ctx. (See the BUGS sec-
tion.)
SSL_set_verify_depth() sets the maximum depth for the certificate
chain verification that shall be allowed for ssl. (See the BUGS sec-
tion.)
NOTES
The verification of certificates can be controlled by a set of logi-
cally or’ed mode flags:
SSL_VERIFY_NONE
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 dis-
abled), 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(3)
function. The handshake will be continued regardless of the veri-
fication result.
SSL_VERIFY_PEER
Server mode: the server sends a client certificate request to the
client. The certificate returned (if any) is checked. If the ver-
ification process fails, the TLS/SSL handshake is immediately ter-
minated with an alert message containing the reason for the veri-
fication failure. The behaviour can be controlled by the addi-
tional SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE
flags.
Client mode: the server certificate is verified. If the verifica-
tion process fails, the TLS/SSL handshake is immediately termi-
nated with an alert message containing the reason for the verifi-
cation failure. If no server certificate is sent, because an
anonymous cipher is used, SSL_VERIFY_PEER is ignored.
SSL_VERIFY_FAIL_IF_NO_PEER_CERT
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_VER-
IFY_PEER.
Client mode: ignored
SSL_VERIFY_CLIENT_ONCE
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
Exactly one of the mode flags SSL_VERIFY_NONE and 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 veri-
fication function set with SSL_CTX_set_cert_verify_callback(3). 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.
SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set the limit up
to which depth certificates in a chain are used during the verifica-
tion 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 behaviour when the
SSL_VERIFY_PEER flag is set. It must be supplied by the application
and receives two arguments: preverify_ok indicates, whether the veri-
fication of the certificate in question was passed (preverify_ok=1) or
not (preverify_ok=0). 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 cer-
tificate. At each level signatures and issuer attributes are checked.
Whenever a verification error is found, the error number is stored in
x509_ctx and verify_callback is called with preverify_ok=0. By apply-
ing X509_CTX_store_* functions verify_callback can locate the certifi-
cate in question and perform additional steps (see EXAMPLES). If no
error is found for a certificate, verify_callback is called with pre-
verify_ok=1 before advancing to the next level.
The return value of verify_callback controls the strategy of the fur-
ther verification process. If verify_callback returns 0, the verifica-
tion process is immediately stopped with "verification failed" state.
If SSL_VERIFY_PEER is set, a verification failure alert is sent to the
peer and the TLS/SSL handshake is terminated. 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 veri-
fication error using SSL_get_verify_result(3) 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 verifica-
tion failure will lead to a termination of the TLS/SSL handshake with
an alert message, if SSL_VERIFY_PEER is set.
BUGS
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 unex-
pected behaviour, if the SSL_VERIFY_PEER and SSL_VERIFY_NONE are not
used as required (exactly one must be set at any time).
The certificate verification depth set with SSL[_CTX]_verify_depth()
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.
RETURN VALUES
The SSL*_set_verify*() functions do not provide diagnostic informa-
tion.
EXAMPLES
The following code sequence realizes an example verify_callback func-
tion that will always continue the TLS/SSL handshake regardless of
verification failure, if wished. The callback realizes a verification
depth limit with more informational output.
All verification errors are printed, informations about the certifi-
cate chain are printed on request. The example is realized for a
server that does allow but not require client certificates.
The example makes use of the ex_data technique to store application
data into/retrieve application data from the SSL structure (see
SSL_get_ex_new_index(3), SSL_get_ex_data_X509_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 */
}
}
SEE ALSO
ssl(3), SSL_new(3), SSL_CTX_get_verify_mode(3), SSL_get_ver-
ify_result(3), SSL_CTX_load_verify_locations(3), SSL_get_peer_certifi-
cate(3), SSL_CTX_set_cert_verify_callback(3),
SSL_get_ex_data_X509_STORE_CTX_idx(3), SSL_get_ex_new_index(3)
0.9.7a 2002-12-04 SSL_CTX_set_verify(3)
