diff --git a/README.md b/README.md
index 128c2b5..2baad97 100644
--- a/README.md
+++ b/README.md
@@ -44,10 +44,11 @@ using your preferred package manager (`npm i keymask`, `yarn add keymask`,
 
 ## Usage
 
-The module exports three classes, `Keymask`, `KeymaskGenerator` (the LCG) and
-`KeymaskEncoder` (the character encoder). These can be used independently of
-each other, but for simple use cases, the main `Keymask` class is typically all
-you need.
+The module exports three main classes, `Keymask`, `KeymaskGenerator` (the LCG)
+and `KeymaskEncoder` (the character encoder). These can be used independently
+of each other, but for simple use cases, the main `Keymask` class is typically
+all you need. There is also an additional class, `StrictKeymask` that can be
+used in special cases (described below under the `safe` option).
 
 **Example (Default settings)**
 
@@ -144,7 +145,11 @@ const unmask = keymask.unmask("xMMJdmtCcf"); // 123456789
 
 ### `safe`
 
-Safe mode is triggered using a boolean flag on the options object.
+Safe mode is triggered using a boolean flag on the options object. This
+prevents encoded keymasks from containing any uppercase characters, making it
+suitable for use in case-insensitive settings (such as a subdomain). It also
+increases the block size from 12 to 14, something to bear in mind when
+configuring the output size.
 
 **Example (Safe mode)**
 
@@ -159,6 +164,18 @@ const masked = keymask.mask(123456789); // "mfwbdg"
 const unmask = keymask.unmask("mfwbdg"); // 123456789
 ```
 
+#### `StrictKeymask`
+
+Some systems, in addition to being case-insensitive, do not allow the first
+character of a string to be a numeral. In these cases, the `StrictKeymask`
+class can be used as a replacement for the main `Keymask` class. This class
+forces `safe` mode, and overrides the `mask` and `unmask` functions so that
+initial numeric characters are replaced with a vowel.
+
+Although this introduces vowels into the encoding, thus the possibility of
+recognizable words, offensive words beginning with `e`, `i`, `o` or `u` (and
+containing no other vowels) are relatively uncommon.
+
 ### `type`
 
 By default, `Keymask` unmasks values as a `number` when possible, while larger
diff --git a/package.json b/package.json
index cbbe34b..2e8694f 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "keymask",
-  "version": "0.10.0",
+  "version": "0.10.1",
   "description": "Map sequential IDs or serial numbers to random-looking strings",
   "type": "module",
   "exports": {
diff --git a/src/Keymask.ts b/src/Keymask.ts
index 98f2fcb..2570b7d 100644
--- a/src/Keymask.ts
+++ b/src/Keymask.ts
@@ -120,8 +120,8 @@ export class Keymask<T extends KeymaskType> {
     } else if (options.seed) {
       this.encoder = new KeymaskEncoder(options.seed, options.safe);
       this.generator = new KeymaskGenerator(
-        options.seed.byteLength < (options.safe ? 20 : 32) 
-          ? void 0 
+        options.seed.byteLength < (options.safe ? 20 : 32)
+          ? void 0
           : options.seed.slice(options.safe ? 12 : 24),
         options.safe
       );
@@ -225,4 +225,45 @@ export class Keymask<T extends KeymaskType> {
           : n
     ) as KeymaskValue<T>;
   }
+}
+
+/**
+ * `StrictKeymask` extends the base `Keymask` class, forcing the `safe: true`
+ * option, while also preventing the first character of a keymask from being a
+ * number (replacing it with a vowel if present).
+ */
+export class StrictKeymask<T extends KeymaskType> extends Keymask<T> {
+
+  constructor(options?: KeymaskOptions<T>) {
+    super({ ...options, safe: true });
+  }
+
+  mask(value: KeymaskData): string {
+    const result = super.mask(value);
+    const first = result.charAt(0);
+    return first === "5"
+      ? "e" + result.substring(1)
+      : first === "9"
+        ? "i" + result.substring(1)
+        : first === "7"
+          ? "o" + result.substring(1)
+          : first === "2"
+            ? "u" + result.substring(1)
+            : result;
+  }
+
+  unmask(value: string): KeymaskValue<T> {
+    const first = value.charAt(0);
+    return super.unmask(
+      first === "e"
+        ? "5" + value.substring(1)
+        : first === "i"
+          ? "9" + value.substring(1)
+          : first === "o"
+            ? "7" + value.substring(1)
+            : first === "u"
+              ? "2" + value.substring(1)
+              : value
+    );
+  }
 }
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diff --git a/src/index.ts b/src/index.ts
index 1ccfd2e..177f95b 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,3 +1,10 @@
-export { Keymask, type KeymaskOptions } from "./Keymask";
+export {
+  Keymask,
+  StrictKeymask,
+  type KeymaskData,
+  type KeymaskOptions,
+  type KeymaskType,
+  type KeymaskValue
+} from "./Keymask";
 export { KeymaskEncoder } from "./KeymaskEncoder";
 export { KeymaskGenerator } from "./KeymaskGenerator";
\ No newline at end of file
diff --git a/test/Keymask.test.ts b/test/Keymask.test.ts
index 57d0ef5..facb25c 100644
--- a/test/Keymask.test.ts
+++ b/test/Keymask.test.ts
@@ -1,5 +1,5 @@
 import { equal, deepEqual } from "node:assert/strict";
-import { KeymaskEncoder, Keymask } from "../src/";
+import { KeymaskEncoder, Keymask, StrictKeymask } from "../src/";
 
 describe("Keymask", () => {
   describe("Default options", () => {
@@ -1387,4 +1387,117 @@ describe("Keymask", () => {
       deepEqual(keymask.unmask("g7qckgtvcghhh2nc"), buffer2);
     });
   });
+});
+
+describe("StrictKeymask", () => {
+  describe("Default options", () => {
+    const keymask = new StrictKeymask();
+
+    it("should mask and unmask in range 1", () => {
+      equal(keymask.mask(1), "k");
+      equal(keymask.mask(22), "w");
+      equal(keymask.unmask("k"), 1);
+      equal(keymask.unmask("w"), 22);
+    });
+
+    it("should mask and unmask in range 2", () => {
+      equal(keymask.mask(23), "t2");
+      equal(keymask.mask(508), "vw");
+      equal(keymask.unmask("t2"), 23);
+      equal(keymask.unmask("vw"), 508);
+    });
+
+    it("should mask and unmask in range 3", () => {
+      equal(keymask.mask(509), "tbn");
+      equal(keymask.mask(8190), "zhq");
+      equal(keymask.unmask("tbn"), 509);
+      equal(keymask.unmask("zhq"), 8190);
+    });
+
+    it("should mask and unmask in range 4", () => {
+      equal(keymask.mask(8191), "wccd");
+      equal(keymask.mask(262138), "jfjr");
+      equal(keymask.unmask("wccd"), 8191);
+      equal(keymask.unmask("jfjr"), 262138);
+    });
+
+    it("should mask and unmask in range 5", () => {
+      equal(keymask.mask(262139), "tm2wh");
+      equal(keymask.mask(4194300), "tcgpk");
+      equal(keymask.unmask("tm2wh"), 262139);
+      equal(keymask.unmask("tcgpk"), 4194300);
+    });
+
+    it("should mask and unmask in range 6", () => {
+      equal(keymask.mask(4194301), "izn2vj");
+      equal(keymask.mask(134217688), "n7dgfq");
+      equal(keymask.unmask("izn2vj"), 4194301);
+      equal(keymask.unmask("n7dgfq"), 134217688);
+    });
+
+    it("should mask and unmask in range 7", () => {
+      equal(keymask.mask(134217689), "jjc2rjd");
+      equal(keymask.mask(4294967290), "rmrd5ft");
+      equal(keymask.unmask("jjc2rjd"), 134217689);
+      equal(keymask.unmask("rmrd5ft"), 4294967290);
+    });
+
+    it("should mask and unmask in range 8", () => {
+      equal(keymask.mask(4294967291), "ohwj72gt");
+      equal(keymask.mask(68719476730), "wghjnphj");
+      equal(keymask.unmask("ohwj72gt"), 4294967291);
+      equal(keymask.unmask("wghjnphj"), 68719476730);
+    });
+
+    it("should mask and unmask in range 9", () => {
+      equal(keymask.mask(68719476731), "xhkztn29v");
+      equal(keymask.mask(2199023255530), "utfrbg7ps");
+      equal(keymask.unmask("xhkztn29v"), 68719476731);
+      equal(keymask.unmask("utfrbg7ps"), 2199023255530);
+    });
+
+    it("should mask and unmask in range 10", () => {
+      equal(keymask.mask(2199023255531), "wyjnjyf5pn");
+      equal(keymask.mask(35184372088776), "k5tvhvs9zm");
+      equal(keymask.unmask("wyjnjyf5pn"), 2199023255531);
+      equal(keymask.unmask("k5tvhvs9zm"), 35184372088776);
+    });
+
+    it("should mask and unmask in range 11", () => {
+      equal(keymask.mask(35184372088777), "eyqxcwjgcfv");
+      equal(keymask.mask(1125899906842596), "s5xtjhkjzgm");
+      equal(keymask.unmask("eyqxcwjgcfv"), 35184372088777);
+      equal(keymask.unmask("s5xtjhkjzgm"), 1125899906842596);
+    });
+
+    it("should mask and unmask in range 12", () => {
+      equal(keymask.mask(1125899906842597n), "kzd2kmfzbksd");
+      equal(keymask.mask(36028797018963912n), "zxdhgvqmsnxp");
+      equal(keymask.unmask("kzd2kmfzbksd"), 1125899906842597n);
+      equal(keymask.unmask("zxdhgvqmsnxp"), 36028797018963912n);
+    });
+
+    it("should mask and unmask in range 13", () => {
+      equal(keymask.mask(36028797018963913n), "fm7mspv5nt2mq");
+      equal(keymask.mask(576460752303423432n), "dgwh9ks2mj55k");
+      equal(keymask.unmask("fm7mspv5nt2mq"), 36028797018963913n);
+      equal(keymask.unmask("dgwh9ks2mj55k"), 576460752303423432n);
+    });
+
+    it("should mask and unmask in range 14", () => {
+      equal(keymask.mask(576460752303423433n), "cfkgghxwykkjsj");
+      equal(keymask.mask(18446744073709551556n), "xfhpdkrn9fvpxp");
+      equal(keymask.unmask("cfkgghxwykkjsj"), 576460752303423433n);
+      equal(keymask.unmask("xfhpdkrn9fvpxp"), 18446744073709551556n);
+    });
+
+    it("should process binary data", () => {
+      const buffer1 = new Uint8Array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]).buffer;
+      const buffer2 = new Uint8Array([11, 22, 33, 44, 55, 66, 77, 88, 99]).buffer;
+      equal(keymask.mask(buffer1), "gjph95tz2792txgrfmn9dvvhn9vm");
+      equal(keymask.mask(buffer2), "ixcxhygbpfrvngnm");
+      deepEqual(keymask.unmask("gjph95tz2792txgrfmn9dvvhn9vm"), buffer1);
+      deepEqual(keymask.unmask("ixcxhygbpfrvngnm"), buffer2);
+    });
+  });
 });
\ No newline at end of file