What limitations arise when using the same key for authentication with a token?

Using the same key for authentication and token can lead to several limitations and risks:

• Security: If the key is compromised, an attacker will gain full access, as the same key is used for generating and verifying tokens.
• Lack of key rotation: It is difficult to securely change the key without disrupting all tokens.
• Lack of responsibility separation: The key should be limited in purpose — for example, one for signing tokens, another for authentication.
• Scalability issues: Using a single key makes it difficult to manage access and auditing.

Therefore, it is recommended to use different keys or secrets for different purposes, as well as implement rotation mechanisms and access restrictions.