You want to encrypt a credit card number using the Transit secrets engine.

Comprehensive and Detailed in Depth Explanation:

The error indicates a problem with the plaintext input format. Let’s analyze:

A:The Transit engine requires plaintext to be base64-encoded for safe transport, as it may include non-text data. The error illegal base64 data occurs because "1234 5678 9101 1121" isn’t base64-encoded. Correct: use plaintext=$(base64 <<< "1234 5678 9101 1121").

B:Permission errors would return a 403, not a base64 error. Incorrect.

C:Transit supports encrypting sensitive data like credit card numbers. Incorrect.

D:Spaces aren’t the issue; the format must be base64. Incorrect.

Overall Explanation from Vault Docs:

“When you send data to Vault for encryption, it must be base64-encoded plaintext… This ensures safe transport of binary or text data.”