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
verify_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
verify_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 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 section.)
SSL_set_verify_depth() sets the maximum
depth for the certificate
chain verification that shall be allowed for
ssl. (See the BUGS
section.)
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(3) 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
behaviour 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
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 verification
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
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
100, allowing for the peer certificate and additional 100 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
verification 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 certificate. 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 applying
X509_CTX_store_* functions
verify_callback can locate the certificate
in question and perform additional steps (see EXAMPLES). 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 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 not be terminated with respect to verification
failures and the connection will be established. The calling process can
however retrieve the error code of the last verification 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 verification
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 unexpected 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 information.
EXAMPLES¶
The following code sequence realizes an example
verify_callback function
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; information about the certificate chain is
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_verify_result(3),
SSL_CTX_load_verify_locations(3),
SSL_get_peer_certificate(3),
SSL_CTX_set_cert_verify_callback(3),
SSL_get_ex_data_X509_STORE_CTX_idx(3),
SSL_get_ex_new_index(3)