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Diffstat (limited to 'doc/doc-txt')
| -rw-r--r-- | doc/doc-txt/GnuTLS-FAQ.txt | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/doc/doc-txt/GnuTLS-FAQ.txt b/doc/doc-txt/GnuTLS-FAQ.txt index 60f402004..4339becac 100644 --- a/doc/doc-txt/GnuTLS-FAQ.txt +++ b/doc/doc-txt/GnuTLS-FAQ.txt @@ -232,6 +232,37 @@ security versus compatibility by raising it. A future release of Exim may even let the administrator tell GnuTLS to ask for more or less than "NORMAL". +To add to the fun, the size of the prime returned by GnuTLS when we call +gnutls_dh_params_generate2() is not limited to be the requested size. GnuTLS +has a tendency to overshoot. 2237 bit primes are common when 2236 is +requested, and higher still have been observed. Further, there is no API to +ask how large the prime bundled up inside the parameter is; the most we can do +is ask how large the DH prime used in an active TLS session is. Since we're +not able to use GnuTLS API calls (and exporting to PKCS3 and then calling +OpenSSL routines would be undiplomatic, plus add a library dependency), we're +left with no way to actually know the size of the freshly generated DH prime. + +Thus we check if the the value returned is at least 10 more than the minimum +we'll accept as a client (EXIM_CLIENT_DH_MIN_BITS, see below, defaults to +1024) and if it is, we subtract 10. Then we reluctantly deploy a strategy +called "hope". This is not guaranteed to be successful; in the first code +pass on this logic, we subtracted 3, asked for 2233 bits and got 2240 in the +first test. + +If you see Thunderbird clients still failing, then as a user who can see into +Exim's spool directory, run: + +$ openssl dhparam -noout -text -in /path/to/spool/gnutls-params-2236 | head + +Ideally, the first line will read "PKCS#3 DH Parameters: (2236 bit)". If the +count is more than 2236, then remove the file and let Exim regenerate it, or +generate one yourself and move it into place. Ideally use "openssl dhparam" +to generate it, and then wait a very long time; at least this way, the size +will be correct. (This developer is now convinced that Exim 4.81 should +bundle the suggested primes from a few RFCs and let the administrator choose +those.) + + A TLS client does not get to choose the DH prime used, but can choose a minimum acceptable value. For Exim, this is a compile-time constant called "EXIM_CLIENT_DH_MIN_BITS" of 1024, which can be overruled in "Local/Makefile". |