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By default OpenSSL 1.1+ only allows signatures and ecdh/ecdhx from the
default list of X25519:secp256r1:X448:secp521r1:secp384r1. In
TLS1.3 key exchange is independent from the signature/key of the
certificates, so allowing all groups per default is not a sensible
choice anymore and instead a shorter list is reasonable.
However, when using certificates with exotic curves that are not on
the group list, the signatures of these certificates will no longer
be accepted.
The tls-groups option allows to modify the group list to account
for these corner cases.
Patch V2: Uses local gc_arena instead of malloc/free, reword commit
message. Fix other typos/clarify messages
Patch V3: Style fixes, adjust code to changes from mbedTLS session
fix
Patch V5: Fix compilation with OpenSSL 1.0.2
Patch V6: Redo the 'while((token = strsep(&tmp_groups, ":"))' change
which accidentally got lost.
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20200721154922.17144-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20521.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
37 lines
1.8 KiB
Plaintext
37 lines
1.8 KiB
Plaintext
Since 2.4.0, OpenVPN has official support for elliptic curve crypto. Elliptic
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curves are an alternative to RSA for asymmetric encryption.
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Elliptic curve crypto ('ECC') can be used for the ('TLS') control channel only
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in OpenVPN; the data channel (encrypting the actual network traffic) uses
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symmetric encryption. ECC can be used in TLS for authentication (ECDSA) and key
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exchange (ECDH).
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Key exchange (ECDH)
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-------------------
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OpenVPN 2.4.0 and newer automatically initialize ECDH parameters. When ECDSA is
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used for authentication, the curve used for the server certificate will be used
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for ECDH too. When autodetection fails (e.g. when using RSA certificates)
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OpenVPN lets the crypto library decide if possible, or falls back to the
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secp384r1 curve. The list of groups/curves that the crypto library will choose
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from can be set with the --tls-groups <grouplist> option.
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An administrator can force an OpenVPN/OpenSSL server to use a specific curve
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using the --ecdh-curve <curvename> option with one of the curves listed as
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available by the --show-groups option. Clients will use the same curve as
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selected by the server.
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Note that not all curves listed by --show-groups are available for use with TLS;
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in that case connecting will fail with a 'no shared cipher' TLS error.
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Authentication (ECDSA)
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----------------------
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Since OpenVPN 2.4.0, using ECDSA certificates works 'out of the box'. Which
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specific curves and cipher suites are available depends on your version and
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configuration of the crypto library. The crypto library will automatically
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select a cipher suite for the TLS control channel.
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Support for generating an ECDSA certificate chain is available in EasyRSA (in
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spite of it's name) since EasyRSA 3.0. The parameters you're looking for are
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'--use-algo=ec' and '--curve=<curve_name>'. See the EasyRSA documentation for
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more details on generating ECDSA certificates.
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