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2ec4e95014
The documentation currently describes SSL_CTX_set1_groups as a preference order, but this does not match the typical interpretation of "preference order" in OpenSSL and TLS. Typically, an application can order more secure options ahead of less secure ones and pick up TLS's usual downgrade protection guarantees. TLS 1.3 servers need to balance an additional consideration: some options will perform worse than others due to key share prediction. The prototypical selection procedure is to first select the set of more secure options, then select the most performant among those. OpenSSL follows this procedure, but it *unconditionally* treats all configured curves as equivalent security. Per discussion on GitHub, OpenSSL's position is that this is an intended behavior. While not supported by built-in providers, OpenSSL now documents that external providers can extend the group list and CHANGES.md explicitly cites post-quantum as a use case. With post-quantum providers, it's unlikely that application developers actually wanted options to be equivalent security. To avoid security vulnerabilities arising from mismatched expectations, update the documentation to clarify the server behavior. Per the OTC decision in https://github.com/openssl/openssl/issues/22203#issuecomment-1744465829, this documentation fix should be backported to stable branches. Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com> Reviewed-by: Tomas Mraz <tomas@openssl.org> (Merged from https://github.com/openssl/openssl/pull/23776)
187 lines
8.4 KiB
Plaintext
187 lines
8.4 KiB
Plaintext
=pod
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=head1 NAME
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SSL_CTX_set1_groups, SSL_CTX_set1_groups_list, SSL_set1_groups,
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SSL_set1_groups_list, SSL_get1_groups, SSL_get0_iana_groups,
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SSL_get_shared_group, SSL_get_negotiated_group, SSL_CTX_set1_curves,
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SSL_CTX_set1_curves_list, SSL_set1_curves, SSL_set1_curves_list,
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SSL_get1_curves, SSL_get_shared_curve
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- EC supported curve functions
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_CTX_set1_groups(SSL_CTX *ctx, int *glist, int glistlen);
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int SSL_CTX_set1_groups_list(SSL_CTX *ctx, char *list);
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int SSL_set1_groups(SSL *ssl, int *glist, int glistlen);
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int SSL_set1_groups_list(SSL *ssl, char *list);
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int SSL_get1_groups(SSL *ssl, int *groups);
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int SSL_get0_iana_groups(SSL *ssl, uint16_t **out);
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int SSL_get_shared_group(SSL *s, int n);
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int SSL_get_negotiated_group(SSL *s);
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int SSL_CTX_set1_curves(SSL_CTX *ctx, int *clist, int clistlen);
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int SSL_CTX_set1_curves_list(SSL_CTX *ctx, char *list);
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int SSL_set1_curves(SSL *ssl, int *clist, int clistlen);
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int SSL_set1_curves_list(SSL *ssl, char *list);
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int SSL_get1_curves(SSL *ssl, int *curves);
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int SSL_get_shared_curve(SSL *s, int n);
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=head1 DESCRIPTION
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For all of the functions below that set the supported groups there must be at
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least one group in the list. A number of these functions identify groups via a
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unique integer NID value. However, support for some groups may be added by
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external providers. In this case there will be no NID assigned for the group.
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When setting such groups applications should use the "list" form of these
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functions (i.e. SSL_CTX_set1_groups_list() and SSL_set1_groups_list).
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SSL_CTX_set1_groups() sets the supported groups for B<ctx> to B<glistlen>
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groups in the array B<glist>. The array consist of all NIDs of supported groups.
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Currently supported groups for B<TLSv1.3> are B<NID_X9_62_prime256v1>,
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B<NID_secp384r1>, B<NID_secp521r1>, B<NID_X25519>, B<NID_X448>,
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B<NID_brainpoolP256r1tls13>, B<NID_brainpoolP384r1tls13>,
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B<NID_brainpoolP512r1tls13>, B<NID_ffdhe2048>, B<NID_ffdhe3072>,
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B<NID_ffdhe4096>, B<NID_ffdhe6144> and B<NID_ffdhe8192>.
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OpenSSL will use this array in different ways depending on TLS role and version:
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=over 4
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=item For a TLS client, the groups are used directly in the supported groups
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extension. The extension's preference order, to be evaluated by the server, is
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determined by the order of the elements in the array.
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=item For a TLS 1.2 server, the groups determine the selected group. If
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B<SSL_OP_CIPHER_SERVER_PREFERENCE> is set, the order of the elements in the
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array determines the selected group. Otherwise, the order is ignored and the
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client's order determines the selection.
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=item For a TLS 1.3 server, the groups determine the selected group, but
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selection is more complex. A TLS 1.3 client sends both a group list as well as a
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predicted subset of groups. Choosing a group outside the predicted subset incurs
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an extra roundtrip. However, in some situations, the most preferred group may
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not be predicted. OpenSSL considers all supported groups to be comparable in
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security and prioritizes avoiding roundtrips above either client or server
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preference order. If an application uses an external provider to extend OpenSSL
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with, e.g., a post-quantum algorithm, this behavior may allow a network attacker
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to downgrade connections to a weaker algorithm.
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=back
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SSL_CTX_set1_groups_list() sets the supported groups for B<ctx> to
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string B<list>. The string is a colon separated list of group names, for example
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"P-521:P-384:P-256:X25519:ffdhe2048". The groups are used as in
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SSL_CTX_set1_groups(), described above. Currently supported groups for
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B<TLSv1.3> are B<P-256>, B<P-384>, B<P-521>, B<X25519>, B<X448>,
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B<brainpoolP256r1tls13>, B<brainpoolP384r1tls13>, B<brainpoolP512r1tls13>,
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B<ffdhe2048>, B<ffdhe3072>, B<ffdhe4096>, B<ffdhe6144> and B<ffdhe8192>. Support
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for other groups may be added by external providers, however note the discussion
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on TLS 1.3 selection criteria above. If a group name is preceded with the C<?>
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character, it will be ignored if an implementation is missing.
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SSL_set1_groups() and SSL_set1_groups_list() are similar except they set
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supported groups for the SSL structure B<ssl>.
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SSL_get1_groups() returns the set of supported groups sent by a client
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in the supported groups extension. It returns the total number of
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supported groups. The B<groups> parameter can be B<NULL> to simply
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return the number of groups for memory allocation purposes. The
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B<groups> array is in the form of a set of group NIDs in preference
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order. It can return zero if the client did not send a supported groups
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extension. If a supported group NID is unknown then the value is set to the
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bitwise OR of TLSEXT_nid_unknown (0x1000000) and the id of the group.
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SSL_get0_iana_groups() retrieves the list of groups sent by the
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client in the supported_groups extension. The B<*out> array of bytes
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is populated with the host-byte-order representation of the uint16_t group
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identifiers, as assigned by IANA. The group list is returned in the same order
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that was received in the ClientHello. The return value is the number of groups,
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not the number of bytes written.
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SSL_get_shared_group() returns the NID of the shared group B<n> for a
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server-side SSL B<ssl>. If B<n> is -1 then the total number of shared groups is
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returned, which may be zero. Other than for diagnostic purposes,
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most applications will only be interested in the first shared group
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so B<n> is normally set to zero. If the value B<n> is out of range,
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NID_undef is returned. If the NID for the shared group is unknown then the value
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is set to the bitwise OR of TLSEXT_nid_unknown (0x1000000) and the id of the
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group.
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SSL_get_negotiated_group() returns the NID of the negotiated group used for
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the handshake key exchange process. For TLSv1.3 connections this typically
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reflects the state of the current connection, though in the case of PSK-only
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resumption, the returned value will be from a previous connection. For earlier
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TLS versions, when a session has been resumed, it always reflects the group
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used for key exchange during the initial handshake (otherwise it is from the
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current, non-resumption, connection). This can be called by either client or
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server. If the NID for the shared group is unknown then the value is set to the
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bitwise OR of TLSEXT_nid_unknown (0x1000000) and the id of the group.
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All these functions are implemented as macros.
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The curve functions are synonyms for the equivalently named group functions and
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are identical in every respect. They exist because, prior to TLS1.3, there was
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only the concept of supported curves. In TLS1.3 this was renamed to supported
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groups, and extended to include Diffie Hellman groups. The group functions
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should be used in preference.
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=head1 NOTES
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If an application wishes to make use of several of these functions for
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configuration purposes either on a command line or in a file it should
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consider using the SSL_CONF interface instead of manually parsing options.
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=head1 RETURN VALUES
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SSL_CTX_set1_groups(), SSL_CTX_set1_groups_list(), SSL_set1_groups() and
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SSL_set1_groups_list(), return 1 for success and 0 for failure.
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SSL_get1_groups() returns the number of groups, which may be zero.
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SSL_get0_iana_groups() returns the number of (uint16_t) groups, which may be zero.
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SSL_get_shared_group() returns the NID of shared group B<n> or NID_undef if there
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is no shared group B<n>; or the total number of shared groups if B<n>
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is -1.
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When called on a client B<ssl>, SSL_get_shared_group() has no meaning and
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returns -1.
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SSL_get_negotiated_group() returns the NID of the negotiated group used for
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key exchange, or NID_undef if there was no negotiated group.
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=head1 SEE ALSO
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L<ssl(7)>,
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L<SSL_CTX_add_extra_chain_cert(3)>
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=head1 HISTORY
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The curve functions were added in OpenSSL 1.0.2. The equivalent group
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functions were added in OpenSSL 1.1.1. The SSL_get_negotiated_group() function
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was added in OpenSSL 3.0.0.
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Support for ignoring unknown groups in SSL_CTX_set1_groups_list() and
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SSL_set1_groups_list() was added in OpenSSL 3.3.
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Earlier versions of this document described the list as a preference order.
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However, OpenSSL's behavior as a TLS 1.3 server is to consider I<all>
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supported groups as comparable in security.
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=head1 COPYRIGHT
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Copyright 2013-2024 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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