295 lines
11 KiB
Plaintext
295 lines
11 KiB
Plaintext
=pod
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=head1 NAME
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SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth, SSL_set_verify_depth - set peer certificate verification parameters
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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void SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
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int (*verify_callback)(int, X509_STORE_CTX *));
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void SSL_set_verify(SSL *s, int mode,
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int (*verify_callback)(int, X509_STORE_CTX *));
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void SSL_CTX_set_verify_depth(SSL_CTX *ctx,int depth);
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void SSL_set_verify_depth(SSL *s, int depth);
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int verify_callback(int preverify_ok, X509_STORE_CTX *x509_ctx);
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=head1 DESCRIPTION
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SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and
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specifies the B<verify_callback> function to be used. If no callback function
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shall be specified, the NULL pointer can be used for B<verify_callback>.
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SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and
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specifies the B<verify_callback> function to be used. If no callback function
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shall be specified, the NULL pointer can be used for B<verify_callback>. In
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this case last B<verify_callback> set specifically for this B<ssl> remains. If
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no special B<callback> was set before, the default callback for the underlying
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B<ctx> is used, that was valid at the the time B<ssl> was created with
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L<SSL_new(3)|SSL_new(3)>.
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SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain
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verification that shall be allowed for B<ctx>. (See the BUGS section.)
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SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain
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verification that shall be allowed for B<ssl>. (See the BUGS section.)
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=head1 NOTES
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The verification of certificates can be controlled by a set of logically
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or'ed B<mode> flags:
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=over 4
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=item SSL_VERIFY_NONE
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B<Server mode:> the server will not send a client certificate request to the
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client, so the client will not send a certificate.
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B<Client mode:> if not using an anonymous cipher (by default disabled), the
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server will send a certificate which will be checked. The result of the
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certificate verification process can be checked after the TLS/SSL handshake
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using the L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> function.
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The handshake will be continued regardless of the verification result.
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=item SSL_VERIFY_PEER
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B<Server mode:> the server sends a client certificate request to the client.
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The certificate returned (if any) is checked. If the verification process
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fails, the TLS/SSL handshake is
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immediately terminated with an alert message containing the reason for
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the verification failure.
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The behaviour can be controlled by the additional
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SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE flags.
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B<Client mode:> the server certificate is verified. If the verification process
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fails, the TLS/SSL handshake is
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immediately terminated with an alert message containing the reason for
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the verification failure. If no server certificate is sent, because an
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anonymous cipher is used, SSL_VERIFY_PEER is ignored.
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=item SSL_VERIFY_FAIL_IF_NO_PEER_CERT
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B<Server mode:> if the client did not return a certificate, the TLS/SSL
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handshake is immediately terminated with a "handshake failure" alert.
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This flag must be used together with SSL_VERIFY_PEER.
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B<Client mode:> ignored
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=item SSL_VERIFY_CLIENT_ONCE
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B<Server mode:> only request a client certificate on the initial TLS/SSL
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handshake. Do not ask for a client certificate again in case of a
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renegotiation. This flag must be used together with SSL_VERIFY_PEER.
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B<Client mode:> ignored
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=back
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Exactly one of the B<mode> flags SSL_VERIFY_NONE and SSL_VERIFY_PEER must be
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set at any time.
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The actual verification procedure is performed either using the built-in
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verification procedure or using another application provided verification
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function set with
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L<SSL_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(3)>.
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The following descriptions apply in the case of the built-in procedure. An
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application provided procedure also has access to the verify depth information
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and the verify_callback() function, but the way this information is used
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may be different.
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SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set the limit up
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to which depth certificates in a chain are used during the verification
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procedure. If the certificate chain is longer than allowed, the certificates
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above the limit are ignored. Error messages are generated as if these
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certificates would not be present, most likely a
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X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY will be issued.
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The depth count is "level 0:peer certificate", "level 1: CA certificate",
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"level 2: higher level CA certificate", and so on. Setting the maximum
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depth to 2 allows the levels 0, 1, and 2. The default depth limit is 9,
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allowing for the peer certificate and additional 9 CA certificates.
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The B<verify_callback> function is used to control the behaviour when the
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SSL_VERIFY_PEER flag is set. It must be supplied by the application and
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receives two arguments: B<preverify_ok> indicates, whether the verification of
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the certificate in question was passed (preverify_ok=1) or not
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(preverify_ok=0). B<x509_ctx> is a pointer to the complete context used
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for the certificate chain verification.
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The certificate chain is checked starting with the deepest nesting level
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(the root CA certificate) and worked upward to the peer's certificate.
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At each level signatures and issuer attributes are checked. Whenever
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a verification error is found, the error number is stored in B<x509_ctx>
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and B<verify_callback> is called with B<preverify_ok>=0. By applying
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X509_CTX_store_* functions B<verify_callback> can locate the certificate
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in question and perform additional steps (see EXAMPLES). If no error is
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found for a certificate, B<verify_callback> is called with B<preverify_ok>=1
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before advancing to the next level.
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The return value of B<verify_callback> controls the strategy of the further
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verification process. If B<verify_callback> returns 0, the verification
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process is immediately stopped with "verification failed" state. If
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SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and
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the TLS/SSL handshake is terminated. If B<verify_callback> returns 1,
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the verification process is continued. If B<verify_callback> always returns
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1, the TLS/SSL handshake will never be terminated because of this application
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experiencing a verification failure. The calling process can however
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retrieve the error code of the last verification error using
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L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> or by maintaining its
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own error storage managed by B<verify_callback>.
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If no B<verify_callback> is specified, the default callback will be used.
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Its return value is identical to B<preverify_ok>, so that any verification
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failure will lead to a termination of the TLS/SSL handshake with an
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alert message, if SSL_VERIFY_PEER is set.
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=head1 BUGS
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In client mode, it is not checked whether the SSL_VERIFY_PEER flag
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is set, but whether SSL_VERIFY_NONE is not set. This can lead to
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unexpected behaviour, if the SSL_VERIFY_PEER and SSL_VERIFY_NONE are not
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used as required (exactly one must be set at any time).
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The certificate verification depth set with SSL[_CTX]_verify_depth()
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stops the verification at a certain depth. The error message produced
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will be that of an incomplete certificate chain and not
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X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected.
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=head1 RETURN VALUES
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The SSL*_set_verify*() functions do not provide diagnostic information.
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=head1 EXAMPLES
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The following code sequence realizes an example B<verify_callback> function
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that will always continue the TLS/SSL handshake regardless of verification
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failure, if wished. The callback realizes a verification depth limit with
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more informational output.
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All verification errors are printed, informations about the certificate chain
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are printed on request.
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The example is realized for a server that does allow but not require client
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certificates.
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The example makes use of the ex_data technique to store application data
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into/retrieve application data from the SSL structure
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(see L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)>,
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L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>).
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...
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typedef struct {
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int verbose_mode;
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int verify_depth;
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int always_continue;
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} mydata_t;
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int mydata_index;
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...
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static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
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{
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char buf[256];
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X509 *err_cert;
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int err, depth;
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SSL *ssl;
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mydata_t *mydata;
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err_cert = X509_STORE_CTX_get_current_cert(ctx);
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err = X509_STORE_CTX_get_error(ctx);
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depth = X509_STORE_CTX_get_error_depth(ctx);
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/*
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* Retrieve the pointer to the SSL of the connection currently treated
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* and the application specific data stored into the SSL object.
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*/
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ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
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mydata = SSL_get_ex_data(ssl, mydata_index);
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X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);
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/*
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* Catch a too long certificate chain. The depth limit set using
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* SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so
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* that whenever the "depth>verify_depth" condition is met, we
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* have violated the limit and want to log this error condition.
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* We must do it here, because the CHAIN_TOO_LONG error would not
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* be found explicitly; only errors introduced by cutting off the
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* additional certificates would be logged.
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*/
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if (depth > mydata->verify_depth) {
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preverify_ok = 0;
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err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
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X509_STORE_CTX_set_error(ctx, err);
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}
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if (!preverify_ok) {
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printf("verify error:num=%d:%s:depth=%d:%s\n", err,
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X509_verify_cert_error_string(err), depth, buf);
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}
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else if (mydata->verbose_mode)
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{
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printf("depth=%d:%s\n", depth, buf);
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}
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/*
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* At this point, err contains the last verification error. We can use
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* it for something special
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*/
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if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)
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{
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X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert), buf, 256);
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printf("issuer= %s\n", buf);
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}
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if (mydata->always_continue)
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return 1;
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else
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return preverify_ok;
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}
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...
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mydata_t mydata;
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...
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mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL);
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...
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SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE,
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verify_callback);
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/*
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* Let the verify_callback catch the verify_depth error so that we get
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* an appropriate error in the logfile.
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*/
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SSL_CTX_set_verify_depth(verify_depth + 1);
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/*
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* Set up the SSL specific data into "mydata" and store it into th SSL
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* structure.
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*/
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mydata.verify_depth = verify_depth; ...
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SSL_set_ex_data(ssl, mydata_index, &mydata);
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...
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SSL_accept(ssl); /* check of success left out for clarity */
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if (peer = SSL_get_peer_certificate(ssl))
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{
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if (SSL_get_verify_result(ssl) == X509_V_OK)
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{
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/* The client sent a certificate which verified OK */
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}
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}
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=head1 SEE ALSO
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L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>,
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L<SSL_CTX_get_verify_mode(3)|SSL_CTX_get_verify_mode(3)>,
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L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>,
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L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>,
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L<SSL_get_peer_certificate(3)|SSL_get_peer_certificate(3)>,
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L<SSL_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(3)>,
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L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>,
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L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)>
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=cut
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