SSL_CTX_set_verify(3)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
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 ver‐
ify_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 cer‐
tificate chain verification that will be allowed for ssl.
NOTES
The verification of certificates can be controlled by a set of logi‐
cally 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 cer‐
tificate.
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_ver‐
ify_result() function. The handshake will be continued regard‐
less 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 veri‐
fication failure. The behavior can be controlled by the addi‐
tional SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VER‐
IFY_CLIENT_ONCE flags.
Client mode: the server certificate is verified. If the verifi‐
cation process fails, the TLS/SSL handshake is immediately ter‐
minated 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_VER‐
IFY_PEER.
Client mode: ignored Server mode: only request a client certifi‐
cate 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 verifi‐
cation function set with the SSL_CTX_set_cert_verify_callback() func‐
tion. The following descriptions apply in the case of the built-in pro‐
cedure. An application provided procedure also has access to the ver‐
ify 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 verifica‐
tion 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 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. The verify_callback() function is called with prever‐
ify_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 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 verifi‐
cation 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 veri‐
fication process is continued. If verify_callback() always returns 1,
the TLS/SSL handshake will never be terminated because of this applica‐
tion 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 veri‐
fication failure will lead to a termination of the TLS/SSL handshake
with an alert message, if SSL_VERIFY_PEER is set.
RESTRICTIONS
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 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]_ver‐
ify_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.
RETURN VALUES
The SSL*_set_verify*() functions do not provide diagnostic information.
EXAMPLES
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 */
}
}
SEE ALSO
Functions: 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_cb(3), SSL_get_ex_data_X509(3)_STORE_CTX_idx(3), SSL_get_ex_new_index(3)SSL_CTX_set_verify(3)
