Two-factor authentication stops most account takeover attempts — credential stuffing, brute force, and leaked password reuse all fail against a second factor. But the type of 2FA matters, and not every free app handles backup and recovery the same way. Four apps cover the main options, each with a specific history worth knowing.
Aegis Authenticator: Best for Android
Aegis is a free, open-source TOTP authenticator for Android. The source code is publicly available on GitHub and has been independently reviewed. It supports encrypted vault backups to your device storage, which is the key feature that separates it from simpler apps: you can export an encrypted backup file, store it in Google Drive or a USB drive, and import it on a new device if your phone is lost or replaced.
Setup: Download from F-Droid or Google Play, create a new vault with a strong password, scan QR codes from services as you enable 2FA. After setup, immediately export an encrypted backup (Aegis menu → Backups → Export) and store it somewhere offline from your phone.
Aegis supports TOTP (30-second rotating codes used by most services), HOTP (counter-based, used by some older systems), and Steam authenticator format. No cloud sync by default — your codes stay on your device. Aegis is Android only; there is no iOS version.
Raivo OTP: Open-Source for iOS
Raivo OTP is a free, open-source authenticator for iOS that stores codes in iCloud Keychain for sync across Apple devices. The source is available on GitHub. For iOS users who want an open-source alternative to proprietary apps, Raivo is the standard recommendation.
One note: Raivo's original developer sold the app in 2023. The new owners made changes that caused concern in the security community regarding transparency. Review current app store reviews and GitHub activity before relying on Raivo for sensitive accounts. As an alternative, some iOS users use Strongbox (open-source password manager with TOTP support) or the built-in iOS Passwords app in iOS 17+, which added native TOTP support.
Google Authenticator: No Backup Until 2023
Google Authenticator is the most-installed 2FA app globally. For most of its history from 2010 to 2023, it stored codes locally on the device with no backup mechanism. Losing or resetting your phone meant losing all 2FA codes and requiring manual recovery on every account. This caused significant account lockout issues and was Google's most significant design failure for this app.
In April 2023, Google added optional Google Account cloud backup. Codes can now sync to your Google Account and restore on new devices. This is enabled by default for accounts signed into Google Authenticator.
The backup uses Google Account sync, which means Google has access to your 2FA seed data. The codes are encrypted in transit but stored in Google's infrastructure tied to your account. For users comfortable with Google's security model, this is a net improvement. For users who want zero cloud exposure of their 2FA seeds, Aegis (Android) or an offline manager is preferable.
Google Authenticator remains functional, widely compatible, and free. Its current version is meaningfully better than pre-2023 versions. The "no backup" criticism applies only to the historical versions.
Authy: Cloud Backup Plus a Breach History
Authy (Twilio Authy) provides free TOTP with multi-device sync and encrypted cloud backup. Your codes sync across all devices linked to your Authy account, which makes device loss recovery significantly easier than local-only apps. It is available on iOS, Android, Windows, and macOS.
The relevant history: in August 2022, Twilio (Authy's parent company) disclosed a breach of its customer support system. The attackers obtained access through a social engineering attack on Twilio employees. The breach exposed phone numbers for some Authy user accounts. Twilio confirmed that TOTP seed data — the actual one-time password secrets — was not exposed, because it is stored encrypted on user devices and on Twilio servers in a form that requires the user's device to decrypt.
Subsequently in 2024, a threat actor claimed to have obtained 33 million Authy user phone numbers through a Twilio API endpoint. Twilio confirmed the endpoint exposure. Phone numbers alone do not compromise 2FA codes, but exposed phone numbers can be used in SIM swapping attacks targeting those accounts.
Authy is still a functioning, widely-used 2FA app. These incidents are risk factors to weigh, not automatic disqualifiers. Users who want cloud backup and multi-device sync with less dependency on a single company's infrastructure may prefer Aegis with a manual backup process.
What 2FA Does Not Protect Against
SIM swapping. SMS 2FA codes can be intercepted if an attacker transfers your phone number. Authenticator apps are not vulnerable to SIM swapping. But the account itself may still be recoverable via SMS fallback if the service allows it. Disabling SMS as a backup option for critical accounts is advisable when possible.
Real-time phishing. A convincing fake login page that captures your 2FA code in real time can relay it to the legitimate site within the 30-second window. Authenticator apps do not prevent this. Hardware keys (YubiKey, Google Titan) use WebAuthn/FIDO2 to verify the domain cryptographically — phishing a hardware key code is physically impossible because the key won't authenticate to a fake domain. Hardware keys start at $25-$50 and are the strongest 2FA method available.
Malware on your device. If malware with sufficient permissions runs on the device where your authenticator app is installed, it can potentially read TOTP codes or seed data. This is why device security (keeping the OS updated, avoiding unknown APKs on Android) matters alongside 2FA selection.
Set up free breach alerts to get notified when your accounts appear in breach databases — catching credential exposure early reduces the window where 2FA is your only protection.
Honest Caveat
Any authenticator app is better than no 2FA. Even SMS 2FA — despite its known vulnerabilities — stops the vast majority of automated account takeover attempts. The attacker who successfully SIM-swaps your number or runs a real-time phishing operation is doing significant manual work targeting you specifically. The common attacks — credential stuffing, password spraying, breach reuse — are stopped by any 2FA implementation. Start with what you will actually use consistently; optimize later.
Frequently Asked Questions
What is the difference between SMS 2FA and authenticator app 2FA?
SMS 2FA sends a one-time code to your phone number via text message. Authenticator app 2FA generates a code locally on your device using the TOTP algorithm (Time-based One-Time Password). SMS 2FA is vulnerable to SIM swapping (where an attacker transfers your phone number to their SIM card) and SS7 network interception. Authenticator app codes are generated without any network communication and cannot be intercepted in transit. Authenticator apps are meaningfully more secure than SMS 2FA for any account worth protecting.
What happens to my 2FA codes if I lose my phone?
It depends on which app you use and whether you backed up. Aegis allows encrypted vault backups to your storage — if you exported a backup, you can import it into a new device. Google Authenticator added optional cloud backup to Google Account in 2023 — if enabled, restoring to a new device signed into the same Google account restores codes. Authy syncs to their cloud servers automatically. Without any backup, losing your phone means losing access to any account where the 2FA was set up — you would need account recovery codes (which services provide at 2FA setup) or contact the service\
Is Authy still safe to use after the Twilio breach?
Authy itself was not breached. The 2022 Twilio breach compromised Twilio\
Can authenticator app codes be phished?
Yes. An attacker who tricks you into entering your username, password, and 2FA code into a fake login page can use all three credentials in real time on the legitimate site within the 30-second TOTP window. This is called a real-time phishing attack or adversary-in-the-middle attack. Authenticator app 2FA protects against remote brute-force and credential stuffing but not against active phishing. Hardware security keys (FIDO2/WebAuthn) are the only 2FA method that prevents real-time phishing because they verify the domain cryptographically.
What is SIM swapping and how does it bypass 2FA?
SIM swapping is a social engineering attack where an attacker contacts your mobile carrier and convinces them to transfer your phone number to a SIM card the attacker controls. Once your number is on their device, they receive all SMS messages sent to your number — including SMS 2FA codes. Carriers vary in how easily they can be socially engineered. Authenticator apps are not vulnerable to SIM swapping because they generate codes locally without involving your phone number.