X509_build_chain() - Re: Request for Openssl APIs to be used to sort the certificate chain

[Viktor Dukhovni]

On Mon, Oct 09, 2023 at 03:31:45PM +0200, David von Oheimb via openssl-users wrote:

> What likely comes very close to what you asked for is the function
> X509_build_chain() added in https://github.com/openssl/openssl/pull/14128.
> You can call it, e.g., like this:
> 
> chain = X509_build_chain(target_cert, candidate_certs, NULL /* truststore
> */, 1, NULL, NULL);
> 
> See https://www.openssl.org/docs/manmaster/man3/X509_build_chain.html for
> its man page.

Long-standing Prior practice is to add all the candidate certificates in
a "store" and/or as a list of candidate "untrusted" chain certificates,
and then attempt to "verify" the EE (leaf) certificate, as was done, for
example, the internal to the SSL library function
ssl_build_cert_chain():

    https://github.com/openssl/openssl/blob/OpenSSL_1_1_1-stable/ssl/ssl_cert.c#L756-L863

That function is quite general, and attends to various details (Suite-B,
security levels, ...) that most users don't need to bother with.  A much
shorter version can be specialised from its core elements:

    1. Initialise a verification context:

        https://github.com/openssl/openssl/blob/OpenSSL_1_1_1-stable/ssl/ssl_cert.c#L801-L804

    2. Request chain verification:

        https://github.com/openssl/openssl/blob/OpenSSL_1_1_1-stable/ssl/ssl_cert.c#L809

    3. Optionally clean up the error stack.

        https://github.com/openssl/openssl/blob/OpenSSL_1_1_1-stable/ssl/ssl_cert.c#L810-L812

    4. Request the constructed chain:

        https://github.com/openssl/openssl/blob/OpenSSL_1_1_1-stable/ssl/ssl_cert.c#L816-L817

The new X509_build_chain() is convenient, but not essential.
Constructing a chain is possible also with earlier releases of OpenSSL.

If you choose to present the candidate chain certificates as a "store",
you can also mark some of them as "trusted" (if not implicitly "trusted"
by virtue of being self-signed), by decorating them with one or more
trusted "purpose" OIDs, and then build a chain "up to" one of the
trusted certificates, rather than a root CA.

To add a trust OID to a certificate, call X509_add1_trust_object(), for
a manpage is sorely missing...

-- 
    Viktor.