Secure Shell (SSH) is a network protocol that is commonly used to provide secure access to network devices. It is an encrypted replacement for Telnet and other insecure remote communication protocols. SSH provides authentication, data integrity, and encryption of data over untrusted networks.
The Diffie-Hellman algorithm is used to securely exchange cryptographic keys over an open network. It is a key agreement protocol that allows two parties to establish a secure communication channel over an insecure network. SSH incorporates the Diffie-Hellman algorithm to establish a secure communication channel between client and server.
The Diffie-Hellman algorithm uses a mathematical group to generate a shared secret between two parties. In SSH, the Diffie-Hellman group is used to generate a shared secret key in order to authenticate the SSH session. The Diffie-Hellman group consists of a prime number, a generator, and an initial secret value.
The Cisco 3750G is a popular switch used in enterprise networks. It supports SSH version 2, which uses the Diffie-Hellman algorithm with Group 1 and SHA1 authentication. SSH Group 1 uses a 1024-bit prime number and a 160-bit generator to generate the shared secret key. SHA1 is a hashing algorithm used to authenticate the SSH session.
Recently, many Cisco 3750G switches have been reporting an error when attempting to authenticate via SSH. The error message is \Diffie-Hellman Group 1 SHA1 Error\. This error is due to a weakness in the SHA1 authentication algorithm used by Group 1 of the Diffie-Hellman algorithm.
The recommended solution is to upgrade to SSH version 2 with the Diffie-Hellman Group 14 algorithm and the SHA256 authentication algorithm. Group 14 uses a 2048-bit prime number and a 256-bit generator to generate the shared secret key. SHA256 is a stronger hashing algorithm than SHA1 and is more secure.
To upgrade to SSH version 2 with the Diffie-Hellman Group 14 algorithm and the SHA256 authentication algorithm, you will need to access the device's configuration mode. From there, you can issue the command \ip ssh version 2\ to enable SSH version 2. You can also issue the command \ip ssh dhm-group14-sha256\ to enable the Diffie-Hellman Group 14 algorithm and the SHA256 authentication algorithm.
Once you have enabled SSH version 2 with the Diffie-Hellman Group 14 algorithm and the SHA256 authentication algorithm, you should be able to connect to the device without experiencing the \Diffie-Hellman Group 1 SHA1 Error\. This upgrade provides more secure authentication and encryption of data over untrusted networks.