Updated: 2022/Sep/29
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SSL_CTX_set_verify(3) OpenSSL SSL_CTX_set_verify(3)
NAME
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
LIBRARY
libcrypto, -lcrypto
SYNOPSIS
#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);
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). 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.
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, 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.
BUGS
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.
RETURN VALUES
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.
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
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 */
}
}
SEE ALSO
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)
HISTORY
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
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>.
3.0.12 2023-05-07 SSL_CTX_set_verify(3)