Final answer:
The most secure method to transmit an API key is to use HTTPS and include the key in the HTTP header, not as a URL parameter. Implementing token-based authentication, limiting key permissions, rotating the key, and monitoring for unauthorized use provide further security measures.
Step-by-step explanation:
The most secure method to transmit an API key is to use HTTPS (Hypertext Transfer Protocol Secure), which is the secure version of HTTP. This ensures that all data transferred between the client and the server is encrypted and difficult to intercept. When transmitting an API key, it is crucial to include it in the HTTP header rather than as a query parameter in the URL, because URLs are often logged in various places and could expose the key. Additionally, utilizing a method called token-based authentication can enhance security. This involves the client requesting an access token using the API key and then using that token for subsequent requests.
Another important security practice is to limit the API key's permissions only to what is necessary for the operation it needs to perform. Regularly rotating the API key and monitoring its usage for any unusual or unauthorized activity is also recommended for maintaining a high-security level. Moreover, it is advisable not to hardcode API keys into source code, especially if that code is stored in version control systems or is publicly accessible.