When you generate an SSH key pair using ssh-keygen, the public key file (typically id_rsa.pub or similar) contains a curious suffix:

ssh-rsa AAAAB3NzaC1yc2EAAA...bunchOfBase64.../CA9gyE8HRhNMG6ZDwyhPBbDfX root@mydomain

The user@host portion serves purely as:

• Identification: Documents which user/host generated the key
• Comment field: Modifiable without affecting key functionality

Example of modifying the comment:

ssh-keygen -c -C "prod-server-key" -f ~/.ssh/id_rsa

The authentication process only verifies:

1. The private key matches the public key
2. The key is authorized in authorized_keys

Proof of non-dependency:

# This works regardless of original comment
ssh-copy-id -i ~/.ssh/id_rsa.pub user@otherhost

Despite being optional, the comment helps with:

# Key management example
ssh-keygen -t ed25519 -C "aws-ec2-key-2024-jane-doe"

When auditing authorized_keys files:

# Sample authorized_keys entry
ssh-rsa AAAAB3...DfX jane-dev-laptop
ssh-ed25519 AAAAC3...Nza alice-ci-server

For complex environments, consider:

# Generate key with detailed comment
ssh-keygen -t ecdsa -C "env=prod|purpose=db-access|expires=2025-01"

View comment separately:

ssh-keygen -l -f ~/.ssh/id_rsa.pub

When examining an SSH public key file (typically id_rsa.pub or similar), you'll notice it follows this structure:

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQ...b64encodedkey... root@hostname

The final segment (root@hostname) is called the key comment. Here's what actually gets transmitted during authentication:

ssh-rsa AAAAB3...user@host

The comment field has zero effect on the cryptographic authentication process. The SSH protocol specification (RFC 4252) explicitly states:

"The comment may be used by the user to identify the key and is included whenever the key is displayed, but it does not affect the authentication process."

To prove this, try these experiments:

# Experiment 1: Change the comment
cp id_rsa.pub modified.pub
sed -i 's/root@mydomain/changed@example/' modified.pub
ssh -i id_rsa user@server # Still works

# Experiment 2: Strip comment entirely
awk '{print $1,$2}' id_rsa.pub > no_comment.pub
ssh -i id_rsa user@server # Still works

While not affecting security, comments serve important administrative purposes:

• Key tracking: jenkins-buildserver-2024@aws
• Environment identification: prod-deploy@azure-eastus
• Expiration dates: temp-access-2024Q2@vpn

ssh-keygen -t ed25519 -C "deploy-key-$(date +%Y%m%d)"

ssh-keygen -c -C "new-comment" -f ~/.ssh/id_rsa

#!/bin/bash
COMMENT="auto-$(hostname)-$(date +%s)"
ssh-keygen -t rsa -b 4096 -f new_key -N "" -C "$COMMENT"

# Generate keys with minimal comments
ssh-keygen -t ed25519 -C ""