Re: [NSE script] SSH2 hostkey

[Sven Klemm <sven () c3d2 de>]

Hi Brandon,

> You might consider only displaying the key fingerprint instead of the
> full key unless verbose is set to at least 2.  Here is what that would
> look like:
>
> > Starting Nmap 4.68 ( http://nmap.org ) at 2008-08-05 16:34 CEST
> > Interesting ports on localhost (127.0.0.1):
> > PORT   STATE SERVICE
> > 22/tcp open  ssh
> > |  SSH2-Hostkey: ssh-rsa 2048 f0:58:ce:f4:aa:a4:59:1c:8e:dd:4d:07:44:c8:25:11
> > |_ ssh-dss 1024 23:23:8c:73:26:22:4a:63:d8:5d:41:eb:86:cf:a0:58
>
> You'll have to dig into the standard spec to figure out what part of
> the key you compute the hash over.  For RSA it is probably the modulus
> but I'm not sure for DSA.

I've attached a new version that implements your proposal.

Sample output:

> ./nmap --script SSH2-hostkey -p 22 localhost

Starting Nmap 4.68 ( http://nmap.org ) at 2008-08-05 19:13 CEST
Interesting ports on localhost (127.0.0.1):
PORT   STATE SERVICE
22/tcp open  ssh
|  SSH2-Hostkey: ssh-rsa 2048 
f0:58:ce:f4:aa:a4:59:1c:8e:dd:4d:07:44:c8:25:11
|_ ssh-dsa 1024 23:23:8c:73:26:22:4a:63:d8:5d:41:eb:86:cf:a0:58

Nmap done: 1 IP address (1 host up) scanned in 0.06 seconds


Sample output with verbosity 2:

> ./nmap --script SSH2-hostkey -p 22 localhost -vv

Starting Nmap 4.68 ( http://nmap.org ) at 2008-08-05 19:12 CEST
Initiating Ping Scan at 19:12
Scanning 127.0.0.1 [1 port]
Completed Ping Scan at 19:12, 0.00s elapsed (1 total hosts)
Initiating Connect Scan at 19:12
Scanning localhost (127.0.0.1) [1 port]
Discovered open port 22/tcp on 127.0.0.1
Completed Connect Scan at 19:12, 0.00s elapsed (1 total ports)
SCRIPT ENGINE: Initiating script scanning.
Initiating SCRIPT ENGINE at 19:12
Completed SCRIPT ENGINE at 19:12, 0.03s elapsed
Host localhost (127.0.0.1) appears to be up ... good.
Scanned at 2008-08-05 19:12:40 CEST for 0s
Interesting ports on localhost (127.0.0.1):
PORT   STATE SERVICE
22/tcp open  ssh
|  SSH2-Hostkey: ssh-rsa 2048 
f0:58:ce:f4:aa:a4:59:1c:8e:dd:4d:07:44:c8:25:11 
AAAAB3NzaC1yc2EAAAABIwAAAQEAwVuv2gcr0maaKQ69VVIEv2ob4OxnuI64fkeOnCXD1lUx5tTA+vefXUWEMxgMuA7iX4irJHy2zer0NQ3Z3yJvr5scPgTYIaEOp5Uo/eGFG9Agpk5wE8CoF0e47iCAPHqzlmP2V7aNURLMODb3jVZuI07A2ZRrMGrD8d888E2ORVORv1rYeTYCqcMMoVFmX9l3gWEdk4yx3w5sD8v501Iuyd1v19mPfyhrI5E1E1nl/Xjp5N0/xP2GUBrdkDMxKaxqTPMie/f0dXBUPQQN697a5q+5lBRPhKYOtn6yQKCd9s1Q22nxn72Jmi1RzbMyYJ52FosDT755Qmb46GLrDMaZMQ==
|_ ssh-dsa 1024 23:23:8c:73:26:22:4a:63:d8:5d:41:eb:86:cf:a0:58 
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

Read data files from: .
Nmap done: 1 IP address (1 host up) scanned in 0.10 seconds


Cheers,
Sven

--
Sven Klemm
http://cthulhu.c3d2.de/~sven/


id = "SSH2-Hostkey"
author = "Sven Klemm <sven () c3d2 de>"
description = "Show SSH2 Hostkey"
license = "Same as Nmap--See http://nmap.org/book/man-legal.html";
categories = {"safe"}

require("stdnse")
require("shortport")
require("openssl")
require("bin")
require("base64")
require("hash")

portrule = shortport.port_or_service(22, "ssh")

local ssh2
ssh2 = {
  transport = {
    build = function( payload )
      local packet_length, padding_length
      padding_length = 8 - ( (payload:len() + 1 + 4 ) % 8 )
      packet_length = payload:len() + padding_length + 1
      return bin.pack( ">IcAA", packet_length, padding_length, payload, openssl.rand_pseudo_bytes( padding_length ) )
    end,

    payload = function( packet )
      local packet_length, padding_length, payload_length, payload, offset
      offset, packet_length, padding_length = bin.unpack( ">Ic", packet )
      payload_length = packet_length - padding_length - 1
      offset, payload = bin.unpack( ">A" .. payload_length, packet, offset )
      return payload
    end,

    dh_gex_request = function( min, opt, max )
      return bin.pack( ">cIII", 34, min, opt, max )
    end,

    kexdh_init = function( e )
      return bin.pack( ">cIA", 30, e:num_bytes(), e:to_bin() )
    end,

    kex_init = function( cookie, options )
      options = options or {}
      kex_algorithms = "diffie-hellman-group1-sha1"
      host_key_algorithms = options['host_key_algorithms'] or "ssh-dss,ssh-rsa"
      encryption_algorithms = "aes128-cbc,3des-cbc,blowfish-cbc,aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr"
      mac_algorithms = "hmac-md5,hmac-sha1,hmac-ripemd160"
      compression_algorithms = "none"
      languages = ""

      local payload = bin.pack( ">cAaa", 20, cookie, kex_algorithms, host_key_algorithms )
      payload = payload .. bin.pack( ">aa", encryption_algorithms, encryption_algorithms )
      payload = payload .. bin.pack( ">aa", mac_algorithms, mac_algorithms )
      payload = payload .. bin.pack( ">aa", compression_algorithms, compression_algorithms )
      payload = payload .. bin.pack( ">aa", languages, languages )
      payload = payload .. bin.pack( ">cI", 0, 0 )

      return payload
    end
  },
  fetch_host_key = function( host, port, key_type )
    local socket = nmap.new_socket()
    local catch = function() socket:close() end
    local try = nmap.new_try(catch)
    -- oakley group 2 prime taken from rfc 2409
    local prime = 
"FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF"

    try(socket:connect(host.ip, port.number))
    -- fetch banner
    try(socket:receive_lines(1))
    -- send our banner
    try(socket:send("SSH-2.0-Nmap-SSH2-Hostkey\r\n"))

    local cookie = openssl.rand_bytes( 16 )
    local packet = ssh2.transport.build( ssh2.transport.kex_init( cookie, {host_key_algorithms=key_type} ) )
    try(socket:send( packet ))

    local kex_init = try(socket:receive_bytes(1))
    kex_init = ssh2.transport.payload( kex_init )
    -- check for proper msg code
    if kex_init:byte(1) ~= 20 then
      return
    end

    local e, g, x, p
    -- e = g^x mod p
    g = openssl.bignum_dec2bn( "2" )
    p = openssl.bignum_hex2bn( prime )
    x = openssl.bignum_pseudo_rand( 1024 )
    e = openssl.bignum_mod_exp( g, p, x )

    packet = ssh2.transport.build( ssh2.transport.kexdh_init( e ) )
    try(socket:send( packet ))

    local kexdh_reply = try(socket:receive_bytes(1))
    kexdh_reply = ssh2.transport.payload( kexdh_reply )
    -- check for proper msg code
    if kexdh_reply:byte(1) ~= 31 then
      return
    end

    local public_host_key
    _, _, public_host_key = bin.unpack( ">ca", kexdh_reply )

    return public_host_key
  end
}

local format_fingerprint = function( fp )
  local s = fp:sub( 1, 2 )
  for i = 3, #fp, 2 do
    s = s .. ':' .. fp:sub( i, i + 1 )
  end
  return s
end

action = function(host, port)
  local output = ""

  local rsa_key = ssh2.fetch_host_key( host, port, "ssh-rsa" )
  if rsa_key then 
    local key_type, e, n
    _, key_type, e, n = bin.unpack( ">aaa", rsa_key )
    output = output .. "ssh-rsa " .. openssl.bignum_bin2bn( n ):num_bits() .. " " .. format_fingerprint( hash.md5( 
rsa_key ) )
    if nmap.verbosity() > 1 then
      output = output .. ' ' .. base64.enc( rsa_key )
    end
    output = output .. '\n'
  end

  local dsa_key = ssh2.fetch_host_key( host, port, "ssh-dss" )
  if dsa_key then 
    local key_type,p,q,g,y
    _, key_type, p, q, g, y = bin.unpack( ">aaaaaa", dsa_key )
    output = output .. "ssh-dsa " .. openssl.bignum_bin2bn( p ):num_bits() .. " " .. format_fingerprint( hash.md5( 
dsa_key ) )
    if nmap.verbosity() > 1 then
      output = output .. ' ' .. base64.enc( dsa_key )
    end
  end

  return output

end


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