Documentation for SSL_check_chain()

Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 6d5f8265ce6c4a8ed528462f519d9e8f2b7cfafd)

diff --git a/doc/ssl/SSL_check_chain.pod b/doc/ssl/SSL_check_chain.pod
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+=pod
+
+=head1 NAME
+
+SSL_check_chain - check certificate chain suitability
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ int SSL_check_chain(SSL *s, X509 *x, EVP_PKEY *pk, STACK_OF(X509) *chain);
+
+=head1 DESCRIPTION
+
+SSL_check_chain() checks whether certificate B<x>, private key B<pk> and
+certificate chain B<chain> is suitable for use with the current session
+B<s>.
+
+=head1 RETURN VALUES
+
+SSL_check_chain() returns a bitmap of flags indicating the validity of the
+chain.
+
+B<CERT_PKEY_VALID>: the chain can be used with the current session.
+If this flag is B<not> set then the certificate will never be used even
+if the application tries to set it because it is inconsistent with the
+peer preferences.
+
+B<CERT_PKEY_SIGN>: the EE key can be used for signing.
+
+B<CERT_PKEY_EE_SIGNATURE>: the signature algorithm of the EE certificate is
+acceptable.
+
+B<CERT_PKEY_CA_SIGNATURE>: the signature algorithms of all CA certificates
+are acceptable.
+
+B<CERT_PKEY_EE_PARAM>: the parameters of the end entity certificate are
+acceptable (e.g. it is a supported curve).
+
+B<CERT_PKEY_CA_PARAM>: the parameters of all CA certificates are acceptable.
+
+B<CERT_PKEY_EXPLICIT_SIGN>: the end entity certificate algorithm
+can be used explicitly for signing (i.e. it is mentioned in the signature
+algorithms extension).
+
+B<CERT_PKEY_ISSUER_NAME>: the issuer name is acceptable. This is only
+meaningful for client authentication.
+
+B<CERT_PKEY_CERT_TYPE>: the certificate type is acceptable. Only meaningful
+for client authentication.
+
+B<CERT_PKEY_SUITEB>: chain is suitable for Suite B use.
+
+=head1 NOTES
+
+SSL_check_chain() must be called in servers after a client hello message or in
+clients after a certificate request message. It will typically be called
+in the certificate callback.
+
+An application wishing to support multiple certificate chains may call this
+function on each chain in turn: starting with the one it considers the
+most secure. It could then use the chain of the first set which returns
+suitable flags.
+
+As a minimum the flag B<CERT_PKEY_VALID> must be set for a chain to be
+usable. An application supporting multiple chains with different CA signature
+algorithms may also wish to check B<CERT_PKEY_CA_SIGNATURE> too. If no
+chain is suitable a server should fall back to the most secure chain which
+sets B<CERT_PKEY_VALID>.
+
+The validity of a chain is determined by checking if it matches a supported
+signature algorithm, supported curves and in the case of client authentication
+certificate types and issuer names.
+
+Since the supported signature algorithms extension is only used in TLS 1.2
+and DTLS 1.2 the results for earlier versions of TLS and DTLS may not be
+very useful. Applications may wish to specify a different "legacy" chain
+for earlier versions of TLS or DTLS.
+
+=head1 SEE ALSO
+
+L<SSL_CTX_set_cert_cb(3)|SSL_CTX_set_cert_cb(3)>,
+L<ssl(3)|ssl(3)>
+
+=cut