SSH User Authentication Protocol in .NET Encoding Quick Response Code in .NET SSH User Authentication Protocol

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SSH User Authentication Protocol generate, create qr codes none for .net projects POSTNET Authenticates the client-side user to the server. SSH Connection Protocol Multiplexes the encrypted tunnel into several logical channels. SSH Transport Layer Protocol Provides se VS .NET QR-Code rver authentication, confidentiality, and integrity. It may optionally also provide compression.

. Transmission control protocol provides reliable, connectionoriented end-to-end delivery. Internet protocol provides datagram delivery across multiple networks. Figure 5.8 SSH Protocol Stack 5.5 / SECURE SHELL (SSH). User Auth entication Protocol: Authenticates the user to the server. Connection Protocol: Multiplexes multiple logical communications channels over a single, underlying SSH connection..

Transport Layer Protocol HOST KEYS S erver authentication occurs at the transport layer, based on the server possessing a public/private key pair. A server may have multiple host keys using multiple different asymmetric encryption algorithms. Multiple hosts may share the same host key.

In any case, the server host key is used during key exchange to authenticate the identity of the host. For this to be possible, the client must have a priori knowledge of the server s public host key. RFC 4251 dictates two alternative trust models that can be used: 1.

The client has a local database that associates each host name (as typed by the user) with the corresponding public host key. This method requires no centrally administered infrastructure and no third-party coordination. The downside is that the database of name-to-key associations may become burdensome to maintain.

2. The host name-to-key association is certified by a trusted certification authority (CA). The client only knows the CA root key and can verify the validity of all host keys certified by accepted CAs.

This alternative eases the maintenance problem, since ideally, only a single CA key needs to be securely stored on the client. On the other hand, each host key must be appropriately certified by a central authority before authorization is possible. PACKET E XCHANGE Figure 5.

9 illustrates the sequence of events in the SSH Transport Layer Protocol. First, the client establishes a TCP connection to the server. This is done via the TCP protocol and is not part of the Transport Layer Protocol.

Once the connection is established, the client and server exchange data, referred to as packets, in the data field of a TCP segment. Each packet is in the following format (Figure 5.10).

Packet length: Length of the packet in bytes, not including the packet length and MAC fields. Padding length: Length of the random padding field. Payload: Useful contents of the packet.

Prior to algorithm negotiation, this field is uncompressed. If compression is negotiated, then in subsequent packets, this field is compressed. Random padding: Once an encryption algorithm has been negotiated, this field is added.

It contains random bytes of padding so that that total length of the packet (excluding the MAC field) is a multiple of the cipher block size, or 8 bytes for a stream cipher. Message authentication code (MAC): If message authentication has been negotiated, this field contains the MAC value. The MAC value is computed over the entire packet plus a sequence number, excluding the MAC field.

The sequence number is an implicit 32-bit packet sequence that is initialized to. CHAPTER 5 / TRANSPORT-LEVEL SECURITY Client Server Establish T CP Connection SSH-protoversion-softwareversion Identification string exchange SSH-protoversion-softwareversion. SSH_MSG_KEXINIT Algorithm negotiation SSH_MSG_KEXINIT Key Exchange SSH_MSG_NEWKEYS End of key exchange SSH_MSG_NEWKEYS Service request SSH_MSG_SERVICE_REQUEST Figure 5.9 SSH Transport Layer Protocol Packet Exchanges zero for th .net framework QR Code ISO/IEC18004 e first packet and incremented for every packet. The sequence number is not included in the packet sent over the TCP connection.

Once an encryption algorithm has been negotiated, the entire packet (excluding the MAC field) is encrypted after the MAC value is calculated. The SSH Transport Layer packet exchange consists of a sequence of steps (Figure 5.9).

The first step, the identification string exchange, begins with the client sending a packet with an identification string of the form: SSH-protoversion-softwareversion SP comments CR LF where SP, CR, and LF are space character, carriage return, and line feed, respectively. An example of a valid string is SSH-2.0-billsSSH_3.

6.3q3<CR><LF>. The server responds with its own identification string.

These strings are used in the DiffieHellman key exchange. Next comes algorithm negotiation. Each side sends an SSH_MSG_KEXINIT containing lists of supported algorithms in the order of preference to the sender.

There is one list for each type of cryptographic algorithm.The algorithms include key exchange, encryption, MAC algorithm, and compression algorithm.Table 5.

3 shows the allowable options for encryption, MAC, and compression. For each category, the algorithm chosen is the first algorithm on the client s list that is also supported by the server..

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