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Asked: 2026-05-30T09:52:30+00:00

I’m trying to create a very simple & lightweight client implementation of the SSH

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I’m trying to create a very simple & lightweight client implementation of the SSH protocol for node.js.

The following documentation confuses me totally:
http://www.snailbook.com/docs/transport.txt

It lacks a full example of the whole key exchange thing. There are many things well explained, but I’m not really sure how to put those things together.

Could you help me to put an example together?

I’m stuck after section 7.1. I successfully receive the list of alorightms of the server, and I send a list with only the required alorithms to the server. Also successful.

So, in this case we have the following alorightms:

  • kex: diffie-hellman-group1-sha1
  • key: ssh-dss
  • encryption: 3des-cbc
  • mac: hmac-sha1

After that, I skipped section 7.2 and 7.3 and continued directly to section 8, since generating a key needs the values H & K, which are generated in section 8.

But section 8 does not make sense to me. It requires both the client & the server to already know the same prime, generator and order value. When have those values been negotiated an sent to each other? Section 8 cleary says that it directly follows the algorithm exchange, so there is nothing between those steps…
Am I missing something?

Thanks really much for your help!

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1 Answer

  1. Editorial Team

    The prime is specified by the key-exchange algorithm. For example, to quote from your source:

    The “diffie-hellman-group1-sha1” method specifies the Diffie-Hellman key exchange with SHA-1 as HASH, and Oakley Group 2 [RFC2409] (1024-bit MODP Group).

    And if you consult RFC 2409 §6.2, you’ll find:

    The prime is 2^1024 – 2^960 – 1 + 2^64 * { [2^894 pi] + 129093 }. Its hexadecimal value is 

        FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
    29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
    EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
    E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
    EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381
    FFFFFFFF FFFFFFFF

    The generator is 2 (decimal)

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