Private cryptographic keys and certificates are used for authentication, encryption/decryption, and digital signatures. 
Adversaries may gather private keys from compromised systems for use in authenticating to Remote Services like SSH or for use in decrypting other collected files such as email. Common key and certificate file extensions include: .key, .pgp, .gpg, .ppk., .p12, .pem, .pfx, .cer, .p7b, .asc. Adversaries may also look in common key directories, such as
~/.ssh for SSH keys on * nix-based systems or
C:\Users(username).ssh\ on Windows.
Use strong passphrases for private keys to make cracking difficult. When possible, store keys on separate cryptographic hardware instead of on the local system. Ensure only authorized keys are allowed access to critical resources and audit access lists regularly. Ensure permissions are properly set on folders containing sensitive private keys to prevent unintended access. Use separate infrastructure for managing critical systems to prevent overlap of credentials and permissions on systems that could be used as vectors for lateral movement. Follow other best practices for mitigating access through use of Valid Accounts.
Monitor access to files and directories related to cryptographic keys and certificates as a means for potentially detecting access patterns that may indicate collection and exfiltration activity. Collect authentication logs and look for potentially abnormal activity that may indicate improper use of keys or certificates for remote authentication.