Good writeup. Here are some security suggestions:

1. Improved File Permissions:
   • Ensure that the key file ($KEYFILE) and the script itself have strict file permissions. This can be done using chmod to restrict access to only the necessary users, typically just the owner.

   chmod 600 "$KEYFILE"
   chmod 700 /path/to/your/script.sh

2. Input Validation:
   • Add validation for the inputs, especially for the service name and key, to prevent injection attacks or accidental misconfiguration.

   if [[ ! "$2" =~ ^[a-zA-Z0-9_]+$ ]]; then
       echo "Invalid service name"
       exit 1
   fi

3. Error Handling and Logging:
   • Implement better error handling and logging to track script usage and errors. This can help in auditing and troubleshooting.

   log_file="/var/log/totp_script.log"
   
   log() {
       echo "$(date '+%Y-%m-%d %H:%M:%S') - $1" >> "$log_file"
   }
   
   # Example usage within the script:
   log "Generating TOTP for service $2"

4. Encrypted Backups:
   • Create a mechanism for encrypted backups of the $KEYFILE. This can be a simple script that encrypts and copies the file to a secure location.

   backup_file="$HOME/.totpkeys_backup_$(date '+%Y%m%d')"
   
   cp "$KEYFILE" "$backup_file"
   gpg --encrypt -r "$UID" "$backup_file"

5. Enhanced GnuPG Handling:
   • Ensure that the GnuPG configuration is secure. This may include setting up a strong key passphrase, using a secure keyring, and keeping the GnuPG software up to date.
6. Avoid Hardcoded Information:
   • Instead of hardcoding the GnuPG user ID and key ID, consider passing them as arguments or setting them as environment variables.

   UID=${TOTP_UID:-"default_user@example.com"}
   KEYID=${TOTP_KEYID:-"default_keyid"}

7. Restrict Script Execution:
   • Restrict the script to be executable only by the intended users. This can be done by checking the user ID at the beginning of the script.

   if [ "$(id -u)" -ne "expected_user_id" ]; then
       echo "This script can only be run by a specific user."
       exit 1
   fi

8. Prompt for Confirmation on Sensitive Actions:
   • For operations like setting a new key, prompt for user confirmation to prevent accidental changes.

   read -p "Are you sure you want to set a new key for $2? [y/N] " response
   if [[ ! "$response" =~ ^[Yy]$ ]]; then
       echo "Operation canceled."
       exit 1
   fi

9. Use Temporary Files for Sensitive Data:
   • Instead of directly writing sensitive data to files, use temporary files with restricted permissions and ensure they are securely deleted after use.

   tmpfile=$(mktemp /tmp/.totp.XXXXXX)
   chmod 600 "$tmpfile"
   # Use $tmpfile for intermediate steps
   rm -f "$tmpfile"