Document purpose and trust setting functions

In particular:
X509_STORE_CTX_set_purpose()
X509_STORE_CTX_set_trust();
X509_STORE_CTX_purpose_inherit();

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/17604)

1 files changed, 68 insertions, 1 deletions

diff --git a/doc/man3/X509_STORE_CTX_new.pod b/doc/man3/X509_STORE_CTX_new.pod
index aba7fff781..bd179e6274 100644
--- a/doc/man3/X509_STORE_CTX_new.pod
+++ b/doc/man3/X509_STORE_CTX_new.pod
@@ -11,7 +11,10 @@ X509_STORE_CTX_get0_untrusted, X509_STORE_CTX_set0_untrusted,
 X509_STORE_CTX_get_num_untrusted,
 X509_STORE_CTX_set_default,
 X509_STORE_CTX_set_verify,
-X509_STORE_CTX_verify_fn
+X509_STORE_CTX_verify_fn,
+X509_STORE_CTX_set_purpose,
+X509_STORE_CTX_set_trust,
+X509_STORE_CTX_purpose_inherit
 - X509_STORE_CTX initialisation
 
 =head1 SYNOPSIS
@@ -44,6 +47,11 @@ X509_STORE_CTX_verify_fn
  typedef int (*X509_STORE_CTX_verify_fn)(X509_STORE_CTX *);
  void X509_STORE_CTX_set_verify(X509_STORE_CTX *ctx, X509_STORE_CTX_verify_fn verify);
 
+ int X509_STORE_CTX_set_purpose(X509_STORE_CTX *ctx, int purpose);
+ int X509_STORE_CTX_set_trust(X509_STORE_CTX *ctx, int trust);
+ int X509_STORE_CTX_purpose_inherit(X509_STORE_CTX *ctx, int def_purpose,
+                                    int purpose, int trust);
+
 =head1 DESCRIPTION
 
 These functions initialise an B<X509_STORE_CTX> structure for subsequent use
@@ -120,6 +128,65 @@ following signature:
 This function should receive the current X509_STORE_CTX as a parameter and
 return 1 on success or 0 on failure.
 
+X509 certificates may contain information about what purposes keys contained
+within them can be used for. For example "TLS WWW Server Authentication" or
+"Email Protection". This "key usage" information is held internally to the
+certificate itself. In addition the trust store containing trusted certificates
+can declare what purposes we trust different certificates for. This "trust"
+information is not held within the certificate itself but is "meta" information
+held alongside it. This "meta" information is associated with the certificate
+after it is issued and could be determined by a system administrator. For
+example a certificate might declare that it is suitable for use for both
+"TLS WWW Server Authentication" and "TLS Client Authentication", but a system
+administrator might only trust it for the former. An X.509 certificate extension
+exists that can record extended key usage information to supplement the purpose
+information described above. This extended mechanism is arbitrarily extensible
+and not well suited for a generic library API; applications that need to
+validate extended key usage information in certifiates will need to define a
+custom "purpose" (see below) or supply a nondefault verification callback
+(L<X509_STORE_set_verify_cb_func(3)>).
+
+X509_STORE_CTX_set_purpose() sets the purpose for the target certificate being
+verified in the I<ctx>. Built-in available values for the I<purpose> argument
+are B<X509_PURPOSE_SSL_CLIENT>, B<X509_PURPOSE_SSL_SERVER>,
+B<X509_PURPOSE_NS_SSL_SERVER>, B<X509_PURPOSE_SMIME_SIGN>,
+B<X509_PURPOSE_SMIME_ENCRYPT>, B<X509_PURPOSE_CRL_SIGN>, B<X509_PURPOSE_ANY>,
+B<X509_PURPOSE_OCSP_HELPER> and B<X509_PURPOSE_TIMESTAMP_SIGN>. It is also
+possible to create a custom purpose value. Setting a purpose will ensure that
+the key usage declared within certificates in the chain being verified is
+consistent with that purpose as well as, potentially, other checks. Every
+purpose also has an associated default trust value which will also be set at the
+same time. During verification this trust setting will be verified to check it
+is consistent with the trust set by the system administrator for certificates in
+the chain.
+
+X509_STORE_CTX_set_trust() sets the trust value for the target certificate
+being verified in the I<ctx>. Built-in available values for the I<trust>
+argument are B<X509_TRUST_COMPAT>, B<X509_TRUST_SSL_CLIENT>,
+B<X509_TRUST_SSL_SERVER>, B<X509_TRUST_EMAIL>, B<X509_TRUST_OBJECT_SIGN>,
+B<X509_TRUST_OCSP_SIGN>, B<X509_TRUST_OCSP_REQUEST> and B<X509_TRUST_TSA>. It is
+also possible to create a custom trust value. Since X509_STORE_CTX_set_purpose()
+also sets the trust value it is normally sufficient to only call that function.
+If both are called then X509_STORE_CTX_set_trust() should be called after
+X509_STORE_CTX_set_purpose() since the trust setting of the last call will be
+used.
+
+It should not normally be necessary for end user applications to call
+X509_STORE_CTX_purpose_inherit() directly. Typically applications should call
+X509_STORE_CTX_set_purpose() or X509_STORE_CTX_set_trust() instead. Using this
+function it is possible to set the purpose and trust values for the I<ctx> at
+the same time. The I<def_purpose> and I<purpose> arguments can have the same
+purpose values as described for X509_STORE_CTX_set_purpose() above. The I<trust>
+argument can have the same trust values as described in
+X509_STORE_CTX_set_trust() above. Any of the I<def_purpose>, I<purpose> or
+I<trust> values may also have the value 0 to indicate that the supplied
+parameter should be ignored. After calling this function the purpose to be used
+for verification is set from the I<purpose> argument, and the trust is set from
+the I<trust> argument. If I<trust> is 0 then the trust value will be set from
+the default trust value for I<purpose>. If the default trust value for the
+purpose is I<X509_TRUST_DEFAULT> and I<trust> is 0 then the default trust value
+associated with the I<def_purpose> value is used for the trust setting instead.
+
 =head1 NOTES
 
 The certificates and CRLs in a store are used internally and should B<not>