Bug 1067633 - ssh provides limited compatibility to embedded ssh servers
Summary: ssh provides limited compatibility to embedded ssh servers
Alias: None
Product: Red Hat Enterprise Linux 7
Classification: Red Hat
Component: openssh
Version: 7.0
Hardware: Unspecified
OS: Unspecified
Target Milestone: rc
: ---
Assignee: Petr Lautrbach
QA Contact: BaseOS QE Security Team
Depends On: 1053107
TreeView+ depends on / blocked
Reported: 2014-02-20 18:58 UTC by Hubert Kario
Modified: 2014-09-30 19:28 UTC (History)
1 user (show)

Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Clone Of:
Last Closed: 2014-09-30 19:28:59 UTC
Target Upstream Version:

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Description Hubert Kario 2014-02-20 18:58:58 UTC
Description of problem:
The new algorithm used in openssh for calculating size of DH parameters provides limited compatibility for embedded SSH servers.
In case the remote server supports only SHA-1 and SHA-2 MACs, openssh will always suggest usage of 7680 bit or 8192 bit DH group.

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
1. ssh -vv -o "MACs hmac-sha1" localhost

Actual results:
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<7680<8192)

Expected results:
Depending on cipher selected:
3des: 2048
aes-128, arcfour, blowfish: 3072
aes-192: 7680
aes-256: 8192

Additional info:
NIST SP 800-57.
See bug 1053107 for details.

Comment 1 Hubert Kario 2014-02-20 19:01:56 UTC
Previous versions of openssh had a hard limit on DH group size at 4096 bit.

Comment 3 Hubert Kario 2014-09-30 17:03:51 UTC
I raised the issue with upstream and they don't consider this to be an issue or willing to fix.

This probably should be closed as won't fix.

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