=pod

=head1 NAME

SSL_get0_shared_sigalg, SSL_get0_sigalg,
SSL_get_shared_sigalgs, SSL_get_sigalgs -
get shared or peer signature algorithms

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_get0_sigalg(SSL *s, int idx, unsigned int *codepoint,
     const char **name);

 int SSL_get0_shared_sigalg(SSL *s, int idx, unsigned int *codepoint,
     const char **name);

 int SSL_get_shared_sigalgs(SSL *s, int idx,
                            int *psign, int *phash, int *psignhash,
                            unsigned char *rsig, unsigned char *rhash);

 int SSL_get_sigalgs(SSL *s, int idx,
                     int *psign, int *phash, int *psignhash,
                     unsigned char *rsig, unsigned char *rhash);

=head1 DESCRIPTION

SSL_get0_shared_sigalg() returns the names and codepoints of signature
algorithms shared with the peer in the SSL connection I<s>.
When I<idx> is negative, the number of algorithms is returned, and the output
parameters remain unmodified.
Otherwise, when I<idx> is nonnegative, it is the index of the shared
signature algorithm to return starting from zero.
The signature algorithm name is written to I<*name> (if I<name>) is not NULL).
The name reported is the IANA registered name for that algorithm, and is
expected to work verbatim if used in a B<SignatureAlgorithms> configuration
setting (see L<SSL_CONF_cmd(3)>).
The signature algorithm codepoint is written to I<*codepoint> (if I<codepoint>
is not NULL).
This function is better suited for inspecting TLS 1.3 signature algorithms than
the older SSL_get_shared_sigalgs().

SSL_get0_sigalg() returns the names and codepoints of signature
algorithms advertised by the peer in the SSL connection I<s>.
When I<idx> is negative, the number of algorithms is returned, and the output
parameters remain unmodified.
Otherwise, when I<idx> is nonnegative, it is the index of the peer
signature algorithm to return starting from zero.
For signature algorithms that are locally known, the name reported is the IANA
registered name for that algorithm, and is expected to work verbatim if used in
a B<SignatureAlgorithms> configuration setting (see L<SSL_CONF_cmd(3)>).
The signature algorithm codepoint is written to I<*codepoint> (if I<codepoint>
is not NULL).
If the signature algorithm is known, its name is written to I<*name> (if
I<name>) is not NULL).
This function is better suited for inspecting TLS 1.3 signature algorithms than
the older SSL_get_sigalgs().

SSL_get_shared_sigalgs() returns information about the supported signature
algorithms shared with the peer in the SSL connection I<s>.
When I<idx> is negative, the number of shared algorithms is returned, and
the output parameters remain unmodified.
Otherwise, when I<idx> is nonnegative, it is the index of the shared
signature algorithm to return starting from zero.
The signature algorithm NID is written to I<*psign>, the hash NID to I<*phash>
and the sign and hash NID to I<*psignhash>.
As of TLS 1.3 signature algorithms are not always a pairing of of a separate
public key algorithm and a digest algorithm, and so I<*phash> and I<*psignhash>
may not always be meaningful.
The raw B<hash> and B<signature> bytes of the signature algorithm codepoint are
written to I<*rhash> and I<*rsig>.
For example, the B<rsa_pkcs1_sha256> signature algorithm, with codepoint B<0x0401>,
gives B<0x04> for I<*rhash> and B<0x01> for I<*rsig>.

SSL_get_sigalgs() is similar to SSL_get_shared_sigalgs() except it returns
information about all signature algorithms advertised by the peer in the order
they were sent by the peer.

=head1 RETURN VALUES

SSL_get0_sigalg(), SSL_get0_shared_sigalg(), SSL_get_shared_sigalgs() and
SSL_get_sigalgs() return the number of signature algorithms or B<0> if the
I<idx> parameter is out of range (too large).

=head1 NOTES

These functions are typically called for debugging purposes (to report
the peer's preferences) or where an application wants finer control over
certificate selection. Most applications will rely on internal handling
and will not need to call them.

If an application is only interested in the highest preference shared
signature algorithm it can just set B<idx> to zero.

Any or all of the parameters B<psign>, B<phash>, B<psignhash>, B<rsig> or
B<rhash> can be set to NULL if the value is not required. By setting
them all to NULL and setting B<idx> to zero the total number of
signature algorithms can be determined: which can be zero.

These functions must be called after the peer has sent a list of supported
signature algorithms: after a client hello (for servers) or a certificate
request (for clients). They can (for example) be called in the certificate
callback.

Only TLS 1.2, TLS 1.3 and DTLS 1.2 currently support signature algorithms.
If these
functions are called on an earlier version of TLS or DTLS zero is returned.

The shared signature algorithms returned by SSL_get_shared_sigalgs() are
ordered according to configuration and peer preferences.

The raw values correspond to the on the wire form as defined by RFC5246 et al.
The NIDs are OpenSSL equivalents. For example if the peer sent sha256(4) and
rsa(1) then B<*rhash> would be 4, B<*rsign> 1, B<*phash> NID_sha256, B<*psig>
NID_rsaEncryption and B<*psignhash> NID_sha256WithRSAEncryption.

If a signature algorithm is not recognised the corresponding NIDs
will be set to B<NID_undef>. This may be because the value is not supported,
is not an appropriate combination (for example MD5 and DSA) or the
signature algorithm does not use a hash (for example Ed25519).

=head1 SEE ALSO

L<SSL_CTX_set_cert_cb(3)>,
L<ssl(7)>

=head1 HISTORY

Prior to OpenSSL 4.0 SSL_get_shared_sigalgs() treated negative I<idx> values
the same way as out-of-range (too large positive) values, and returned 0.

SSL_get0_shared_sigalg() and SSL_get0_sigalg() were added in OpenSSL
4.0.

=head1 COPYRIGHT

Copyright 2015-2026 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
L<https://www.openssl.org/source/license.html>.

=cut
