Resolving SSH Key Error on Amazon Linux 2023

Issue

SSH connections using `.pem` files generated from the AWS console fail on EC2 instances running Amazon Linux 2023. However, the same key works without issue on Ubuntu OS.

Cause

Amazon Linux 2023 enforces a strict cryptographic policy by default, which disables older RSA algorithms (such as RSA1 and ssh-rsa). This causes authentication failures when attempting to use traditional RSA-based PEM keys.

Resolution

To resolve this issue, the cryptographic policy on the Amazon Linux 2023 instance needs to be changed to 'LEGACY'. This re-enables support for older RSA algorithms.

Steps to Fix

Run the following commands on the EC2 instance:

sudo dnf install crypto-policies-scripts
sudo update-crypto-policies --set LEGACY
sudo systemctl restart sshd

Summary