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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
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.
NOTES
The verification of certificates can be controlled by a set of logically
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
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.
SSL_VERIFY_PEER
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
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_VERIFY_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
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.
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 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.
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_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)
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