The payload.json file that has the correct contents is C. This file contains a JSON object with a single key, “plaintext”, and a value that is the base64-encoded string of the data to be encrypted. This is the format that the Vault API expects for the transit encrypt endpoint1. The other files are not correct because they either have the wrong key name, the wrong value format, or the wrong JSON syntax.

References:

• Encrypt Data - Transit Secrets Engine | Vault | HashiCorp Developer

The PKI secrets engine is designed to support the use case of reducing and ultimately removing the use of long lived X.509 certificates. The PKI secrets engine can generate dynamic X.509 certificates on demand, with short time-to-live (TTL) and automatic revocation. This eliminates the need for manual processes of generating, signing, and rotating certificates, and reduces the risk of certificate compromise or misuse. The PKI secrets engine can also act as a certificate authority (CA) or an intermediate CA, and can integrate with external CAs or CRLs. The PKI secrets engine can issue certificates for various purposes, such as TLS, SSH, code signing, email encryption, etc. References: https://developer.hashicorp.com/vault/docs/secrets/pki 1, https://developer.hashicorp.com/vault/tutorials/getting-started/getting-started-dynamic-secrets

Secrets engines are components that store, generate, or encrypt data in Vault. They are enabled at a specific path in Vault and have their own API and configuration. Some of the statements that describe the secrets engines in Vault are:

• Some secrets engines simply store and read data, such as the key/value secrets engine, which acts like an encrypted Redis or Memcached. Other secrets engines perform more complex operations, such as generating dynamic credentials, encrypting data, issuing certificates, etc1.
• You can build your own custom secrets engine by using the plugin system, which allows you to write and run your own secrets engine as a separate process that communicates with Vault over gRPC. You can also use the SDK to create your own secrets engine in Go and compile it into Vault2.
• Each secrets engine is isolated to its path, which means that the secrets engine cannot access or interact with other secrets engines or data outside its path. The path where the secrets engine is enabled can be customized and can have multiple segments. For example, you can enable the AWS secrets engine at aws/ or aws/prod/ or aws/dev/3.

The statements that are not true about the secrets engines in Vault are:

• You can disable an existing secrets engine by using the vault secrets disable command or the sys/mounts API endpoint. When a secrets engine is disabled, all of its secrets are revoked and all of its data is deleted from the storage backend4.
• A secrets engine can be enabled at multiple paths, with a few exceptions, such as the system and identity secrets engines. Each secrets engine enabled at a different path is independent and isolated from others. For example, you can enable the KV secrets engine at kv/ and secret/ and they will not share any data3.

References: 1(https://developer.hashicorp.com/vault/docs/secrets),  2(https://developer.hashicorp.com/vault/docs/secrets),  3(https://developer.hashicorp.com/vault/docs/secrets),  4(https://developer.hashicorp.com/vault/docs/secrets)