English
This negotiation of the cryptographic algorithms allows the implementations to evolve when new algorithms are proposed. If a client is upgraded, it can announce a new algorithm as its preferred one even if the server is not yet upgraded.
This is a slightly modified authenticated Diffie Hellman key exchange with two interesting points. The first point is that when the server authenticates the key exchange it does not provide a certificate. This is because ``ssh`` assumes that the client will store inside its cache the public key of the servers that it uses on a regular basis. This assumption is valid for a protocol like ``ssh`` because users typically use it to interact with a small number of servers, typically a few or a few tens. Storing this information does not require a lot of storage. In practice, most ``ssh`` clients will accept to connect to remote servers without knowing their public key before the connection. In this case, the client issues a warning to the user who can decide to accept or reject the key. This warning can be associated with a fingerprint of the key, either as a sequence of letters or as an ASCII art which can be posted on the web or elsewhere [#fdnsssh]_ by the system administrator of the server. If a client connects to a server whose public key does not match the stored one, a stronger warning is issued because this could indicate a man-in-the-middle attack or that the remote server has been compromised. It can also indicate that the server has been upgraded and that a new key has been generated during this upgrade.
The :term:`ssh` protocol runs directly above the TCP protocol. Once the TCP bytestream has been established, the client and the server exchange messages. The first message exchanged is an ASCII line that announces the version of the protocol and the version of the software implementation used by the client and the server. These two lines are useful when debugging interoperability problems and other issues.
The ``ssh`` protocol includes other features that are beyond the scope of this book. Additional details may be found in [BS2005]_.
the server generates the random number :math:`b`. It then computes :math:`B=g^{b} \mod p`, :math:`K=B^{a} \mod p` and signs with its private key :math:`hash(V_{Client} || V_{Server} || KEX\_INIT_{Client} || KEX\_INIT_{Server} || Server_{pub} || A || B || K )` where :math:`V_{Server}` (resp. :math:`V_{Client}`) is the initial messages sent by the client (resp. server), :math:`KEX\_INIT_{Client}` (resp. :math:`KEX\_INIT_{Server}`) is the key exchange message sent by the client (resp. server) and :math:`A`, :math:`B` and :math:`K` are the messages of the Diffie Hellman key exchange
The secure shell (ssh)
The secure shell protocol was designed in the mid 1990s by T. Ylonen to counter the eavesdropping attacks against :term:`telnet` and similar protocols [Ylonen1996]_. :term:`ssh` became quickly popular and system administrators encouraged its usage. The original version of :term:`ssh` was freely available. After a few years, his author created a company to distribute it commercially, but other programmers continued to develop an open-source version of :term:`ssh` called `OpenSSH <http://www.openssh.com>`_. Over the years, :term:`ssh` evolved and became a flexible applicable whose usage extends beyond remote login to support features such as file transfers, protocol tunneling, ... In this section, we only discuss the basic features of :term:`ssh` and explain how it differs from :term:`telnet`. Entire books have been written to describe :term:`ssh` in details [BS2005]_. An overview of the protocol appeared in [Stallings2009]_.
The second point is that the server authenticates not only the result of the Diffie Hellman exchange but also a hash of all the information sent and received during the exchange. This is important to prevent `downgrade attacks`. A `downgrade attack` is an attack where an active attacker modifies the messages sent by the communicating hosts (typically the client) to request the utilization of weaker encryption algorithms. Consider a client that supports two encryption schemes. The preferred one uses 128 bits secret keys and the second one is an old encryption scheme that uses 48 bits keys. This second algorithm is kept for backward compatibility with older implementations. If an attacker can remove the preferred algorithm from the list of encryption algorithms supported by the client, he can force the server to use a weaker encryption scheme that will be easier to break. Thanks to the hash that covers all the messages exchanged by the server, the downgrade attack cannot occur against ``ssh``. Algorithm agility is a key requirement for security protocols that need to evolve when encryption algorithms are broken by researchers. This agility cannot be used without care and signing a hash of all the messages exchanged is a technique that is frequently used to prevent downgrade attacks.
The next message is the ``SSH_MSG_KEX_INIT`` message that is used to negotiate the cryptographic algorithms that will be used for the ``ssh`` session. It is very important for security protocols to include mechanisms that enable a negotiation of the cryptographic algorithms that are used. First, these algorithms provide different levels of security. Some algorithms might be considered totally secure and are recommended today while they could become deprecated a few years later after the publication of some attacks. Second, these algorithms provide different levels of performance and have different CPU and memory impacts.
The IANA_ maintains a `list of the cryptographic algorithms <http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xhtml#ssh-parameters-16>`_ that can be used by ``ssh`` implementations. For each type of algorithm, the client provides an ordered list of the algorithms that it supports and agrees to use. The server compares the received list with its own list. The outcome of the negotiation is a set of four algorithms [#fnull]_ that will be combined for this session.
The exact algorithms used for the computation of these keys are defined in :rfc:`4253`
the client generates the random number :math:`a` and sends :math:`A=g^{a} \mod p` to the server
the client can recompute :math:`K=A^{b} \mod p` and verify the signature provided by the server
Thanks to the Diffie Hellman key exchange, the client and the servers share key :math:`K`. A naive implementation would probably directly use this key for all the cryptographic algorithms that have been negotiated for this session. Like most security protocols, ``ssh`` does not directly use key :math:`K`. Instead, it uses the negotiated hash function with different parameters [#fsshkeys]_ to allow the client and the servers to compute six keys from :math:`K` :
`ssh` is one example of a protocol that uses Message Authentication Codes (MAC) to authenticates the messages that are sent. A naive implementation of such a MAC would be to simply use a hash function like SHA-1. However, such a construction would not be safe from a security viewpoint. Internet protocols usually rely on the HMAC construction defined in :rfc:`2104`. It works with any hash function (`H`) and a key (`K`). As an example, let us consider HMAC with the SHA-1 hash function. SHA-1 uses 20 bytes blocks and the block size will play an important role in the operation of HMAC. We first require the key to be as long as the block size. Since this key is the output of the key generation algorithm, this is one parameter of this algorithm.
Single use keys
Remote login
``payload`` : the data (after optional compression) passed by the user
``padding`` : random bytes added in each message (at least four) to ensure that the message length is a multiple of the block size used by the negotiated encryption algorithm
``padding length`` : this is the number of random bytes that have been added at the end of the message.