SSL_CTX_SET_VERIFY(3) BSD Library Functions Manual 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);
DESCRIPTIONSSL_CTX_set_verify() sets the verification flags for ctx to be mode and
specifies the verify_callback function to be used. If no callback func‐
tion 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 spec‐
ifies 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 underly‐
ing 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 sec‐
tion.)
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 bitwise
ORed 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 ver‐
ification 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 verifi‐
cation 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_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 pro‐
cedure. If the certificate chain is longer than allowed, the certifi‐
cates 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, allow‐
ing for the peer certificate and an 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 (pre‐
verify_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 equal to 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
equal to 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 verifica‐
tion 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.
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 verifica‐
tion 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 the 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 ALSOssl(3), SSL_CTX_get_verify_mode(3), SSL_CTX_load_verify_locations(3),
SSL_CTX_set_cert_verify_callback(3),
SSL_get_ex_data_X509_STORE_CTX_idx(3), SSL_get_ex_new_index(3),
SSL_get_peer_certificate(3), SSL_get_verify_result(3), SSL_new(3)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.
BSD April 24, 2024 BSD
