Document signature algorithm setting functions.

Reviewed-by: NMatt Caswell <matt@openssl.org>

doc/ssl/SSL_CTX_set1_sigalgs.pod

+=pod
+
+=head1 NAME
+
+SSL_CTX_set1_sigalgs, SSL_set1_sigalgs, SSL_CTX_set1_sigalgs_list,
+SSL_set1_sigalgs_list, SSL_CTX_set1_client_sigalgs,
+SSL_set1_client_sigalgs, SSL_CTX_set1_client_sigalgs_list,
+SSL_set1_client_sigalgs_list - set supported signature algorithms
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ long SSL_CTX_set1_sigalgs(SSL_CTX *ctx, const int *slist, long slistlen);
+ long SSL_set1_sigalgs(SSL *ssl, const int *slist, long slistlen);
+ long SSL_CTX_set1_sigalgs_list(SSL_CTX *ctx, const char *str);
+ long SSL_set1_sigalgs_list(SSL *ssl, const char *str);
+
+ long SSL_CTX_set1_client_sigalgs(SSL_CTX *ctx, const int *slist, long slistlen);
+ long SSL_set1_client_sigalgs(SSL *ssl, const int *slist, long slistlen);
+ long SSL_CTX_set1_client_sigalgs_list(SSL_CTX *ctx, const char *str);
+ long SSL_set1_client_sigalgs_list(SSL *ssl, const char *str);
+
+=head1 DESCRIPTION
+
+SSL_CTX_set1_sigalgs() and SSL_set1_sigalgs() set the supported signature
+algorithms for B<ctx> or B<ssl>. The array B<slist> of length B<slistlen>
+must consist of pairs of NIDs corresponding to digest and public key
+algorithms.
+
+SSL_CTX_set1_sigalgs_list() and SSL_set1_sigalgs_list() set the supported
+signature algorithms for B<ctx> or B<ssl>. The B<str> parameter
+must be a null terminated string consisting or a colon separated list of
+public key algorithms and digests separated by B<+>.
+
+SSL_CTX_set1_client_sigalgs(), SSL_set1_client_sigalgs(),
+SSL_CTX_set1_client_sigalgs_list() and SSL_set1_client_sigalgs_list() set
+signature algorithms related to client authentication, otherwise they are
+identical to SSL_CTX_set1_sigalgs(), SSL_set1_sigalgs(),
+SSL_CTX_set1_sigalgs_list() and SSL_set1_sigalgs_list().
+
+All these functions are implemented as macros. The signature algorithm
+parameter (integer array or string) is not freed: the application should
+free it, if necessary.
+
+=head1 NOTES
+
+If an application wishes to allow the setting of signature algorithms
+as one of many user configurable options it should consider using the more
+flexible SSL_CONF API instead.
+
+The signature algorithms set by a client are used directly in the supported
+signature algorithm in the client hello message.
+
+The supported signature algorithms set by a server are not sent to the
+client but are used to determine the set of shared signature algorithms
+and (if server preferences are set with SSL_OP_CIPHER_SERVER_PREFERENCE)
+their order.
+
+The client authentication signature algorithms set by a server are sent
+in a certificate request message if client authentication is enabled,
+otherwise they are unused.
+
+Similarly client authentication signature algorithms set by a client are
+used to determined the set of client authentication shared signature
+algorithms.
+
+Signature algorithms will neither be advertised nor used if the security level
+prohibits them (for example SHA1 if the security level is 4 or more).
+
+Currently the NID_md5, NID_sha1, NID_sha224, NID_sha256, NID_sha384 and
+NID_sha512 digest NIDs are supported and the public key algorithm NIDs
+EVP_PKEY_RSA, EVP_PKEY_DSA and EVP_PKEY_EC.
+
+The short or long name values for digests can be used in a string (for
+example "MD5", "SHA1", "SHA224", "SHA256", "SHA384", "SHA512") and
+the public key algorithm strings "RSA", "DSA" or "ECDSA".
+
+The use of MD5 as a digest is strongly discouraged due to security weaknesses.
+
+=head1 EXAMPLES
+
+Set supported signature algoritms to SHA256 with ECDSA and SHA256 with RSA
+using an array:
+
+    const int slist[] = {NID_sha256, EVP_PKEY_EC, NID_sha256, EVP_PKEY_RSA};
+
+    SSL_CTX_set1_sigalgs(ctx, slist, 4);
+
+Set supported signature algoritms to SHA256 with ECDSA and SHA256 with RSA
+using a string:
+
+    SSL_CTX_set1_sigalgs_list(ctx, "ECDSA+SHA256:RSA+SHA256");
+
+=head1 RETURN VALUES
+
+All these functions return 1 for success and 0 for failure.
+
+=head1 SEE ALSO
+
+L<ssl(3)|ssl(3)>, L<SSL_get_shared_sigalgs(3)|SSL_get_shared_sigalgs(3)>,
+L<SSL_CONF_CTX_new(3)|SSL_CONF_CTX_new(3)>
+
+=cut