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Bug 1067633

Summary:	ssh provides limited compatibility to embedded ssh servers
Product:	Red Hat Enterprise Linux 7	Reporter:	Alicja Kario <hkario>
Component:	openssh	Assignee:	Petr Lautrbach <plautrba>
Status:	CLOSED WONTFIX	QA Contact:	BaseOS QE Security Team <qe-baseos-security>
Severity:	high	Docs Contact:
Priority:	low
Version:	7.0	CC:	pvrabec
Target Milestone:	rc
Target Release:	---
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:		Doc Type:	Bug Fix
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2014-09-30 19:28:59 UTC	Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	1053107
Bug Blocks:

Description Alicja 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):
openssh-6.4p1-7.el7.x86_64

How reproducible:
Always

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 Alicja 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 Alicja 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.