.\" $OpenBSD: X509_STORE_CTX_set_verify_cb.3,v 1.4 2018/03/22 17:38:08 schwarze Exp $ .\" OpenSSL a528d4f0 Oct 27 13:40:11 2015 -0400 .\" .\" This file was written by Dr. Stephen Henson . .\" Copyright (c) 2009, 2016 The OpenSSL Project. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: March 22 2018 $ .Dt X509_STORE_CTX_SET_VERIFY_CB 3 .Os .Sh NAME .Nm X509_STORE_CTX_set_verify_cb .Nd set verification callback .Sh SYNOPSIS .In openssl/x509_vfy.h .Ft void .Fo X509_STORE_CTX_set_verify_cb .Fa "X509_STORE_CTX *ctx" .Fa "int (*verify_cb)(int ok, X509_STORE_CTX *ctx)" .Fc .Sh DESCRIPTION .Fn X509_STORE_CTX_set_verify_cb sets the verification callback of .Fa ctx to .Fa verify_cb overwriting any existing callback. .Pp The verification callback can be used to customise the operation of certificate verification, either by overriding error conditions or logging errors for debugging purposes. .Pp However a verification callback is .Sy not essential and the default operation is often sufficient. .Pp The .Fa ok parameter to the callback indicates the value the callback should return to retain the default behaviour. If it is zero then an error condition is indicated. If it is 1 then no error occurred. If the flag .Dv X509_V_FLAG_NOTIFY_POLICY is set, then .Fa ok is set to 2 to indicate the policy checking is complete. .Pp The .Fa ctx parameter to the callback is the .Vt X509_STORE_CTX structure that is performing the verification operation. A callback can examine this structure and receive additional information about the error, for example by calling .Xr X509_STORE_CTX_get_current_cert 3 . Additional application data can be passed to the callback via the .Sy ex_data mechanism. .Pp The verification callback can be set and inherited from the parent structure performing the operation. In some cases (such as S/MIME verification) the .Vt X509_STORE_CTX structure is created and destroyed internally and the only way to set a custom verification callback is by inheriting it from the associated .Vt X509_STORE . .Sh RETURN VALUES .Fn X509_STORE_CTX_set_verify_cb does not return a value. .Sh EXAMPLES Default callback operation: .Bd -literal int verify_callback(int ok, X509_STORE_CTX *ctx) { return ok; } .Ed .Pp Simple example, suppose a certificate in the chain is expired and we wish to continue after this error: .Bd -literal int verify_callback(int ok, X509_STORE_CTX *ctx) { /* Tolerate certificate expiration */ if (X509_STORE_CTX_get_error(ctx) == X509_V_ERR_CERT_HAS_EXPIRED) return 1; /* Otherwise don't override */ return ok; } .Ed .Pp More complex example, we don't wish to continue after .Sy any certificate has expired just one specific case: .Bd -literal int verify_callback(int ok, X509_STORE_CTX *ctx) { int err = X509_STORE_CTX_get_error(ctx); X509 *err_cert = X509_STORE_CTX_get_current_cert(ctx); if (err == X509_V_ERR_CERT_HAS_EXPIRED) { if (check_is_acceptable_expired_cert(err_cert) return 1; } return ok; } .Ed .Pp Full featured logging callback. In this case the .Fa bio_err is assumed to be a global logging .Vt BIO , an alternative would to store a .Vt BIO in .Fa ctx using .Sy ex_data . .Bd -literal int verify_callback(int ok, X509_STORE_CTX *ctx) { X509 *err_cert; int err,depth; err_cert = X509_STORE_CTX_get_current_cert(ctx); err = X509_STORE_CTX_get_error(ctx); depth = X509_STORE_CTX_get_error_depth(ctx); BIO_printf(bio_err,"depth=%d ",depth); if (err_cert) { X509_NAME_print_ex(bio_err, X509_get_subject_name(err_cert), 0, XN_FLAG_ONELINE); BIO_puts(bio_err, "\en"); } else BIO_puts(bio_err, "\en"); if (!ok) BIO_printf(bio_err, "verify error:num=%d:%s\en", err, X509_verify_cert_error_string(err)); switch (err) { case X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT: BIO_puts(bio_err, "issuer= "); X509_NAME_print_ex(bio_err, X509_get_issuer_name(err_cert), 0, XN_FLAG_ONELINE); BIO_puts(bio_err, "\en"); break; case X509_V_ERR_CERT_NOT_YET_VALID: case X509_V_ERR_ERROR_IN_CERT_NOT_BEFORE_FIELD: BIO_printf(bio_err, "notBefore="); ASN1_TIME_print(bio_err, X509_get_notBefore(err_cert)); BIO_printf(bio_err, "\en"); break; case X509_V_ERR_CERT_HAS_EXPIRED: case X509_V_ERR_ERROR_IN_CERT_NOT_AFTER_FIELD: BIO_printf(bio_err, "notAfter="); ASN1_TIME_print(bio_err, X509_get_notAfter(err_cert)); BIO_printf(bio_err, "\en"); break; case X509_V_ERR_NO_EXPLICIT_POLICY: policies_print(bio_err, ctx); break; } if (err == X509_V_OK && ok == 2) /* print out policies */ BIO_printf(bio_err,"verify return:%d\en",ok); return(ok); } .Ed .Sh SEE ALSO .Xr X509_STORE_CTX_get_error 3 , .Xr X509_STORE_CTX_get_ex_new_index 3 , .Xr X509_STORE_set_verify_cb_func 3 .Sh HISTORY .Fn X509_STORE_CTX_set_verify_cb first appeared in OpenSSL 0.9.6c and has been available since .Ox 3.2 . .Sh CAVEATS In general a verification callback should .Sy NOT unconditionally return 1 in all circumstances because this will allow verification to succeed no matter what the error. This effectively removes all security from the application because .Sy any certificate (including untrusted generated ones) will be accepted.