Use a random impl with rejection sampling

spec/unit/randomstring.spec.ts

@@ -24,6 +24,10 @@ import {
 } from "../../src/randomstring";
 
 describe("Random strings", () => {
+    afterEach(() => {
+        jest.restoreAllMocks();
+    });
+
     it.each([8, 16, 32])("secureRandomBase64 generates %i valid base64 bytes", (n: number) => {
         const randb641 = secureRandomBase64Url(n);
         const randb642 = secureRandomBase64Url(n);
@@ -70,4 +74,41 @@ describe("Random strings", () => {
             expect(rand1.toUpperCase()).toEqual(rand1);
         },
     );
+
+    it("throws if given character set less than 2 characters", () => {
+        expect(() => secureRandomStringFrom(8, "a")).toThrow();
+    });
+
+    it("throws if given character set more than 256 characters", () => {
+        const charSet = Array.from({ length: 257 }, (_, i) => "a").join("");
+
+        expect(() => secureRandomStringFrom(8, charSet)).toThrow();
+    });
+
+    it("throws if given length less than 1", () => {
+        expect(() => secureRandomStringFrom(0, "abc")).toThrow();
+    });
+
+    it("throws if given length more than 32768", () => {
+        expect(() => secureRandomStringFrom(32769, "abc")).toThrow();
+    });
+
+    it("asks for more entropy if given entropy is unusable", () => {
+        // This is testing the internal implementation details of the function rather
+        // than strictly the public API. The intention is to have some assertion that
+        // the rejection sampling to make the distribution even over all possible characters
+        // is doing what it's supposed to do.
+
+        // mock once to fill with 255 the first time: 255 should be unusable because
+        // we give 10 possible characters below and 256 is not evenly divisible by 10, so
+        // this should force it to call for more entropy.
+        jest.spyOn(globalThis.crypto, "getRandomValues").mockImplementationOnce((arr) => {
+            if (arr === null) throw new Error("Buffer is null");
+            new Uint8Array(arr.buffer).fill(255);
+            return arr;
+        });
+
+        secureRandomStringFrom(8, "0123456789");
+        expect(globalThis.crypto.getRandomValues).toHaveBeenCalledTimes(2);
+    });
 });