How to use the jsrsasign.b64utoutf8 function in jsrsasign

const idToken = tokenInfo.id_token;

    // A token must exist to be valid.
    if (!idToken) {
        return null;
    }

    const tokenParts = idToken.split('.');

    if (tokenParts.length !== 3) {
        return null;
    }

    const payload
         = rs.KJUR.jws.JWS.readSafeJSONString(rs.b64utoutf8(tokenParts[1]));

    if (payload.nonce !== guids.authNonce
        || payload.aud !== appId
        || payload.iss
            !== `https://login.microsoftonline.com/${payload.tid}/v2.0`) {
        return null;
    }

    const now = new Date();

    // Adjust by 5 minutes to allow for inconsistencies in system clocks.
    const notBefore = new Date((payload.nbf - 300) * 1000);
    const expires = new Date((payload.exp + 300) * 1000);

    if (now < notBefore || now > expires) {
        return null;

static parse(plain: Response): ?Session {
    const token = plain.data.metadata ? plain.data.metadata.jwt : null;
    let authorizations = [];

    // Add authorizations from JWT
    if (token) {
      const isValid = jws.JWS.verifyJWT(token, swarmKey, { alg: ['RS256'], verifyAt: new Date() });
      if (isValid) {
        const decodedToken = jws.JWS.readSafeJSONString(b64utoutf8(token.split('.')[1]));
        authorizations = decodedToken ? decodedToken.authorizations : [];
      }
    }

    return new Session({
      token: plain.data.token,
      refreshToken: plain.data.refresh_token || null,
      uuid: plain.data.metadata ? plain.data.metadata.uuid : null,
      sessionUuid: plain.data.session_uuid,
      authorizations,
      tenantUuid: plain.data.metadata ? plain.data.metadata.tenant_uuid : undefined,
      expiresAt: new Date(`${plain.data.utc_expires_at}z`),
    });
  }