X509_build_chain, X509_verify_cert, X509_STORE_CTX_verify - build and verify X509 certificate chain

       Copyright 2009-2023 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>.

3.5.0                                              2025-06-04                             X509_VERIFY_CERT(3SSL)

X509_build_chain() builds a certificate chain starting from target using the optional list of
       intermediate CA certificates certs.  If store is NULL it builds the chain as far down as possible,
       ignoring errors.  Else the chain must reach a trust anchor contained in store.  It internally uses a
       X509_STORE_CTX structure associated with the library context libctx and property query string propq, both
       of which may be NULL.  In case there is more than one possibility for the chain, only one is taken.

       On success it returns a pointer to a new stack of (up_ref'ed) certificates starting with target and
       followed by all available intermediate certificates.  A self-signed trust anchor is included only if
       target is the trust anchor of with_self_signed is 1.  If a non-NULL stack is returned the caller is
       responsible for freeing it.

       The X509_verify_cert() function attempts to discover and validate a certificate chain based on parameters
       in ctx.  The verification context, of type X509_STORE_CTX, can be constructed using X509_STORE_CTX_new(3)
       and X509_STORE_CTX_init(3).  It usually includes a target certificate to be verified, a set of
       certificates serving as trust anchors, a list of non-trusted certificates that may be helpful for chain
       construction, flags such as X509_V_FLAG_X509_STRICT, and various other optional components such as a
       callback function that allows customizing the verification outcome.  A complete description of the
       certificate verification process is contained in the openssl-verification-options(1) manual page.

       Applications rarely call this function directly but it is used by OpenSSL internally for certificate
       validation, in both the S/MIME and SSL/TLS code.

       A negative return value from X509_verify_cert() can occur if it is invoked incorrectly, such as with no
       certificate set in ctx, or when it is called twice in succession without reinitialising ctx for the
       second call.  A negative return value can also happen due to internal resource problems or because an
       internal inconsistency has been detected.  Applications must interpret any return value <= 0 as an error.

       The X509_STORE_CTX_verify() behaves like X509_verify_cert() except that its target certificate is the
       first element of the list of untrusted certificates in ctx unless a target certificate is set explicitly.

       When the verification target is a raw public key, rather than a certificate, both functions validate the
       target raw public key.  In that case the number of possible checks is significantly reduced.  The raw
       public key can be authenticated only via DANE TLSA records, either locally synthesised or obtained by the
       application from DNS.  Raw public key DANE TLSA records may be added via SSL_add_expected_rpk(3) or
       SSL_dane_tlsa_add(3).

X509_build_chain() and X509_STORE_CTX_verify() were added in OpenSSL 3.0.

       X509_build_chain, X509_verify_cert, X509_STORE_CTX_verify - build and verify X509 certificate chain

X509_build_chain() returns NULL on error, else a stack of certificates.

       Both X509_verify_cert() and X509_STORE_CTX_verify() return 1 if a complete chain can be built and
       validated, otherwise they return 0, and in exceptional circumstances (such as malloc failure and internal
       errors) they can also return a negative code.

       If a complete chain can be built and validated both functions return 1.  If the certificate must be
       rejected on the basis of the data available or any required certificate status data is not available they
       return 0.  If no definite answer possible they usually return a negative code.

       On error or failure additional error information can be obtained by examining ctx using, for example,
       X509_STORE_CTX_get_error(3).  Even if verification indicated success, the stored error code may be
       different from X509_V_OK, likely because a verification callback function has waived the error.

SSL_add_expected_rpk(3), SSL_CTX_dane_enable(3), SSL_dane_tlsa_add(3), X509_STORE_CTX_new(3),
       X509_STORE_CTX_init(3), X509_STORE_CTX_init_rpk(3), X509_STORE_CTX_get_error(3)

        #include <openssl/x509_vfy.h>

        STACK_OF(X509) *X509_build_chain(X509 *target, STACK_OF(X509) *certs,
                                         X509_STORE *store, int with_self_signed,
                                         OSSL_LIB_CTX *libctx, const char *propq);
        int X509_verify_cert(X509_STORE_CTX *ctx);
        int X509_STORE_CTX_verify(X509_STORE_CTX *ctx);