The Confluent REST Proxy exposes an HTTP API for clients to interact with a Kafka cluster. To secure the communication channel between a REST client and the REST Proxy server, the standard web security protocol, Transport Layer Security (TLS), is used. This encrypts the HTTP traffic, resulting in HTTPS. Configuring HTTPS on the REST Proxy ensures that all data in transit between the client and the proxy is encrypted, providing confidentiality and integrity. This is the primary and standard method for securing this specific communication link.

B. MD5: This is a cryptographic hash function used for verifying data integrity, not a protocol for securing a communication channel.

C. SCRAM: This is a SASL (Simple Authentication and Security Layer) mechanism used for authenticating clients to Kafka brokers. The REST Proxy itself may use SCRAM to authenticate to the brokers, but this is not used for the REST client-to-proxy connection.

D. Kerberos: This is a network authentication protocol, also typically used via SASL for authenticating components within the Kafka ecosystem (e.g., REST Proxy to brokers), not for encrypting the transport layer between a standard REST client and the proxy's HTTP endpoint.

1. Confluent Documentation, "REST Proxy Security": In the section "Encrypt traffic between REST clients and the REST Proxy," the documentation explicitly states: "To encrypt traffic between REST clients and the REST Proxy, configure HTTPS for the REST Proxy. To enable HTTPS for the REST Proxy, set the listeners configuration property to use HTTPS..." This directly confirms that TLS (HTTPS) is the supported method.

Source: Confluent Documentation, REST Proxy Security, Section: "Encrypt traffic between REST clients and the REST Proxy".

2. Confluent Documentation, "REST Proxy Configuration Options": The configuration options for the REST Proxy include a suite of ssl. properties such as ssl.keystore.location, ssl.keystore.password, and ssl.key.password. These properties are standard for configuring TLS/SSL on a Java-based server, which is necessary to enable HTTPS.

Source: Confluent Documentation, REST Proxy Configuration Options, Section: "SSL".