diff --git a/src/cli/index.ts b/src/cli/index.ts
index 8e42f2a..0491742 100644
--- a/src/cli/index.ts
+++ b/src/cli/index.ts
@@ -4,4 +4,5 @@ export * as splatnet2 from './splatnet2.js';
 export * as nooklink from './nooklink.js';
 export * as pctl from './pctl.js';
 export * as androidZncaApiServerFrida from './android-znca-api-server-frida.js';
+export * as util from './util.js';
 export * as app from './app.js';
diff --git a/src/cli/util.ts b/src/cli/util.ts
new file mode 100644
index 0000000..0959a6c
--- /dev/null
+++ b/src/cli/util.ts
@@ -0,0 +1,20 @@
+import createDebug from 'debug';
+import type { Arguments as ParentArguments } from '../cli.js';
+import { Argv, YargsArguments } from '../util.js';
+import * as commands from './util/index.js';
+
+const debug = createDebug('cli:util');
+
+export const command = 'util <command>';
+export const desc = 'Utilities';
+
+export function builder(yargs: Argv<ParentArguments>) {
+    for (const command of Object.values(commands)) {
+        // @ts-expect-error
+        yargs.command(command);
+    }
+
+    return yargs;
+}
+
+export type Arguments = YargsArguments<ReturnType<typeof builder>>;
diff --git a/src/cli/util/captureid.ts b/src/cli/util/captureid.ts
new file mode 100644
index 0000000..857c386
--- /dev/null
+++ b/src/cli/util/captureid.ts
@@ -0,0 +1,82 @@
+import * as crypto from 'crypto';
+import createDebug from 'debug';
+import type { Arguments as ParentArguments } from '../../cli.js';
+import { Argv } from '../../util.js';
+
+const debug = createDebug('cli:util:captureid');
+
+export const command = 'captureid';
+export const desc = 'Encrypt/decrypt capture IDs';
+
+export function builder(yargs: Argv<ParentArguments>) {
+    return yargs.demandCommand().command('encrypt <titleid>', 'Title ID to Capture ID', yargs => {
+        return yargs.positional('titleid', {
+            describe: 'Title ID',
+            type: 'string',
+            demandOption: true,
+        });
+    }, argv => {
+        console.log(encrypt(argv.titleid));
+    }).command('decrypt <captureid>', 'Capture ID to Title ID', yargs => {
+        return yargs.positional('captureid', {
+            describe: 'Capture ID',
+            type: 'string',
+            demandOption: true,
+        });
+    }, argv => {
+        console.log(decrypt(argv.captureid));
+}
+
+const key = Buffer.from('b7ed7a66c80b4b008baf7f0589c08224', 'hex');
+
+/**
+ * @param {string} tid Hex-encoded 8-byte title ID
+ * @return {string} Hex-encoded 16-byte capture ID
+ */
+export function encrypt(tid: string) {
+    if (typeof tid !== 'string' || !tid.match(/^[0-9A-Fa-f]{16}$/)) {
+        throw new Error('tid must be a valid title ID');
+    }
+
+    const tidb = Buffer.from('0000000000000000' + tid, 'hex').reverse();
+
+    const cipher = crypto.createCipheriv('aes-128-ecb', key, null);
+    cipher.setAutoPadding(false);
+
+    const cidb = Buffer.concat([
+        cipher.update(tidb),
+        cipher.final(),
+    ]);
+
+    const cid = cidb.toString('hex').toUpperCase();
+
+    return cid;
+}
+
+/**
+ * @param {string} cid Hex-encoded 16-byte capture ID
+ * @return {string} Hex-encoded 8-byte title ID
+ */
+export function decrypt(cid: string) {
+    if (typeof cid !== 'string' || !cid.match(/^[0-9A-Fa-f]{32}$/)) {
+        throw new Error('cid must be a valid capture ID');
+    }
+
+    const cidb = Buffer.from(cid, 'hex');
+
+    const cipher = crypto.createDecipheriv('aes-128-ecb', key, null);
+    cipher.setAutoPadding(false);
+
+    const tidb = Buffer.concat([
+        cipher.update(cidb),
+        cipher.final(),
+    ]).reverse();
+
+    if (!Buffer.alloc(8).equals(tidb.slice(0, 8))) {
+        throw new Error('Invalid title ID');
+    }
+
+    const tid = tidb.slice(8, 16).toString('hex');
+
+    return tid;
+}
diff --git a/src/cli/util/index.ts b/src/cli/util/index.ts
new file mode 100644
index 0000000..7560de2
--- /dev/null
+++ b/src/cli/util/index.ts
@@ -0,0 +1 @@
+export * as captureid from './captureid.js';