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  • Committer: Teddy Hogeborn
  • Date: 2015-03-10 18:03:38 UTC
  • mto: (237.7.304 trunk)
  • mto: This revision was merged to the branch mainline in revision 325.
  • Revision ID: teddy@recompile.se-20150310180338-pcxw6r2qmw9k6br9
Add ":!RSA" to GnuTLS priority string, to disallow non-DHE kx.

If Mandos was somehow made to use a non-ephemeral Diffie-Hellman key
exchange algorithm in the TLS handshake, any saved network traffic
could then be decrypted later if the Mandos client key was obtained.
By default, Mandos uses ephemeral DH key exchanges which does not have
this problem, but a non-ephemeral key exchange algorithm was still
enabled by default.  The simplest solution is to simply turn that off,
which ensures that Mandos will always use ephemeral DH key exchanges.

There is a "PFS" priority string specifier, but we can't use it because:

1. Security-wise, it is a mix between "NORMAL" and "SECURE128" - it
   enables a lot more algorithms than "SECURE256".

2. It is only available since GnuTLS 3.2.4.

Thanks to Andreas Fischer <af@bantuX.org> for reporting this issue.

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  <para id="priority_compat">
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    GnuTLS priority string for the <acronym>TLS</acronym> handshake.
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    The default is <quote><literal
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    >SECURE256:!CTYPE-X.509:+CTYPE-OPENPGP:+SIGN-RSA-SHA224:</literal>
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    <literal>+SIGN-RSA-RMD160</literal></quote>.
 
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    >SECURE256:!CTYPE-X.509:+CTYPE-OPENPGP:!RSA</literal>
 
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    <literal>:+SIGN-RSA-SHA224:+SIGN-RSA-RMD160</literal></quote>.
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    See <citerefentry><refentrytitle
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    >gnutls_priority_init</refentrytitle>
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    <manvolnum>3</manvolnum></citerefentry> for the syntax.
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    <emphasis>Warning</emphasis>: changing this may make the
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    <acronym>TLS</acronym> handshake fail, making server-client
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    communication impossible.
 
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    communication impossible.  Changing this option may also make the
 
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    network traffic decryptable by an attacker.
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  </para>
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  <para id="priority">
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    GnuTLS priority string for the <acronym>TLS</acronym> handshake.
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    The default is <quote><literal
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    >SECURE256:!CTYPE-X.509:+CTYPE-OPENPGP</literal></quote>.  See
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    <citerefentry><refentrytitle >gnutls_priority_init</refentrytitle>
 
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    >SECURE256:!CTYPE-X.509:+CTYPE-OPENPGP:!RSA</literal></quote>.
 
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    See <citerefentry><refentrytitle
 
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    >gnutls_priority_init</refentrytitle>
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    <manvolnum>3</manvolnum></citerefentry> for the syntax.
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    <emphasis>Warning</emphasis>: changing this may make the
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    <acronym>TLS</acronym> handshake fail, making server-client
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    communication impossible.
 
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    communication impossible.  Changing this option may also make the
 
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    network traffic decryptable by an attacker.
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  </para>
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  <para id="servicename">