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.\" ========================================================================
.\"
.IX Title "SSL_CTX_set_verify 3"
.TH SSL_CTX_set_verify 3 "2019-12-20" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth, SSL_set_verify_depth \- set peer certificate verification parameters
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #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);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBSSL_CTX_set_verify()\fR sets the verification flags for \fBctx\fR to be \fBmode\fR and
specifies the \fBverify_callback\fR function to be used. If no callback function
shall be specified, the \s-1NULL\s0 pointer can be used for \fBverify_callback\fR.
.PP
\&\fBSSL_set_verify()\fR sets the verification flags for \fBssl\fR to be \fBmode\fR and
specifies the \fBverify_callback\fR function to be used. If no callback function
shall be specified, the \s-1NULL\s0 pointer can be used for \fBverify_callback\fR. In
this case last \fBverify_callback\fR set specifically for this \fBssl\fR remains. If
no special \fBcallback\fR was set before, the default callback for the underlying
\&\fBctx\fR is used, that was valid at the time \fBssl\fR was created with
\&\fBSSL_new\fR\|(3).
.PP
\&\fBSSL_CTX_set_verify_depth()\fR sets the maximum \fBdepth\fR for the certificate chain
verification that shall be allowed for \fBctx\fR. (See the \s-1BUGS\s0 section.)
.PP
\&\fBSSL_set_verify_depth()\fR sets the maximum \fBdepth\fR for the certificate chain
verification that shall be allowed for \fBssl\fR. (See the \s-1BUGS\s0 section.)
.SH "NOTES"
.IX Header "NOTES"
The verification of certificates can be controlled by a set of logically
or'ed \fBmode\fR flags:
.IP "\s-1SSL_VERIFY_NONE\s0" 4
.IX Item "SSL_VERIFY_NONE"
\&\fBServer mode:\fR the server will not send a client certificate request to the
client, so the client will not send a certificate.
.Sp
\&\fBClient mode:\fR 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 \s-1TLS/SSL\s0 handshake
using the \fBSSL_get_verify_result\fR\|(3) function.
The handshake will be continued regardless of the verification result.
.IP "\s-1SSL_VERIFY_PEER\s0" 4
.IX Item "SSL_VERIFY_PEER"
\&\fBServer mode:\fR the server sends a client certificate request to the client.
The certificate returned (if any) is checked. If the verification process
fails, the \s-1TLS/SSL\s0 handshake is
immediately terminated with an alert message containing the reason for
the verification failure.
The behaviour can be controlled by the additional
\&\s-1SSL_VERIFY_FAIL_IF_NO_PEER_CERT\s0 and \s-1SSL_VERIFY_CLIENT_ONCE\s0 flags.
.Sp
\&\fBClient mode:\fR the server certificate is verified. If the verification process
fails, the \s-1TLS/SSL\s0 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, \s-1SSL_VERIFY_PEER\s0 is ignored.
.IP "\s-1SSL_VERIFY_FAIL_IF_NO_PEER_CERT\s0" 4
.IX Item "SSL_VERIFY_FAIL_IF_NO_PEER_CERT"
\&\fBServer mode:\fR if the client did not return a certificate, the \s-1TLS/SSL\s0
handshake is immediately terminated with a \*(L"handshake failure\*(R" alert.
This flag must be used together with \s-1SSL_VERIFY_PEER.\s0
.Sp
\&\fBClient mode:\fR ignored
.IP "\s-1SSL_VERIFY_CLIENT_ONCE\s0" 4
.IX Item "SSL_VERIFY_CLIENT_ONCE"
\&\fBServer mode:\fR only request a client certificate on the initial \s-1TLS/SSL\s0
handshake. Do not ask for a client certificate again in case of a
renegotiation. This flag must be used together with \s-1SSL_VERIFY_PEER.\s0
.Sp
\&\fBClient mode:\fR ignored
.PP
Exactly one of the \fBmode\fR flags \s-1SSL_VERIFY_NONE\s0 and \s-1SSL_VERIFY_PEER\s0 must be
set at any time.
.PP
The actual verification procedure is performed either using the built-in
verification procedure or using another application provided verification
function set with
\&\fBSSL_CTX_set_cert_verify_callback\fR\|(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 \fBverify_callback()\fR function, but the way this information is used
may be different.
.PP
\&\fBSSL_CTX_set_verify_depth()\fR and \fBSSL_set_verify_depth()\fR 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 \*(L"level 0:peer certificate\*(R", \*(L"level 1: \s-1CA\s0 certificate\*(R",
\&\*(L"level 2: higher level \s-1CA\s0 certificate\*(R", 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 \s-1CA\s0 certificates.
.PP
The \fBverify_callback\fR function is used to control the behaviour when the
\&\s-1SSL_VERIFY_PEER\s0 flag is set. It must be supplied by the application and
receives two arguments: \fBpreverify_ok\fR indicates, whether the verification of
the certificate in question was passed (preverify_ok=1) or not
(preverify_ok=0). \fBx509_ctx\fR is a pointer to the complete context used
for the certificate chain verification.
.PP
The certificate chain is checked starting with the deepest nesting level
(the root \s-1CA\s0 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 \fBx509_ctx\fR
and \fBverify_callback\fR is called with \fBpreverify_ok\fR=0. By applying
X509_CTX_store_* functions \fBverify_callback\fR can locate the certificate
in question and perform additional steps (see \s-1EXAMPLES\s0). If no error is
found for a certificate, \fBverify_callback\fR is called with \fBpreverify_ok\fR=1
before advancing to the next level.
.PP
The return value of \fBverify_callback\fR controls the strategy of the further
verification process. If \fBverify_callback\fR returns 0, the verification
process is immediately stopped with \*(L"verification failed\*(R" state. If
\&\s-1SSL_VERIFY_PEER\s0 is set, a verification failure alert is sent to the peer and
the \s-1TLS/SSL\s0 handshake is terminated. If \fBverify_callback\fR returns 1,
the verification process is continued. If \fBverify_callback\fR always returns
1, the \s-1TLS/SSL\s0 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
\&\fBSSL_get_verify_result\fR\|(3) or by maintaining its
own error storage managed by \fBverify_callback\fR.
.PP
If no \fBverify_callback\fR is specified, the default callback will be used.
Its return value is identical to \fBpreverify_ok\fR, so that any verification
failure will lead to a termination of the \s-1TLS/SSL\s0 handshake with an
alert message, if \s-1SSL_VERIFY_PEER\s0 is set.
.SH "BUGS"
.IX Header "BUGS"
In client mode, it is not checked whether the \s-1SSL_VERIFY_PEER\s0 flag
is set, but whether \s-1SSL_VERIFY_NONE\s0 is not set. This can lead to
unexpected behaviour, if the \s-1SSL_VERIFY_PEER\s0 and \s-1SSL_VERIFY_NONE\s0 are not
used as required (exactly one must be set at any time).
.PP
The certificate verification depth set with SSL[_CTX]\fB_verify_depth()\fR
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.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
The SSL*_set_verify*() functions do not provide diagnostic information.
.SH "EXAMPLES"
.IX Header "EXAMPLES"
The following code sequence realizes an example \fBverify_callback\fR function
that will always continue the \s-1TLS/SSL\s0 handshake regardless of verification
failure, if wished. The callback realizes a verification depth limit with
more informational output.
.PP
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.
.PP
The example makes use of the ex_data technique to store application data
into/retrieve application data from the \s-1SSL\s0 structure
(see \fBSSL_get_ex_new_index\fR\|(3),
\&\fBSSL_get_ex_data_X509_STORE_CTX_idx\fR\|(3)).
.PP
.Vb 10
\& ...
\& 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\en", err,
\& X509_verify_cert_error_string(err), depth, buf);
\& }
\& else if (mydata\->verbose_mode)
\& {
\& printf("depth=%d:%s\en", 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\en", 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 */
\& }
\& }
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBssl\fR\|(3), \fBSSL_new\fR\|(3),
\&\fBSSL_CTX_get_verify_mode\fR\|(3),
\&\fBSSL_get_verify_result\fR\|(3),
\&\fBSSL_CTX_load_verify_locations\fR\|(3),
\&\fBSSL_get_peer_certificate\fR\|(3),
\&\fBSSL_CTX_set_cert_verify_callback\fR\|(3),
\&\fBSSL_get_ex_data_X509_STORE_CTX_idx\fR\|(3),
\&\fBSSL_get_ex_new_index\fR\|(3)