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  • Putty.exe v0.76 key exchange support diffie-hellman-group16-sha1

    From Antony Somerville@21:1/5 to All on Thu Aug 12 20:11:56 2021
    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

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Simon Tatham@21:1/5 to antony.somerville@gmail.com on Fri Aug 13 19:00:05 2021
    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>

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Antony Somerville@21:1/5 to All on Sat Aug 14 01:03:34 2021
    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?

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Simon Tatham@21:1/5 to antony.somerville@gmail.com on Sat Aug 14 11:43:51 2021
    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>" ))

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)