Cisco IOS XE: Secure Shell Version 2 Support Guide PDF (EN)

Prerequisites for Secure Shell Version 2 Support

Secure Shell Version 2 Support The Secure Shell Version 2 Support feature allows you to configure Secure Shell (SSH) Version 2. (SSH Version 1 support was implemented in an earlier Cisco software release.) SSH runs on top of a reliable transport layer and provides strong authentication and encryption capabilities. The only reliable transport that is defined for SSH is TCP. SSH provides a means to securely access and securely execute commands on another computer over a network. The Secure Copy Protocol (SCP) feature that is provided with SSH allows for the secure transfer of files.

Prerequisites for Secure Shell Version 2 Support, on page 1 Restrictions for Secure Shell Version 2 Support, on page 2 Information About Secure Shell Version 2 Support, on page 2 How to Configure Secure Shell Version 2 Support, on page 6 Configuration Examples for Secure Shell Version 2 Support, on page 17 Additional References for Secure Shell Version 2 Support, on page 21 Feature History for Secure Shell Version 2 Support, on page 22 Prerequisites for Secure Shell Version 2 Support • Before configuring SSH, ensure that the required image is loaded on your device. The SSH server requires you to have a k9 (Triple Data Encryption Standard [3DES]) software image depending on your release.

• You have to use a SSH remote device that supports SSH Version 2 and connect to a Cisco device. • SCP relies on authentication, authorization, and accounting (AAA) to function correctly. Therefore, AAA must be configured on the device to enable the secure copy protocol on the SSH Server. Note The SSH Version 2 server and the SSH Version 2 client are supported on your Cisco software, depending on your release. (The SSH client runs both the SSH Version 1 protocol and the SSH Version 2 protocol. The SSH client is supported in k9 images depending on your release.) Secure Shell Version 2 Support 1

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Secure Shell Version 2

Restrictions for Secure Shell Version 2 Support • Secure Shell (SSH) servers and SSH clients are supported in Triple Data Encryption Standard (3DES) software images. • Execution Shell, remote command execution, and Secure Copy Protocol (SCP) are the only applications supported. • Rivest, Shamir, and Adleman (RSA) key generation is an SSH server-side requirement. Devices that act as SSH clients need not generate RSA keys. • The RSA key pair size must be greater than or equal to 768 bits. • The following features are not supported: • Port forwarding • Compression • Starting from Cisco IOS XE Release 17.10, the following Key Exchange and MAC algorithms are removed from the default list:

Key Exchange algorithm: • diffie-hellman-group14-sha1 MAC algorithms: • hmac-sha1 • hmac-sha2-256 • hmac-sha2-512 Note You can use the ip ssh server algorithm kex command to configure the Key Exchange algorithm and the ip ssh server algorithm mac command to configure the MAC algorithms. Information About Secure Shell Version 2 Support Secure Shell Version 2 The Secure Shell Version 2 Support feature allows you to configure SSH Version 2. The configuration for the SSH Version 2 server is similar to the configuration for SSH Version 1. The ip ssh version command defines the SSH version to be configured.

If you do not configure this command, SSH by default runs in compatibility mode; that is, both SSH Version 1 and SSH Version 2 connections are honored. Secure Shell Version 2 Support Restrictions for Secure Shell Version 2 Support Secure Shell Version 2 Support 2

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Secure Shell Version 2 Enhancements for RSA Keys

Note SSH Version 1 is a protocol that has never been defined in a standard. If you do not want your device to fall back to the undefined protocol (Version 1), you should use the ip ssh version command and specify Version 2. The ip ssh rsa keypair-name command enables an SSH connection using the Rivest, Shamir, and Adleman (RSA) keys that you have configured. Previously, SSH was linked to the first RSA keys that were generated (that is, SSH was enabled when the first RSA key pair was generated). This behavior still exists, but by using the ip ssh rsa keypair-name command, you can overcome this behavior.

If you configure the ip ssh rsa keypair-name command with a key pair name, SSH is enabled if the key pair exists or SSH will be enabled if the key pair is generated later. If you use this command to enable SSH, you are not forced to configure a hostname and a domain name, which was required in SSH Version 1 of the Cisco software. The login banner is supported in SSH Version 2, but it is not supported in Secure Shell Version 1. Note Secure Shell Version 2 Enhancements The SSH Version 2 Enhancements feature includes a number of additional capabilities such as supporting Virtual Routing and Forwarding (VRF)-Aware SSH, SSH debug enhancements, and Diffie-Hellman (DH) group exchange support.

The VRF-Aware SSH feature is supported depending on your release. Note The Cisco SSH implementation has traditionally used 768-bit modulus, but with an increasing need for higher key sizes to accommodate DH Group 14 (2048 bits) and Group 16 (4096 bits) cryptographic applications, a message exchange between the client and the server to establish the favored DH group becomes necessary. The ip ssh dh min size command configures the modulus size on the SSH server.

In addition to this, the ssh command was extended to add VRF awareness to the SSH client-side functionality through which the VRF instance name in the client is provided with the IP address to look up the correct routing table and establish a connection. Debugging was enhanced by modifying SSH debug commands. The debug ip ssh command was extended to simplify the debugging process. Before the simplification of the debugging process, this command printed all debug messages related to SSH regardless of what was specifically required. The behavior still exists, but if you configure the debug ip ssh command with a keyword, messages are limited to information specified by the keyword.

Secure Shell Version 2 Enhancements for RSA Keys Cisco SSH Version 2 supports keyboard-interactive and password-based authentication methods. The SSH Version 2 Enhancements for RSA Keys feature also supports RSA-based public key authentication for the client and the server. Secure Shell Version 2 Support Secure Shell Version 2 Enhancements Secure Shell Version 2 Support 3

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SSH Ed25519 Encryption

• User authentication: RSA-based user authentication uses a private/public key pair associated with each user for authentication. The user must generate a private/public key pair on the client and configure a public key on the Cisco SSH server to complete the authentication. An SSH user trying to establish credentials provides an encrypted signature using the private key. The signature and the user’s public key are sent to the SSH server for authentication. The SSH server computes a hash over the public key provided by the user. The hash is used to determine if the server has a matching entry. If a match is found, an RSA-based message verification is performed using the public key.

Hence, the user is authenticated or denied access based on the encrypted signature. • Server authentication: While establishing an SSH session, the Cisco SSH client authenticates the SSH server by using the server host keys available during the key exchange phase. SSH server keys are used to identify the SSH server. These keys are created at the time of enabling SSH and must be configured on the client. For server authentication, the Cisco SSH client must assign a host key for each server. When the client tries to establish an SSH session with a server, the client receives the signature of the server as part of the key exchange message.

If the strict host key checking flag is enabled on the client, the client checks if it has the host key entry corresponding to the server. If a match is found, the client tries to validate the signature by using the server host key. If the server is successfully authenticated, the session establishment continues; otherwise, it is terminated and displays a “Server Authentication Failed” message. Note • Storing public keys on a server uses memory; therefore, the number of public keys configurable on an SSH server is restricted to ten users, with a maximum of two public keys per user.

• RSA-based user authentication is supported by the Cisco server, but Cisco clients cannot propose public key as an authentication method. If the Cisco server receives a request from an open SSH client for RSA-based authentication, the server accepts the authentication request. • For server authentication, configure the RSA public key of the server manually and configure the ip ssh stricthostkeycheck command on the Cisco SSH client. SSH Ed25519 Encryption In Cisco IOS XE 17.16.1, along with SSH-RSA key, SSH Ed25519 public key is also supported for server and device authentication. Ed25519 is a public-key signature system provides features such as.

• fast single-signature verification • fast batch verification • fast signing • fast key generation • high security level • foolproof session keys • mall signatures, and Secure Shell Version 2 Support SSH Ed25519 Encryption Secure Shell Version 2 Support 4

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