The warning message you mentioned is related to the openssl_private_decrypt() function in PHP. This warning indicates that the key parameter provided to the function is not a valid private key.

To successfully use the openssl_private_decrypt() function, you need to ensure that you are providing a valid private key in the correct format. Here are a few steps you can follow to troubleshoot and resolve this issue:

Check the private key:

Make sure that the private key you are using is correct and accessible. Verify that the key file exists in the specified location and that the file permissions allow read access. Additionally, ensure that the key file contains a valid private key in the expected format (such as PEM or DER).

Verify the key format:

Check the format of the private key you are using. The openssl_private_decrypt() function expects the private key to be in PEM format. If your key is in a different format, such as DER, you may need to convert it to PEM format using appropriate conversion tools or functions.

Validate the key content:

Open the private key file and examine its contents to ensure that it contains the correct data and is not corrupted. It’s possible that the key file may have been modified, truncated, or saved incorrectly, leading to an invalid key format.

Use the correct key:

Double-check that you are passing the correct variable or file path containing the private key as the key parameter to the openssl_private_decrypt() function. Any mistake in referencing the key variable or file path can result in the warning you mentioned.

Debug and error handling:

Implement proper error handling and debugging techniques to track the source of the issue. You can use functions like openssl_error_string() to retrieve detailed error messages related to OpenSSL functions and identify the specific problem with the private key.