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.)
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 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
andSSL_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 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 (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 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 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.
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 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 ALSO
ssl(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.