Utilizing HMAC (hash-based message authentication code) for the keys is the best option for securing the REST API connection to the database while preventing the use of a hard-coded string in the request string. HMAC is a technique that uses a secret key and a hash function to generate a code that can verify the authenticity and integrity of a message, preventing unauthorized modifications or tampering. Utilizing HMAC for the keys can prevent the use of a hard-coded string in the request string, as it can dynamically generate a unique code for each request based on the secret key and the message content, making it difficult to forge or replay. Implementing a VPN (virtual private network) for all APIs is not a good option for securing the REST API connection to the database, as it could introduce latency or performance issues for API requests, as well as not prevent the use of a hard-coded string in the request string. Signing the key with DSA (Digital Signature Algorithm) is not a good option for securing the REST API connection to the database, as it could be vulnerable to attacks or forgery if the key is compromised or weak, as well as not prevent the use of a hard-coded string in the request string. Deploying MFA (multi-factor authentication) for the service accounts is not a good option for securing the REST API connection to the database, as it could affect the usability or functionality of API requests, as well as not prevent the use of a hard-coded string in the request string. Verified Reference: https://www.comptia.org/blog/what-is-hmac https://partners.comptia.org/docs/default-source/resources/casp-content-guide

The most likely cause of the signature failing is that the certificate is set for the wrong key usage. Key usage is an extension of a certificate that defines the purpose and functionality of the public key contained in the certificate. Key usage can include digital signature, key encipherment, data encipherment, certificate signing, and others. If the certificate is set for a different key usage than digital signature, it will not be able to sign the applications properly. The administrator should check the key usage extension of the certificate and make sure it matches the intended purpose. Verified Reference:
https://www.wintips.org/how-to-fix-windows-cannot-verify-the-digital-signature-for-this-file-error-in-windows-8-7-vista/
https://softwaretested.com/mac/how-to-fix-a-digital-signature-error-on-windows-10/
https://support.microsoft.com/en-us/office/digital-signatures-and-certificates-8186cd15-e7ac-4a16-8597-22bd163e8e96

The clients may not trust idapt by default because it is a self-signed certificate authority that is not in the trusted root store of the clients. Secure LDAP does not support wildcard certificates because they do not match the fully qualified domain name of the server. Verified Reference: https://www.professormesser.com/security-plus/sy0-401/ldap-and-secure-ldap/ , https://www.comptia.org/training/books/casp-cas-004-study-guide