TypeScript: create new type that overrides types of some fields of base type

index.ts

/**
 * Let's say our base type is `Person` and we want to create a type like it,
 * where some fields are encrypted into a Buffer instead
 */

type Encrypted<Base, EncryptedFields extends keyof Base> = {
  [field in keyof Base]: field extends EncryptedFields ? Buffer : Base[field];
};

interface Person {
  fname: string;
  lname: string;
  nonencrypted: string;
}

// compiles:
const e: Encrypted<Person, "fname" | "lname"> = {
  fname: Buffer.alloc(0),
  lname: Buffer.alloc(0),
  nonencrypted: "dddd",
};

// Does not compile:
const t: Encrypted<Person, "fname"> = {
  fname: Buffer.alloc(0),
  lname: Buffer.alloc(0), // should be string
  nonencrypted: "dddd",
};