[REQUIRED] Describe your environment

• Operating System version: OSX
• Browser version: Firefox 83
• Firebase SDK version: 8.1.2
• Firebase Product: auth

[REQUIRED] Describe the problem

The token created with with custom claims by admin.auth().createCustomToken(uid, additionalClaims) are encoded in base64 and getIdTokenResult is decoding it on the client but the base64 decoding causes issues with the UTF-8 encoded strings.

Steps to reproduce:

Authenticate user with a custom token with utf-8 claims:

admin.auth()
        .createCustomToken("123", {role: "Админ"})
       ..

And on the JS side try to read it with:

 user.getIdTokenResult(false)
                    .then(idToken => {
                      const { role } = idToken.claims
                      // role here is bad encoded 

As a workaround the encoded role with base64 on the server and decoding it with the following function:

function b64DecodeUnicode(str) {
    // Going backwards: from bytestream, to percent-encoding, to original string.
    return decodeURIComponent(atob(str).split('').map(function (c) {
        return '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2);
    }).join('')).trim();
}

is working properly.