Re: [PATCH] vpnmain.cgi: Mark MODP-1536 as broken, phase out MODP-2048

[Michael Tremer <michael.tremer@ipfire.org>]

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Hello Peter,

> On 6 Aug 2022, at 08:17, Peter Müller <peter.mueller(a)ipfire.org> wrote:
> 
> https://weakdh.org/imperfect-forward-secrecy-ccs15.pdf (released in
> 2015) recommends "to use primes of 2048 bits or larger", to which BSI's
> techical guideline BSI-TR-02102 (https://www.bsi.bund.de/SharedDocs/Downloads/DE/BSI/Publikationen/TechnischeRichtlinien/TR02102/BSI-TR-02102.pdf?__blob=publicationFile&v=5)
> concurs. The latter also recommends not to use DH groups comprising of
> less than 2000 bits after 2022, and shift to 3000 bit DH groups earlier
> as a precaution.
> 
> According to RFC 3526, section 8, MODP-1536 provides an estimated
> security between 90 and 120 bits, a value that can be reasonably
> considered broken today, as it has been so for other types of
> cryptographic algorithms already, and per section 2.4 in the
> aforementioned paper, breaking 1024-bit DH is considered feasible for
> the NSA in 2015, which does not inspire confidence for MODP-1536 in
> 2022.

I generally agree with that statement.

In my personal opinion, I believe that we should not be using any of the MODP options any more. Not because they are inherently broken, but we would need longer key lengths which are incredibly hard to generate for >=4096 bits - depending on your hardware. Very often, IPFire is not talking to another IPFire instance, and that is where it gets really interesting.

However, having said that there are lots of bad devices out there, plenty don’t support any ECC - not even speaking of Curve25519. They have no HWRNGs and slow CPUs which are unable to even produce a 2048 bit prime - but many of them barely manage.

The reason why MODP-2048 is enabled is, because it makes IPFire more compatible with other devices out there. The intention is that a tunnel comes up without the user visiting the “Advanced Settings”. We do not show any indication in the web UI at all if there is a cipher mismatch. So how else is the user supposed to find out?

> Therefore, this patch suggests to mark MODP-1536 as broken, since it
> de facto is, and tag MODP-2048 as weak. The latter is also removed from
> the default selection, so newly created VPN connections won't use it
> anymore, to follow BSI's recommendations of using DH groups >= 3000 bits
> in 2022 and later.

I agree with the recommendation, although MODP-3072 is probably a bit pointless. It feels too odd to me, and I am sure that devices that support anything better than 2048 would support 4096 as well.

I agree with 2048 being marked as weak, but I do not agree with taking it out of the default list. It shouldn’t be on there, but we would make the setup process a lot more complicated for users.

-Michael

> Signed-off-by: Peter Müller <peter.mueller(a)ipfire.org>
> ---
> html/cgi-bin/vpnmain.cgi | 14 +++++++-------
> 1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/html/cgi-bin/vpnmain.cgi b/html/cgi-bin/vpnmain.cgi
> index 3652627e9..9828b2f9e 100644
> --- a/html/cgi-bin/vpnmain.cgi
> +++ b/html/cgi-bin/vpnmain.cgi
> @@ -2,7 +2,7 @@
> ###############################################################################
> #                                                                             #
> # IPFire.org - A linux based firewall                                         #
> -# Copyright (C) 2007-2020  IPFire Team  <info(a)ipfire.org>                     #
> +# Copyright (C) 2007-2022  IPFire Team  <info(a)ipfire.org>                     #
> #                                                                             #
> # This program is free software: you can redistribute it and/or modify        #
> # it under the terms of the GNU General Public License as published by        #
> @@ -2360,11 +2360,11 @@ END
> 	#use default advanced value
> 	$cgiparams{'IKE_ENCRYPTION'}	= 'chacha20poly1305|aes256gcm128|aes256gcm96|aes256gcm64|aes256|aes192gcm128|aes192gcm96|aes192gcm64|aes192|aes128gcm128|aes128gcm96|aes128gcm64|aes128'; #[18];
> 	$cgiparams{'IKE_INTEGRITY'}		= 'sha2_512|sha2_256'; #[19];
> -	$cgiparams{'IKE_GROUPTYPE'}             = 'curve448|curve25519|4096|3072|2048'; #[20];
> +	$cgiparams{'IKE_GROUPTYPE'}             = 'curve448|curve25519|4096|3072'; #[20];
> 	$cgiparams{'IKE_LIFETIME'}		= '3'; #[16];
> 	$cgiparams{'ESP_ENCRYPTION'}	= 'chacha20poly1305|aes256gcm128|aes256gcm96|aes256gcm64|aes256|aes192gcm128|aes192gcm96|aes192gcm64|aes192|aes128gcm128|aes128gcm96|aes128gcm64|aes128'; #[21];
> 	$cgiparams{'ESP_INTEGRITY'}		= 'sha2_512|sha2_256'; #[22];
> -	$cgiparams{'ESP_GROUPTYPE'}             = 'curve448|curve25519|4096|3072|2048'; #[23];
> +	$cgiparams{'ESP_GROUPTYPE'}             = 'curve448|curve25519|4096|3072'; #[23];
> 	$cgiparams{'ESP_KEYLIFE'}		= '1'; #[17];
> 	$cgiparams{'COMPRESSION'}		= 'off'; #[13];
> 	$cgiparams{'ONLY_PROPOSED'}		= 'on'; #[24];
> @@ -3146,8 +3146,8 @@ if(($cgiparams{'ACTION'} eq $Lang::tr{'advanced'}) ||
> 					<option value='6144' $checked{'IKE_GROUPTYPE'}{'6144'}>MODP-6144</option>
> 					<option value='4096' $checked{'IKE_GROUPTYPE'}{'4096'}>MODP-4096</option>
> 					<option value='3072' $checked{'IKE_GROUPTYPE'}{'3072'}>MODP-3072</option>
> -					<option value='2048' $checked{'IKE_GROUPTYPE'}{'2048'}>MODP-2048</option>
> -					<option value='1536' $checked{'IKE_GROUPTYPE'}{'1536'}>MODP-1536</option>
> +					<option value='2048' $checked{'IKE_GROUPTYPE'}{'2048'}>MODP-2048 ($Lang::tr{'vpn weak'})</option>
> +					<option value='1536' $checked{'IKE_GROUPTYPE'}{'1536'}>MODP-1536 ($Lang::tr{'vpn broken'})</option>
> 					<option value='1024' $checked{'IKE_GROUPTYPE'}{'1024'}>MODP-1024 ($Lang::tr{'vpn broken'})</option>
> 					<option value='768' $checked{'IKE_GROUPTYPE'}{'768'}>MODP-768 ($Lang::tr{'vpn broken'})</option>
> 				</select>
> @@ -3169,8 +3169,8 @@ if(($cgiparams{'ACTION'} eq $Lang::tr{'advanced'}) ||
> 					<option value='6144' $checked{'ESP_GROUPTYPE'}{'6144'}>MODP-6144</option>
> 					<option value='4096' $checked{'ESP_GROUPTYPE'}{'4096'}>MODP-4096</option>
> 					<option value='3072' $checked{'ESP_GROUPTYPE'}{'3072'}>MODP-3072</option>
> -					<option value='2048' $checked{'ESP_GROUPTYPE'}{'2048'}>MODP-2048</option>
> -					<option value='1536' $checked{'ESP_GROUPTYPE'}{'1536'}>MODP-1536</option>
> +					<option value='2048' $checked{'ESP_GROUPTYPE'}{'2048'}>MODP-2048 ($Lang::tr{'vpn weak'})</option>
> +					<option value='1536' $checked{'ESP_GROUPTYPE'}{'1536'}>MODP-1536 ($Lang::tr{'vpn broken'})</option>
> 					<option value='1024' $checked{'ESP_GROUPTYPE'}{'1024'}>MODP-1024 ($Lang::tr{'vpn broken'})</option>
> 					<option value='768' $checked{'ESP_GROUPTYPE'}{'768'}>MODP-768 ($Lang::tr{'vpn broken'})</option>
> 					<option value='none' $checked{'ESP_GROUPTYPE'}{'none'}>- $Lang::tr{'none'} -</option>
> -- 
> 2.35.3