SSL_get_ex_data_X509_STORE_CTX_idx, SSL_CTX_set_verify, SSL_set_verify,
SSL_CTX_set_verify_depth, SSL_set_verify_depth, SSL_verify_cb,
SSL_verify_client_post_handshake, SSL_set_post_handshake_auth,
SSL_CTX_set_post_handshake_auth - set various SSL/TLS parameters for peer
certificate verification
#include <openssl/ssl.h>
typedef int (*SSL_verify_cb)(int preverify_ok, X509_STORE_CTX *x509_ctx);
void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, SSL_verify_cb verify_callback);
void SSL_set_verify(SSL *ssl, int mode, SSL_verify_cb verify_callback);
SSL_get_ex_data_X509_STORE_CTX_idx(void);
void SSL_CTX_set_verify_depth(SSL_CTX *ctx, int depth);
void SSL_set_verify_depth(SSL *ssl, int depth);
int SSL_verify_client_post_handshake(SSL *ssl);
void SSL_CTX_set_post_handshake_auth(SSL_CTX *ctx, int val);
void SSL_set_post_handshake_auth(SSL *ssl, int val);
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). Within
the callback function,
SSL_get_ex_data_X509_STORE_CTX_idx can be called
to get the data index of the current SSL object that is doing the
verification.
In client mode
verify_callback may also call the
SSL_set_retry_verify(3) function on the
SSL object set in the
x509_store_ctx ex data (see
SSL_get_ex_data_X509_STORE_CTX_idx(3)) and return 1. This would be
typically done in case the certificate verification was not yet able to
succeed. This makes the handshake suspend and return control to the calling
application with
SSL_ERROR_WANT_RETRY_VERIFY. The application can for
instance fetch further certificates or cert status information needed for the
verification. Calling
SSL_connect(3) again resumes the connection
attempt by retrying the server certificate verification step. This process may
even be repeated if need be. Note that the handshake may still be aborted if a
subsequent invocation of the callback (e.g., at a lower depth, or for a
separate error condition) returns 0.
SSL_CTX_set_verify_depth() sets the maximum
depth for the
certificate chain verification that shall be allowed for
ctx.
SSL_set_verify_depth() sets the maximum
depth for the certificate
chain verification that shall be allowed for
ssl.
SSL_CTX_set_post_handshake_auth() and
SSL_set_post_handshake_auth() enable the Post-Handshake Authentication
extension to be added to the ClientHello such that post-handshake
authentication can be requested by the server. If
val is 0 then the
extension is not sent, otherwise it is. By default the extension is not sent.
A certificate callback will need to be set via
SSL_CTX_set_client_cert_cb() if no certificate is provided at
initialization.
SSL_verify_client_post_handshake() causes a CertificateRequest message to
be sent by a server on the given
ssl connection. The SSL_VERIFY_PEER
flag must be set; the SSL_VERIFY_POST_HANDSHAKE flag is optional.
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, SSL_VERIFY_CLIENT_ONCE and
SSL_VERIFY_POST_HANDSHAKE 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 (see BUGS)
- SSL_VERIFY_CLIENT_ONCE
-
Server mode: only request a client certificate once
during the connection. Do not ask for a client certificate again during
renegotiation or post-authentication if a certificate was requested during
the initial handshake. This flag must be used together with
SSL_VERIFY_PEER.
Client mode: ignored (see BUGS)
- SSL_VERIFY_POST_HANDSHAKE
-
Server mode: the server will not send a client
certificate request during the initial handshake, but will send the
request via SSL_verify_client_post_handshake(). This allows the
SSL_CTX or SSL to be configured for post-handshake peer verification
before the handshake occurs. This flag must be used together with
SSL_VERIFY_PEER. TLSv1.3 only; no effect on pre-TLSv1.3 connections.
Client mode: ignored (see BUGS)
If the
mode is SSL_VERIFY_NONE none of the other flags may be set.
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 a limit
on the number of certificates between the end-entity and trust-anchor
certificates. Neither the end-entity nor the trust-anchor certificates count
against
depth. If the certificate chain needed to reach a trusted
issuer is longer than
depth+2, X509_V_ERR_CERT_CHAIN_TOO_LONG 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, 2 and 3 (0 being
the end-entity and 3 the trust-anchor). The default depth limit is 100,
allowing for the peer certificate, at most 100 intermediate CA certificates
and a final trust anchor certificate.
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.
After calling
SSL_set_post_handshake_auth(), the client will need to add
a certificate or certificate callback to its configuration before it can
successfully authenticate. This must be called before
SSL_connect().
SSL_verify_client_post_handshake() requires that verify flags have been
previously set, and that a client sent the post-handshake authentication
extension. When the client returns a certificate the verify callback will be
invoked. A write operation must take place for the Certificate Request to be
sent to the client, this can be done with
SSL_do_handshake() or
SSL_write_ex(). Only one certificate request may be outstanding at any
time.
When post-handshake authentication occurs, a refreshed NewSessionTicket message
is sent to the client.
In client mode, it is not checked whether the SSL_VERIFY_PEER flag is set, but
whether any flags other than SSL_VERIFY_NONE are set. This can lead to
unexpected behaviour if SSL_VERIFY_PEER and other flags are not used as
required.
The SSL*_set_verify*() functions do not provide diagnostic information.
The
SSL_verify_client_post_handshake() function returns 1 if the request
succeeded, and 0 if the request failed. The error stack can be examined to
determine the failure reason.
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
CRYPTO_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(err_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 */
}
}
ssl(7),
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_CTX_set_client_cert_cb(3),
CRYPTO_get_ex_new_index(3)
The SSL_VERIFY_POST_HANDSHAKE option, and the
SSL_verify_client_post_handshake() and
SSL_set_post_handshake_auth() functions were added in OpenSSL 1.1.1.
Copyright 2000-2022 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy in the
file LICENSE in the source distribution or at
<
https://www.openssl.org/source/license.html>.