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computers / comp.security.ssh / Re: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1

SubjectAuthor
* Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1Antony Somerville
`* Re: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1Simon Tatham
 `* Re: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1Antony Somerville
  `- Re: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1Simon Tatham

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Subject: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1
From: Antony Somerville
Newsgroups: comp.security.ssh
Date: Fri, 13 Aug 2021 03:11 UTC
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Path: i2pn2.org!i2pn.org!weretis.net!feeder8.news.weretis.net!proxad.net!feeder1-2.proxad.net!209.85.160.216.MISMATCH!news-out.google.com!nntp.google.com!postnews.google.com!google-groups.googlegroups.com!not-for-mail
Newsgroups: comp.security.ssh
Date: Thu, 12 Aug 2021 20:11:56 -0700 (PDT)
Injection-Info: google-groups.googlegroups.com; posting-host=203.34.228.49; posting-account=_zUo5woAAAA_DJW5-D9qY9deDeTm7wvv
NNTP-Posting-Host: 203.34.228.49
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Message-ID: <5d730762-a3d7-4a0d-a920-b16e4e56c7ben@googlegroups.com>
Subject: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1
From: antony.s...@gmail.com (Antony Somerville)
Injection-Date: Fri, 13 Aug 2021 03:11:56 +0000
Content-Type: text/plain; charset="UTF-8"
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Hello,

I have run into an isue where putty key exchange is unable to agree with a device I only have ssh access to.

How can I update putty.exe to handle diffie-hellman-group16-sha1 ?

OR is there a client out there I can use in the mean time until putty updates come through ?

The error when I SSH is:
Key exchange failed.
No compatible key-exchange method. The server supports these methods: diffie-hellman-group16-sha1

Regards
Antony


Subject: Re: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1
From: Simon Tatham
Newsgroups: comp.security.ssh
Date: Fri, 13 Aug 2021 18:00 UTC
References: 1
Path: i2pn2.org!i2pn.org!usenet.goja.nl.eu.org!nntp.terraraq.uk!nntp-feed.chiark.greenend.org.uk!ewrotcd!.POSTED!not-for-mail
From: ana...@pobox.com (Simon Tatham)
Newsgroups: comp.security.ssh
Subject: Re: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1
Date: 13 Aug 2021 19:00:05 +0100 (BST)
Lines: 21
Message-ID: <rXe*jZAry@news.chiark.greenend.org.uk>
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NNTP-Posting-Host: chiark.greenend.org.uk
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Originator: simon@tunnel.thyestes.tartarus.org ([172.31.80.4])
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Antony Somerville  <antony.somerville@gmail.com> wrote:
How can I update putty.exe to handle diffie-hellman-group16-sha1 ?

It's not very difficult, provided you know the group parameters. All
the code is in crypto/diffie-hellman.c.

But where does this method come from? The SSH assigned numbers page at
http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xhtml
lists diffie-hellman-group16-sha512, but not -sha1. Even under the
assumption that it's the same group with a different hash function,
that's going outside the bounds of the standard - there's no
expectation that you should be able to make up new names in that
namespace by re-composing existing names in that way.

What SSH server is supporting this nonstandard name as its only kex
method?
--
for k in [pow(x,37,0x1a1298d262b49c895d47f) for x in [0x50deb914257022de7fff,
0x213558f2215127d5a2d1, 0x90c99e86d08b91218630, 0x109f3d0cfbf640c0beee7,
0xc83e01379a5fbec5fdd1, 0x19d3d70a8d567e388600e, 0x534e2f6e8a4a33155123]]:
 print("".join([chr(32+3*((k>>x)&1))for x in range(79)])) # <anakin@pobox.com>


Subject: Re: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1
From: Antony Somerville
Newsgroups: comp.security.ssh
Date: Sat, 14 Aug 2021 08:03 UTC
References: 1 2
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Newsgroups: comp.security.ssh
Date: Sat, 14 Aug 2021 01:03:34 -0700 (PDT)
In-Reply-To: <rXe*jZAry@news.chiark.greenend.org.uk>
Injection-Info: google-groups.googlegroups.com; posting-host=180.150.111.1; posting-account=_zUo5woAAAA_DJW5-D9qY9deDeTm7wvv
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Subject: Re: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1
From: antony.s...@gmail.com (Antony Somerville)
Injection-Date: Sat, 14 Aug 2021 08:03:34 +0000
Content-Type: text/plain; charset="UTF-8"
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Hi Simon,

Thanks for replying. The kex on a Cisco access point with a DH min key size of 4096 and the IOS prompted for this particular cipher.

I remedied for myself by changing that parameter to min size 2048. The IOS then prompted for the DH kex that already is present in putty.

Regards
Antony.

But where does this method come from? The SSH assigned numbers page at
http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xhtml
lists diffie-hellman-group16-sha512, but not -sha1. Even under the
assumption that it's the same group with a different hash function,
that's going outside the bounds of the standard - there's no
expectation that you should be able to make up new names in that
namespace by re-composing existing names in that way.

What SSH server is supporting this nonstandard name as its only kex
method?


Subject: Re: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1
From: Simon Tatham
Newsgroups: comp.security.ssh
Date: Sat, 14 Aug 2021 10:43 UTC
References: 1 2 3
Path: i2pn2.org!i2pn.org!aioe.org!nntp.terraraq.uk!nntp-feed.chiark.greenend.org.uk!ewrotcd!.POSTED!not-for-mail
From: ana...@pobox.com (Simon Tatham)
Newsgroups: comp.security.ssh
Subject: Re: Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1
Date: 14 Aug 2021 11:43:51 +0100 (BST)
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X-Newsreader: trn 4.0-test77 (Sep 1, 2010)
Originator: simon@tunnel.thyestes.tartarus.org ([172.31.80.4])
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Antony Somerville  <antony.somerville@gmail.com> wrote:
Thanks for replying. The kex on a Cisco access point with a DH min key
size of 4096 and the IOS prompted for this particular cipher.

Please report a bug to Cisco - they should not be unilaterally
inventing kex names in the IANA-controlled (non-domain-suffixed)
namespace, no matter how obvious they think the meaning of those names
are!

I remedied for myself by changing that parameter to min size 2048. The
IOS then prompted for the DH kex that already is present in putty.

That's good to hear.
--
import hashlib; print((lambda p,q,g,y,r,s,m: (lambda w:(pow(g,int(hashlib.sha1(
m.encode('ascii')).hexdigest(),16)*w%q,p)*pow(y,r*w%q,p)%p)%q)(pow(s,q-2,q))==r
and m)(0xb80b5dacabab6145,0xf70027d345023,0x7643bc4018957897,0x11c2e5d9951130c9
,0xa54d9cbe4e8ab,0x746c50eaa1910,      "Simon Tatham <anakin@pobox.com>"     ))


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