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Minimal TOTP generator in 20 lines of Python
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MinTOTP

MinTOTP is a minimal TOTP generator written in Python.

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Contents

Introduction

TOTP stands for Time-Based One-Time Password. Many websites and services require two-factor authentication (2FA) or multi-factor authentication (MFA) where the user is required to present two or more pieces of evidence:

  • Something only the user knows, e.g., password, passphrase, etc.
  • Something only the user has, e.g., hardware token, mobile phone, etc.
  • Something only the user is, e.g., biometrics.

A TOTP value serves as the second factor, i.e., it proves that the user is in possession of a device (e.g., mobile phone) that contains a TOTP secret key from which the TOTP value is generated. Usually the service provider that provides a user's account also issues a secret key encoded either as a Base32 string or as a QR code. This secret key is added to an authenticator app (e.g., Google Authenticator) on a mobile device. The app can then generate TOTP values based on the current time. By default, it generates a new TOTP value every 30 seconds.

MinTOTP is a Python tool that can be used to generate TOTP values from a secret key. Additionally, it exposes its functionality as module-level functions for Python developers. It can be used on any system with Python 3.4 or later installed on it.

Source Code

At the heart of the TOTP algorithm lies the HOTP algorithm. HOTP stands for HMAC-based One-Time Password. HMAC stands for Hash-based Message Authentication Code. Here are the relevant RFCs to learn more about these algorithms:

  • RFC 2104: HMAC: Keyed-Hashing for Message Authentication
  • RFC 4226: HOTP: An HMAC-Based One-Time Password Algorithm
  • RFC 6238: TOTP: Time-Based One-Time Password Algorithm

The source code in mintotp.py generates TOTP values from a secret key and current time. It's just 30 lines of code (actually 20 lines if we ignore the shebang and blank lines). There are no comments in the code, so a brief description of the code is presented in this section. Here is the entire code presented once again for convenience:

#!/usr/bin/python3

import base64
import hmac
import struct
import sys
import time


def hotp(key, counter, digits=6, digest='sha1'):
    key = base64.b32decode(key.upper() + '=' * ((8 - len(key)) % 8))
    counter = struct.pack('>Q', counter)
    mac = hmac.new(key, counter, digest).digest()
    offset = mac[-1] & 0x0f
    binary = struct.unpack('>L', mac[offset:offset+4])[0] & 0x7fffffff
    return str(binary)[-digits:].rjust(digits, '0')


def totp(key, time_step=30, digits=6, digest='sha1'):
    return hotp(key, int(time.time() / time_step), digits, digest)


def main():
    args = [int(x) if x.isdigit() else x for x in sys.argv[1:]]
    for key in sys.stdin:
        print(totp(key.strip(), *args))


if __name__ == '__main__':
    main()

In the code above, we use the hmac module available in the Python standard library to implement HOTP. The implementation can be found in the hotp() function. It is a pretty straightforward implementation of RFC 2104: Section 5: HOTP Algorithm. It takes a Base32-encoded secret key and a counter as input. It returns a 6-digit HOTP value as output.

The totp() function implements the TOTP algorithm. It is a thin wrapper around the HOTP algorithm. The TOTP value is obtained by invoking the HOTP function with the secret key and the number of time intervals (30 second intervals by default) that have elapsed since Unix epoch (1970-01-01 00:00:00 UTC).

Install

MinTOTP requires Python 3.4 or later. If Python 3.4 or later is present on your system, follow one of the two sections below to get MinTOTP on your system.

From PyPI

If you want to install the MinTOTP package from PyPI as a Python module on your system, then follow the steps provided below. Doing so makes MinTOTP available as the mintotp command that you can run on the terminal. A module named mintotp also becomes available that you can import in your own Python code.

  1. Enter the following command to install MinTOTP on your system:

    pip3 install mintotp
  2. Test that MinTOTP works fine as a command:

    mintotp <<< ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS

    A 6-digit TOTP value should appear as the output.

  3. Test that MinTOTP can be used as a library module:

    $ python3
    >>> import mintotp
    >>> mintotp.totp('ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS')
    >>> mintotp.hotp('ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS', 42)

    The totp() function call should return a 6-digit TOTP value based on the current time. The hotp() call should return the following HOTP value: 626854.

From GitHub

If you do not want to install MinTOTP to your system as a command but you want to work with the mintotp.py source file directly clone the GitHub repository of this project.

  1. Clone GitHub repository of this project and enter its top-level directory.

    git clone https://github.com/susam/mintotp.git
    cd mintotp
  2. Test that mintotp.py works fine:

    python3 mintotp.py <<< ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS

    A 6-digit TOTP value should appear as the output.

  3. Test that mintotp.py can be imported as a module:

    $ python3
    >>> import mintotp
    >>> mintotp.totp('ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS')
    >>> mintotp.hotp('ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS', 42)

    The totp() function call should return a 6-digit TOTP value based on the current time. The hotp() call should return the following HOTP value: 626854.

All examples provided in the sections below assume that MinTOTP has been installed from PyPI. If you choose to use MinTOTP from GitHub instead, replace all occurrences of mintotp in the example commands below with python3 mintotp.py.

Get Started

This section presents a few examples to quickly get started with MinTOTP.

Note that this section uses a few example secret keys and QR codes. They are merely examples that come with this project for you to quickly test the program with. They should not be used for any real account that requires TOTP-based two-factor authentication. Usually, the issuer of a real account (such as an account on a website or an organization) would also issue a secret key or a secret QR code to you which you must use to generate TOTP values for the purpose of logging into that account.

With Base32 Key

  1. Enter this command:

    mintotp <<< ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS

    The output should be a 6-digit TOTP value.

  2. Add the following key to a TOTP-based authenticator app:

    ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS
    

    For example, if you have Google Authenticator on your mobile phone, open it, tap the button with plus sign, select "Enter a provided key", enter any account name and "Time-based" and enter the above key. Set the dropdown menu to "Time-based" and tap the "Add" button. A 6-digit TOTP value should appear for the new key.

  3. Run the command in step 1 again and verify that the TOTP value printed by the Python program matches the TOTP value that appears in the authenticator app.

With Encrypted Base32 Key

The previous section uses an example key to show how this tool works. If you use this tool to generate TOTP values from a real secret key for a real account, you must encrypt your secret key to keep it safe.

The steps below show the usage of GPG to encrypt our example secret key. You would have to replace this example secret key with a real secret key that you want to use to generate TOTP values.

  1. Install GNU Privacy Guard (also known as GnuPG or GPG):

    # On macOS
    brew install gnupg
    
    # On Debian, Ubuntu, etc.
    apt-get update
    apt-get install gnupg
  2. Encrypt the secret key using GPG. First enter this command:

    gpg -c -o secret.gpg

    Then enter a strong passphrase when it prompts for it. Re-enter the passphase to confirm it. Then paste the following key as input:

    ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS
    

    Press enter to end the line. Press control + d to end input. The encrypted secret key would be saved in a file named secret.gpg.

  3. Generate TOTP value from the encrypted key:

    gpg -q -o - secret.gpg | mintotp
  4. You can also generate TOTP value and copy it to system clipboard:

    # On macOS
    gpg -q -o - secret.gpg | mintotp | pbcopy
    
    # On Linux
    gpg -q -o - secret.gpg | mintotp | xclip

    Now you can easily paste the TOTP value to any login form that requires it. On Linux, of course, you need to have xclip installed to use it. On Debian, Ubuntu, etc. it can be installed with the apt-get install xclip command. To paste the value copied into the clipboard by xclip, middle-click on mouse.

  5. In case you want to see the TOTP value on the terminal while it is also copied to the system clipboard, use one of these commands:

    # On macOS
    gpg -q -o - secret.gpg | mintotp | tee /dev/stderr | pbcopy
    
    # On Linux
    gpg -q -o - secret.gpg | mintotp | tee /dev/stderr | xclip

With QR Code

  1. Install zbarimg to scan QR codes:

    # On macOS
    brew install zbar
    
    # On Debian, Ubuntu, etc.
    apt-get install zbar-tools
  2. Download and save the following QR code on your system:
    QR code for TOTP secret key
    The QR code above can also be found in this file: secret1.png.

  3. Enter this command to read the data in the QR code:

    zbarimg -q secret1.png

    The output should be:

    QR-Code:otpauth://totp/alice:bob?secret=ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS
    

    Note that the secret key in the URI is same as the secret key we used in the previous sections.

  4. Now enter this command to extract the secret key from the QR code and feed it to MinTOTP.

    zbarimg -q secret1.png | sed 's/.*secret=\([^&]*\).*/\1/' | mintotp
  5. Scan the QR code shown above in step 3 with a TOTP-based authenticator app. For example, if you have Google Authenticator on your mobile phone, open it, tap the button with plus sign, select "Scan a barcode", and scan the QR code shown above in step 3. A 6-digit TOTP value should appear for the new key.

  6. Run the command in step 3 again and verify that the TOTP value printed by MinTOTP matches the TOTP value that appears in the authenticator app.

With Encrypted QR Code

The previous example uses an example QR code to show how this tool works. If you use this tool to generate TOTP values from a real QR code for a real account, you must encrypt your QR code to keep it safe.

The steps below show the usage of GPG to encrypt our example QR code. You would have to replace the example QR code with a real QR code that you want to use to generate TOTP values.

  1. Install GNU Privacy Guard (also known as GnuPG or GPG):

    # On macOS
    brew install gnupg
    
    # On Debian, Ubuntu, etc.
    apt-get update
    apt-get install gnupg
  2. Encrypt the QR code using GPG. First enter this command:

    gpg -c secret1.png

    Then enter a strong passphrase when it prompts for it. Re-enter the passphase to confirm it. The encrypted QR code would be saved in a file named secret1.png.gpg.

  3. Delete the unencrypted QR code file securely:

    # On macOS
    rm -P secret1.png
    
    # On Linux
    shred -u secret1.png
  4. Generate TOTP value from the encrypted QR code file:

    zbarimg -q <(gpg -q -o - secret1.png.gpg) | sed 's/.*secret=\([^&]*\).*/\1/' | mintotp
  5. You can also generate the TOTP value and copy it to system clipboard:

    # On macOS
    zbarimg -q <(gpg -q -o - secret1.png.gpg) | sed 's/.*secret=\([^&]*\).*/\1/' | mintotp | pbcopy
    
    # On Linux
    zbarimg -q <(gpg -q -o - secret1.png.gpg) | sed 's/.*secret=\([^&]*\).*/\1/' | mintotp | xclip
  6. In case you want to see the TOTP value on the terminal while it is also copied to the system clipboard, use one of these commands:

    # On macOS
    zbarimg -q <(gpg -q -o - secret1.png.gpg) | sed 's/.*secret=\([^&]*\).*/\1/' | mintotp | tee /dev/stderr | pbcopy
    
    # On Linux
    zbarimg -q <(gpg -q -o - secret1.png.gpg) | sed 's/.*secret=\([^&]*\).*/\1/' | mintotp | tee /dev/stderr | xclip

Multiple Keys

This tool accepts one or more Base32 secret keys as standard input. Each key must occur in its own line.

  1. Generate multiple TOTP values, one for each of multiple Base32 keys:

    mintotp <<eof
    ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS
    PW4YAYYZVDE5RK2AOLKUATNZIKAFQLZO
    eof
  2. Generate TOTP values for multiple keys in multiple QR codes:

    zbarimg -q *.png | sed 's/.*secret=\([^&]*\).*/\1/' | mintotp

Command Line Arguments

In order to keep this tool as minimal as possible, it does not come with any command line options. In fact, it does not even have the --help option. It does support a few command line arguments though. Since there is no help output from the tool, this section describes the command line arguments for this tool.

Here is a synopsis of the command line arguments supported by this tool:

mintotp [TIME_STEP [DIGITS [DIGEST]]]

Here is a description of each argument:

  • TIME_STEP

    TOTP time-step duration (in seconds) during which a TOTP value is valid. A new TOTP value is generated after time-step duration elapses. (Default: 30)

  • DIGITS

    Number of digits in TOTP value. (Default: 6)

  • DIGEST

    Cryptographic hash algorithm to use while generating TOTP value. (Default: `sha1)

    Possible values are sha1, sha224, sha256, sha384, and sha512.

Here are some usage examples of these command line arguments:

  1. Generate TOTP value with a time-step size of 60 seconds:

    mintotp 60 <<< ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS
  2. Generate 8-digit TOTP value:

    mintotp 60 8 <<< ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS
  3. Use SHA-256 hash algorithm to generate TOTP value:

    mintotp 60 6 sha256 <<< ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS

The behaviour of the tool is undefined if it is used in any way other than what is described above. For example, although surplus command line arguments are ignored currently, this behaviour may change in future, so what should happen in case of surplus arguments is left undefined in this document.

Tradeoff

If you use this tool to generate TOTP values on a desktop/laptop device while logging into a website that requires TOTP-based two-factor (2FA) or multi-factor authentication (MFA) from the same device, you should be aware that doing so trades off some security for convenience.

2FA or MFA relies on the user presenting at least two pieces of evidence (factors) to an authentication system: something only the user knows and something only the user has.

If this tool is run to generate TOTP values on the same desktop/laptop device that you are using to log into a website, then you should consider that if your desktop/laptop device is compromised, then both authentication factors can be compromised. The attacker can steal the first authentication factor that only you should know (e.g., password) by running a key logger on the compromised device. The attacker can also steal the second authentication factor that only you should have (e.g., TOTP secret key) because it would be read by this tool on the same compromised device; if this tool can read the TOTP secret key on the compromised device, so can the attacker.

In other words, for higher security, it is good to generate TOTP values on a separate device. However, if the inconvenience of getting a separate device prevents you from using 2FA or MFA altogether, then you might find this tool helpful. It allows to trade off some security for convenience which is still more secure than not having 2FA or MFA at all. Whether trading some security for convenience is acceptable to you or not is something you need to decide for yourself.

Alternative: OATH Toolkit

There is an oathtool command available in OATH Toolkit that can also generate TOTP values from TOTP secret keys. However, one of the issues currently with oathtool is that it requires the secret key to be provided as a command line argument which is generally insecure because command line arguments are visible to all system users and processes. See the following two posts to read more about this:

The oathtool command is intended to be used as a debugging tool and not for generating real TOTP values for real accounts. It would remain so until the issue described in the two URLs above is fixed. However, it is still good to be aware that oathtool can do what MinTOTP does. This section presents some examples about this:

  1. Install OATH Toolkit:

    # On macOS
    brew install oath-toolkit
    
    # On Debian, Ubuntu, etc.
    apt-get update
    apt-get install oathtool
  2. Generate TOTP from a Base32 key:

    oathtool --totp -b ZYTYYE5FOAGW5ML7LRWUL4WTZLNJAMZS
  3. Generate TOTP from an encrypted Base32 key:

    oathtool --totp -b $(gpg -q -o - secret.gpg)

    Section With Encrypted Base32 Key explains how to create the encrypted key (secret.gpg) with GPG.

  4. Generate TOTP from a QR code:

    oathtool --totp -b $(zbarimg -q secret1.png | sed 's/.*secret=\([^&]*\).*/\1/')
  5. Generate TOTP from an encrypted QR code:

    oathtool --totp -b $(zbarimg -q <(gpg -q -o - secret1.png.gpg) | sed 's/.*secret=\([^&]*\).*/\1/')

    Section With Encrypted QR Code explains how to create the encrypted QR code (secret1.png.gpg) with GPG.

  6. Generate TOTP value and copy it to system clipboard:

    # On macOS
    oathtool --totp -b $(gpg -q -o - secret.gpg) | pbcopy
    oathtool --totp -b $(zbarimg -q <(gpg -q -o - secret1.png.gpg) | sed 's/.*secret=\([^&]*\).*/\1/') | pbcopy
    
    # On Linux
    oathtool --totp -b $(gpg -q -o - secret.gpg) | xclip
    oathtool --totp -b $(zbarimg -q <(gpg -q -o - secret1.png.gpg) | sed 's/.*secret=\([^&]*\).*/\1/') | xclip

Resources

Here is a list of useful links about this project:

License

This is free and open source software. You can use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of it, under the terms of the MIT License. See LICENSE.md for details.

This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND, express or implied. See LICENSE.md for details.

Support

To report bugs, suggest improvements, or ask questions, please create a new issue at http://github.com/susam/mintotp/issues.

Thanks

Thanks to Prateek Nischal for getting me involved with TOTP. I referred to his TOTP implementation at prateeknischal/qry/util/totp.py while writing my own.

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