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Description of problem:
If the server is using 2047 or 3071 bit RSA keys, mod_ssl will choose 1024 and 2048 bit DHE params respectively.
Version-Release number of selected component (if applicable):
httpd-2.4.6-16.el7
How reproducible:
Always
Steps to Reproduce:
SSLCRT=/etc/pki/tls/certs/localhost.crt
SSLKEY=/etc/pki/tls/private/localhost.key
openssl req -x509 -newkey rsa:2047 -keyout ${SSLKEY} -out ${SSLCRT} -subj /CN=localhost -nodes -batch
systemctl restart httpd
openssl s_client -CAfile ${SSLCRT} -cipher 'kEDH:!aNULL' -connect localhost:443
Actual results:
---
No client certificate CA names sent
Server Temp Key: DH, 1024 bits
---
SSL handshake has read 1459 bytes and written 422 bytes
---
New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-GCM-SHA384
Server public key is 2047 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
Protocol : TLSv1.2
Cipher : DHE-RSA-AES256-GCM-SHA384
Session-ID: 33D1A9CFA4B2489CA470D92F06F66FF999C19191F2CC9EAF6A21AEB43CCCC3DD
Session-ID-ctx:
Master-Key: 8398AF230C3E4178307E487F350FCF33651B850DD27B9E6B4F1B113BC7ECEE38140034B0CE1BD077BA3A6B3A947D839B
Key-Arg : None
Krb5 Principal: None
PSK identity: None
PSK identity hint: None
Start Time: 1394039024
Timeout : 300 (sec)
Verify return code: 0 (ok)
Expected results:
---
No client certificate CA names sent
Server Temp Key: DH, 2048 bits
---
Additional info:
This is problematic because there are multiple applications that sometimes generate 2047 keys while asked for 2048 bit keys (similarly for 1024 bit keys, etc.) [2,3]. On the other hand, there are no known applications that will generate 2049 bit keys when asked for 2048 bit keys.
We also don't want the DHE key exchange to be the weakest link in the connection, see NIST SP 800-56 [1] Section 5.6.3 for rationale.
1 - http://csrc.nist.gov/publications/nistpubs/800-57/sp800-57_part1_rev3_general.pdf
2 - https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=661152
3 - https://groups.google.com/forum/#!msg/mozilla.dev.security.policy/fOtt14fiDaM/W69BdOagitAJ
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://rhn.redhat.com/errata/RHSA-2015-0325.html
Description of problem: If the server is using 2047 or 3071 bit RSA keys, mod_ssl will choose 1024 and 2048 bit DHE params respectively. Version-Release number of selected component (if applicable): httpd-2.4.6-16.el7 How reproducible: Always Steps to Reproduce: SSLCRT=/etc/pki/tls/certs/localhost.crt SSLKEY=/etc/pki/tls/private/localhost.key openssl req -x509 -newkey rsa:2047 -keyout ${SSLKEY} -out ${SSLCRT} -subj /CN=localhost -nodes -batch systemctl restart httpd openssl s_client -CAfile ${SSLCRT} -cipher 'kEDH:!aNULL' -connect localhost:443 Actual results: --- No client certificate CA names sent Server Temp Key: DH, 1024 bits --- SSL handshake has read 1459 bytes and written 422 bytes --- New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-GCM-SHA384 Server public key is 2047 bit Secure Renegotiation IS supported Compression: NONE Expansion: NONE SSL-Session: Protocol : TLSv1.2 Cipher : DHE-RSA-AES256-GCM-SHA384 Session-ID: 33D1A9CFA4B2489CA470D92F06F66FF999C19191F2CC9EAF6A21AEB43CCCC3DD Session-ID-ctx: Master-Key: 8398AF230C3E4178307E487F350FCF33651B850DD27B9E6B4F1B113BC7ECEE38140034B0CE1BD077BA3A6B3A947D839B Key-Arg : None Krb5 Principal: None PSK identity: None PSK identity hint: None Start Time: 1394039024 Timeout : 300 (sec) Verify return code: 0 (ok) Expected results: --- No client certificate CA names sent Server Temp Key: DH, 2048 bits --- Additional info: This is problematic because there are multiple applications that sometimes generate 2047 keys while asked for 2048 bit keys (similarly for 1024 bit keys, etc.) [2,3]. On the other hand, there are no known applications that will generate 2049 bit keys when asked for 2048 bit keys. We also don't want the DHE key exchange to be the weakest link in the connection, see NIST SP 800-56 [1] Section 5.6.3 for rationale. 1 - http://csrc.nist.gov/publications/nistpubs/800-57/sp800-57_part1_rev3_general.pdf 2 - https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=661152 3 - https://groups.google.com/forum/#!msg/mozilla.dev.security.policy/fOtt14fiDaM/W69BdOagitAJ