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twofa

TouchID-aware 2-factor authentication for macOS

Licenced under an MIT licence. | Maintainer: Jānis Kiršteins <janis@sqreen.com> (@jkirsteins)

Copyright © 2019 Sqreen

Screenshot of twofa asking for authentication

Table of Contents

Overview

twofa is written in Swift. It uses the macOS keychain API to store one-time password generation parameters using the .userPresence and .whenPasscodeSetThisDeviceOnly constraints.

This constraint will require the user's password or TouchID to access the data.

Note: It is possible to register both time-based (TOTP) and HMAC-based (HOTP) accounts, but only TOTP code generation is implemented.

Rationale

While commonly 2FA (2-factor authentication) is used via a smartphone app, for heavy use a computer-based code generation can be more efficient. While there are some existing TOTP (time-based one time password) generating applications for computers, they have one or more drawbacks:

  • they require you to enter a password every time you want to use it
  • no command line interface

Sidenote: there probably exist some authenticator apps that store the secrets in plaintext on the filesystem, and do not have the first problem. However, if such a tool existed, it would be far too insecure to use in a serious capacity

It seems that there are no good solutions available currently that mitigate these, so this application was built to:

  • allow (on TouchID-equipped Macs) to use the fingerprint sensor, instead of requiring a password. This provides a much better user experience, especially if you need to generate codes often.
  • be usable from the terminal. This is a subjective point and not a problem perse, but some people prefer the simplicity of Terminal.app. While it is not inconceivable that a GUI (graphical user interface) is added to this project down the line, it seems much less likely that a GUI-first project would add a complementary command-line utility down the line.

Sidenote: originally the idea was to have TouchID mandatory, but the macOS keychain API does not support this at the moment. If such an API became available, it could be adopted to gain much stronger security guarantees. The current approach of entangling the secrets with the user's password is likely brute-forceable, whereas a hardware-backed approach would not be (or, at the very least, would make it considerably more difficult to perform an attack).

Supported macOS Versions

Built and tested on macOS Catalina 10.15.5.

Info.plist

An Info.plist is embedded into the binary during linking, and the version command will read the bundle version from that.

macOS entitlements

The macOS keychain API requires the binary to be codesigned and contain appropriate entitlements.

Since this is a command-line utility, some extra work needs to be done to perform a functioning build:

  • obtain a valid macOS codesigning key (obtainable from the Apple Developer website)
    • install the corresponding provisioning profile locally, or the binary will not run for you
    • to have the binary work when distributed, you will need to sign with a Developer ID app distribution key
  • find out your team ID (e.g. similar to N2JEMR5FZG)
  • configure the file Supporting/twofa.entitlements to contain:
    • a valid application identifier
    • your team identifier
    • a valid keychain access group
  • codesign the binary after building using codesign and embed the entitlements from Supporting/twofa.entitlements into it

NOTE: codesigning is required even when developing locally, otherwise the keychain API will not work

Platform Support

Only macOS is supported and tested.

However, it should be possible to add Linux support using a different underlying keychain API.

Usage

If you want to test the app with a test account, you can use http://security-totp.appspot.com

What you want to do normally is run:

$ twofa add

And then take a screenshot (to the pasteboard, not to a file - i.e. using Command-Control-Shift-4) of a QR code containing your OTP (one-time password) seed.

It should print out that some data was found, and that it is adding an account with the name from the QR code. E.g.:

Found: test@security-totp.appspot.com

Then, to get the OTP, type:

# substitute the last argument with your actual account name
twofa get test@security-totp.appspot.com

The OTP will not be output by default, and instead will be put directly into the pasteboard.

(After the application terminates, the previous pasteboard content will be restored.)

add

$ twofa add [--label <label> --secret <secret>] [--uri <otpauth_uri>] [--debug]

You can call this with no extra arguments, and the default behaviour will be to wait for a valid screenshot to appear in the pasteboard of your Mac.

However, you can also provide the label and secret directly:

twofa add --label account_name --secret LZYSI2TSMRSWOYJSPEYSM5Q

You can also provide an otpauth:// URI:

twofa add --uri otpauth://totp/avi-9605?secret=LZYSI2TSMRSWOYJSPEYSM5Q&issuer=SparkPost

Finally, if you add --debug then the parsed data will be output to stdout so you can see what is being stored in the keychain. This is not recommended normally, as it will leave sensitive data in your history.

This command does not require authentication.

list

$ twofa list

twofa list will list all the account labels that you have added. It will require authentication (unless there are no accounts added, in which case it will print out nothing and not ask for authentication).

rm

$ twofa rm <label>

twofa rm account_name will remove the specified account name from the keychain.

This will require authentication (although the keychain itself does not require authentication for item removal)

test

$ twofa test

This is a handy method just to check if the entitlements are correct, and the application is authorized to access the keychain.

It should ask for authentication, and print OK.

get

$ twofa get <label> [--stdout]

This will calculate the current OTP for the specified account, and copy it to the pasteboard.

Pasteboard content will be restored once the application closes.

If the application is not closed, it will automatically terminate in a short while on its own (so you can not forget it running in the background).

By default, it will not output the codes to stdout, however if you pass the --stdout flag, then the code will be output in addition to being put into the pasteboard.

Installation

To install a pre-built binary, you will need Homebrew:

$ brew tap sqreen/speakeasy
$ brew install twofa

Building from Source

Before Building

Before you build, you must:

  • edit Supporting/Info.plist (optional)
  • edit Supporting/twofa.entitlements (important)
  • edit build_mac.sh and replace the code sign identity string with the value corresponding to your Apple-issued key that you will use to codesign

Linux Dependencies

$ apt-get install openssl libssl-dev

swift build

NOTE: Package.resolved is kept in .gitignore because it gets regenerated when switching Platforms (removing Mac/Linux specific packages, and adding Linux/Mac specific packages, and vice versa)

If you want to build using swift build, you can use the script build.sh. It will:

  • build a release version
  • embed the Info.plist into it
  • codesign it with the entitlements file

Xcode

If you want to use Xcode, you can use the accompanying regen.rb script to generate a valid Xcode project.

It invokes swift package generate-xcodeproj and then applies configuration changes to:

  • embed the Info.plist into the binary after building
  • use the correct entitlements file
  • set the correct signing identity

Testing

$ swift test

Contributing

Contributions are welcomed and encouraged! Take a look at the contribution guidelines.