Mastering Linux Security and Hardening - Second Edition

Understanding SSH encryption algorithms

SSH works with a combination of symmetric and asymmetric cryptography, similar to how Transport Layer Security works. The SSH client starts the process by using the public key method to set up an asymmetric session with an SSH server. Once this session has been set up, the two machines can agree on and exchange a secret code, which they'll use to set up a symmetric session. (As we saw previously with TLS, we want to use symmetric cryptography for performance reasons, but we need an asymmetric session to perform the secret key exchange.) To perform this magic, we need four classes of encryption algorithms, which we'll configure on the server side. These are as follows:

• Ciphers: These are the symmetric ...