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Introduction[edit]

Tor Browser is installed in Whonix by default to browse the Internet anonymously. Tor Browser is optimized for safe browsing via pre-configured security and anonymity settings that are quite restrictive. Users are recommended to read the entire Tor Browser chapter for tips on basic usage before undertaking any high-risk activities.

Whonix-Workstation contains all the necessary tools to post or run a blog anonymously. It is recommended to review the following chapters / sections, as well as follow all the recommendations on this page:

Anonymous File Sharing[edit]

Audio Recordings[edit]

It is possible for adversaries to link audio recordings to the specific hardware (microphone) that is used. It is also trivial to fingerprint the embedded audio acoustics associated with the particular speaker device; for example, consider ringtones and video playback in public spaces. [1] For these reasons it is recommended to follow the operational security measures in the Photographs section when sharing audio files.

This recommendation equally applies to any data that is recorded by each and every other sensor component, such as accelermoeters. [2] The best way to defend against this threat is to deny all access to the hardware in question, while also avoid the sharing of unencrypted data recorded by sensors. Similarly, it is inadvisable to share audio with third parties who have limited technical ability or if they are potentially malicious.

Documents[edit]

Digital watermarks are a subset of the science of steganography and can be applied to any type of digital media, including audio, pictures, video, texts or 3D models. [3] In basic terms, covert markers are embedded into the "noise" of data which are imperceptible to humans: [4]

Digital watermarking is defined as inserted bits into a digital image, audio or video file that identify the copyright information; the digital watermarking is intended to be totally invisible unlike the printed ones, bits are scattered in different areas of the digital file in such a way that they cannot be identified and reproduced, otherwise the whole goal of watermarking is compromised.

A digital watermark is said to be robust if it remains intact even if modifications are made to the files. [5] [6] In addition to protecting copyright, another watermarking goal is to trace back information leaks to the specific source. A good countermeasure to this threat is to run documents through an optical character recognition (OCR) reader and share the output instead.

According to a talk by Sarah Harrison from WikiLeaks, [7] source tracing can also happen through much simpler techniques such as inspecting the access lists for the materials that have been leaked. For example, if only three people have access to a set of documents then the hunt is narrowed down considerably.

Redacting identifying information in electronic documents by means of image transformation (blurring or pixelization) has proven inadequate for concealing the intended text; the words can be reconstructed by machine learning algorithms. Solid bars are sufficient but they must be large enough to fully cover the original text. Otherwise, clues are left about the length of underlying word(s) which makes it easier to infer the censored text based on the sentence remainder. [8]

Photographs[edit]

Every camera's sensor has a unique noise signature because of subtle hardware differences. The sensor noise is detectable in the pixels of every image and video shot with the camera and could be fingerprinted. In the same way ballistics forensics can trace a bullet to the barrel it came from, the same can be accomplished with adversarial digital forensics for all images and videos. [9] [10] Note this effect is different from file Metadata that is easily sanitized with the Metadata Anonymization Toolkit.

Photo-Response NonUniformity[edit]

A camera fingerprint arises for the following reason: [11]

Photo-Response NonUniformity (PRNU) is an intrinsic property of all digital imaging sensors due to slight variations among individual pixels in their ability to convert photons to electrons. Consequently every sensor casts a weak noise-like pattern onto every image it takes and this pattern plays the role of a sensor fingerprint.

The reason for this phenomenon is all devices have manufacturing imperfections that lead to small variation in camera sensors, causing some pixels to project colors a little brighter or darker than normal. When extracted by filters, this leads to a unique pattern. [12] Simply put, the type of sensor being used, along with shot and pattern noise leads to a specific fingerprint.

The threat to privacy is obvious: if the camera reference pattern can be determined and the noise of an image is calculated, a correlation between the two can be formed. For example, recent research suggests that only one image is necessary to uniquely identify a smartphone based on the particular PRNU of the built-in camera's image sensor. [13] Major data mining corporations are starting to use this technique to associate identities of camera owners with everything or everyone else they shoot. [14] It follows that governments have had the same capabilities for some time now and can apply them to their vast troves of data.

There are methods to destroy, forge or remove PRNU, but these should only be used with caution. The reason is related research on the question of spoofing sensor fingerprints in image files has proven non-trivial and easily defeated. [15] [16]

Operational Security Advice[edit]

This advice assumes the user wants to preserve their anonymity, even when publicly sharing media on networks that are monitored by the most sophisticated adversaries on the Internet. Always conduct a realistic threat assessment before proceeding. These steps do not apply for communications that never leave anonymous encrypted channels between trusted and technically competent parties.

Current Devices[edit]

It is almost a certainty that photos and videos have been shared from your current devices through non-anonymous channels. Do not use any of these devices to shoot media that will be shared anonymously.

Suitable Devices[edit]

Most users will probably want to avoid phones altogether and use tablets instead, but for most situations phones are a reasonable choice:

  • Buy a new Android phone with cash if possible.
  • Avoid other choices because a proprietary operating system is a nonstarter.
  • Users must flash a freedom and privacy-respecting ROM before using the camera. Be aware that the glorified corporate malware that comes pre-installed on the phone will leak a range of data to the cloud.

Safe Use[edit]

  • The camera must only be reserved for anonymous media.
  • Do not commit serious mistakes like taking "selfies" or photographing places or people associated with you.
  • Sanitize metadata with MAT before sharing photographs anonymously online.
  • Completely obscure faces with solid fills using an image manipulation program. Advancements in neural nets and deep machine learning make pixelated or gaussian blurred faces reconstructable. [17] [18]
  • Consider using the ObscuraCam app from The Guardian Project to protect the identities of protestors: [19]
    • It pixelates images using a technique resistant to facial reconstruction.
    • ObscuraCam also offers a full pixel removal "black bar" option.

Keystroke Fingerprinting[edit]

Keystroke biometric algorithms have advanced to the point where it is viable to fingerprint users based on soft biometric traits. This is a privacy risk because masking spatial information -- such as the IP address via Tor -- is insufficient to anonymize users. [20]

Users can be uniquely fingerprinted based on: [21]

  • Typing speed.
  • Exactly when each key is located and pressed (seek time), how long it is held down before release (hold time), and when the next key is pressed (flight time).
  • How long the breaks/pauses are in typing.
  • How many errors are made and the most common errors produced.
  • How errors are corrected during the drafting of material.
  • The type of local keyboard that is being used.
  • Whether they are likely right or left-handed.
  • Rapidity of letter sequencing indicating the user's likely native language.

A unique neural algorithm generates a primary pattern for future comparison. It is thought that most individuals produce keystrokes that are as unique as handwriting or signatures. This technique is imperfect; typing styles can vary during the day and between different days depending on the user's emotional state and energy level. [21]

Unless protective steps are taken to obfuscate the time intervals between key press and release events, it is likely most users can be deanonymized based on their keystroke manner and rhythm biometrics. Adversaries are likely to have samples of clearnet keystroke fingerprinting which they can compare with "anonymous" Tor samples. At a minimum users should not type into browsers with Javascript enabled, since this opens up this deanonymization vector. Text should be written in an offline text editor and then copied and pasted into the web interface when it is complete.

In addition, users must also disguise their linguistic style to combat stylometric analysis and be aware of mouse tracking techniques available to adversaries.

Mouse Fingerprinting[edit]

Mouse or cursor tracking occurs when software collects the positions of the mouse cursor and click data on the computer. While this can have benefits for web designers, it also poses a privacy (profiling) threat. Without explicit user consent or awareness, a range of data can be leaked via JavaScript, plug-ins or other software: [22]

  • JavaScript readily allows developers to track users' mouse movements by simply entering relevant code on the webpage. This has already been employed on high-traffic websites, such as search engines.
  • Similar to JavaScript, installed and enabled software modules (plug-ins) can track mouse movements.
  • Specific mouse tracking software can reveal:
    • Mouse location.
    • Time stamps.
    • Mouse clicks.
    • A mouse cursor hovering over embedded links and its duration.
    • The amount of time spent in certain webpage areas.
    • Heat maps.
    • Full playbacks which retrace the mouse's trajectory.

For a practical example of deanonymization, consider a user who regularly uses both clearnet and Tor with JavaScript enabled. Individuals have distinctly unique characteristics associated with mouse movements and mouse clicks. Therefore, if these research methods are used in the public domain, supervised learning methods are likely to "learn" the typical behavior of individuals. Over time, it may be possible to link "anonymous" activities with the known profile of a clearnet user with a high degree of probability. [22] [23]

Stylometry[edit]

Whonix does not obfuscate a user's writing style. Consequently, unless precautions are taken (see below), users are at risk from stylometric analysis based on their linguistic style. Research suggests only a few thousand words (or less) may be enough to positively identify an author and there are a host of software tools available to conduct this analysis.

This technique is used by advanced adversaries to attribute authorship to anonymous documents, online texts (web pages, blogs etc.), electronic messages (emails, tweets, posts etc.) and more. The field is dominated by A.I. techniques like neural networks and statistical pattern recognition, and is critical to privacy and security. Current anonymity and circumvention systems are focused on location-based privacy, but ignore leakage of identification via the content of data which has a high accuracy in authorship recognition (90%+ probability). [24]

There are multiple ways to conduct statistical analysis on "anonymous" texts, including: [24] [25]

  • Keystroke fingerprinting, for example in conjunction with Javascript.
  • Stylistic flourishes.
  • Abbreviations.
  • Spelling preferences and misspellings.
  • Language preferences.
  • Word frequency.
  • Number of unique words.
  • Regional linguistic preferences in slang, idioms and so on.
  • Sentence/phrasing patterns.
  • Word co-location (pairs).
  • Use of formal/informal language.
  • Function words.
  • Vocabulary usage and lexical density.
  • Character count with white space.
  • Average sentence length.
  • Average syllables per word.
  • Synonym choice.
  • Expressive elements like colors, layout, fonts, graphics, emoticons and so on.
  • Analysis of grammatical structure and syntax.

Fortunately research suggests that if users purposefully obfuscate their linguistic style or imitate the style of other known authors, this is largely successful in defeating all stylometric analysis methods so they are no better than randomly guessing the correct author of a document. However, using automated methods like machine translation services do not appear to be a viable method of circumvention. [24]

Tips for Anonymous Posting, Blogging and Uploading[edit]

Before undertaking any anonymous activities, be sure to understand and exercise a healthy dose of Operational Security (OpSec). Even the best anonymity software available today cannot prevent catastrophic mistakes by end users.

Blogging[edit]

  • Activity Partitioning: Separate all online activities and only use a dedicated email address for the blog.
  • Blog Administration: Usually the blog is administrated via a web interface only. Use Tor Browser for all blog activities.
  • Blog Posting: Every type of blog software offers the option to select a point in time when new postings are published. It is safer to delay the publishing of new posts to a time when you are not online anymore, rather than publishing immediately. [26]
  • Email Address Registration: For anonymous blogs hosted on third-party services, register it with a new and anonymous e-mail address (see E-Mail) that has never been used before and which has been exclusively paired with Tor for logins and other related activity. [27]
    • Different Providers: The blog can be registered with different providers anonymously; for example, to utilize https://wordpress.com/
    • Payments: If using a premium product, keep the option open to pay anonymously via BitCoin or cash cards like Paysafecard. Note that cash card codes differ by country and could theoretically contain an ID which is linked to the shop where it was bought.

Browser Input[edit]

  • Accidental Searches: Text can be accidentally pasted into the search or URL bar, which triggers an unintended search across the public internet.
  • JavaScript: With JavaScript enabled, user behavior can be tracked and profiled as already noted in this chapter:
    • Keystroke Fingerprinting.
    • Mouse Fingerprinting. See additional footnotes. [28] [29]
    • When the methods above are combined with Stylometry, a user will be de-anonymized unless countermeasures are implemented, like faking one's authorship style [30] and confusing stylometry with a spell checker. [31]
    • Tor Browser defenses are based on skewing JavaScript's perception of time. [32] [33] The eventual packaging of kloak (keystroke-level online anonymization kernel; under development) in Whonix will provide a system-wide solution for keystroke and mouse profiling.
  • Text Editors: It is recommended to prepare text in an offline text editor like KWrite and then copy and paste the content into the web interface once finished.
    • External Devices: Avoid typing in places where open microphones are used, otherwise recorded keyboard sounds might provide enough information to accurately reconstruct what was typed. [34]

Hardware Threat Mitigation[edit]

  • Disable Dangerous Peripherals: It is advisable to shut off the speakers and microphone at all times, as newer methods of advertisement tracking can link multiple devices via ultrasound covert channels. [35] It is also possible to decrease the risk by playing video and audio from untrusted sources with headphones connected and adjusted at a low volume. [36]
    • LCD Coil Whine: The coil whining of LCD screens is unique enough to leak the information presented on the screen as reconstructed by machine learning applied on wiretapped data (via the webcam microphone). [37]
  • Remove External Devices: Remove all phones, tablets and so on from the room to avoid them issuing watermarked sounds as well as listening to keystroke sounds and watermarked sounds. [38] [39] Similarly, do not make / take calls in the same room where anonymous browsing is underway, or run sensitive applications (like Orfox) or have documents open on the phone before calls.
  • Side-channel Attacks: Energy leaks that reveal sensitive information are a long studied area of cryptography research; see footnotes. [40] [41] [42] There is no need for alarm, as all these attacks were foiled by software countermeasures in cryptographic libraries and GPG. Also bear in mind this vector is targeted and not a dragnet surveillance threat, since it requires dedicated and skilled attackers.
  • Wi-Fi Signal Emitters: Another keystroke snooping technique involves a WiFi signal emitter (router) and malicious receiver (laptop) that detects changes in the signal that correspond to movements of the target's hands on their keyboard. [43] This attack has many limitations in the real-world which make it unpractical and susceptible to noise, but it is important to remember that public places are riskier computing environments. [44]

User Habits[edit]

  • CAPTCHA / reCAPTCHA: Google's CAPTCHAs fingerprint the behavior of individuals. Only disable JavaScript if it must absolutely be solved. [45] [46]
  • Cookies: Remember to purge the browser's cookie and history cache periodically. When running Tor Browser, it is recommended to simply close Tor Browser after online activities are finished, then restart it.
  • Environment: Avoid public places where people are likely to shoulder surf or where CCTV cameras are deployed.
  • File Sanitization: Generally, any blog pictures, documents or other files must have unique Metadata removed (anonymized) before they are uploaded - check the file format is compatible with the MAT software: [47]
  • Passwords:
    • Detection: Although a remote threat, thermal imaging can capture body heat remains from keys touched to input passwords up to one minute after the fact. [48] Also avoid places with CCTV or those which risk shoulder surfing.
    • Generation: Use random usernames and strong Diceware passwords for anonymous accounts. pwgen should only be used to generate usernames and not passphrases because its emphasis is on generating phonemes. Such a bias means the program does what it is designed to do: produce pronounceable passwords rather than pure line noise. [49]
    • Retention: Consider the password-retention policy of the browser. If it supports a master password that encrypts every password it saves, then use that feature. It is generally safest not to save any blog or other passwords in the browser, but instead use a password manager and cut and paste passwords into the browser.
  • Pseudonym Isolation: For advanced separation of discrete activities, use Multiple Whonix-Workstations.
  • Publishing Time: Over time, pseudonymous activity can be profiled to provide an accurate estimate of the timezone, reducing the user's anonymity set. It is better to restrict posting activity to a fixed time that fits the daily activity pattern of people across many places.
  • Tor Browser Censorship: In most cases, Tor blocks by destination servers can be easily bypassed with simple proxies.

Footnotes[edit]

  1. Do You Hear What I Hear? Fingerprinting Smart Devices Through Embedded Acoustic Components
  2. Mobile Device Identification via Sensor Fingerprinting.
  3. For detailed information on this topic, see: Steganography and Digital Watermarking.
  4. https://www.daoudisamir.com/steganography-and-watermarking/
  5. https://en.wikipedia.org/wiki/Digital_watermarking
  6. Notably the watermark does not change the size of the carrier signal.
  7. Missing footnote.
  8. On the (In)effectiveness of Mosaicing and Blurring as Tools for Document Redaction
  9. http://dde.binghamton.edu/download/camera_fingerprint/
  10. Fingerprintable Camera Anomalies
  11. https://www.slideshare.net/justestadipera/digital-image-forensics-camera-fingerprint-and-its-robustness
  12. https://www.futurity.org/smartphones-cameras-prnu-1634712-2/
  13. The error rates is less than 0.5%
  14. https://www.google.com/patents/US20150124107
  15. Sensor Noise Camera Identification: Countering Counter-Forensics
  16. Anonymizing the PRNU noise pattern of pictures remains a promising area of research.
  17. https://github.com/david-gpu/srez/blob/master/README.md
  18. Defeating Image Obfuscation with Deep Learning
  19. https://lists.mayfirst.org/pipermail/guardian-dev/2016-September/004895.html
  20. https://github.com/vmonaco/keystroke-obfuscation
  21. 21.0 21.1 https://en.wikipedia.org/wiki/Keystroke_dynamics
  22. 22.0 22.1 https://en.wikipedia.org/wiki/Mouse_tracking
  23. This deanonymization technique is likely to succeed, since it is already used to lock persons out of secure accounts (pending identity verification) when their monitored behavior significantly deviates from behavior that has been learned.
  24. 24.0 24.1 24.2 https://www.cs.drexel.edu/~sa499/papers/adversarial_stylometry.pdf
  25. https://en.wikipedia.org/wiki/Stylometry
  26. This will trick lesser adversaries, who cannot force the blog service provider to reveal exactly when and for how long a blog administrator logged in. This will not fool the blog service provider nor an adversary capable of recording all internet traffic.
  27. Do not use personal or identifying data as part of the account creation.
  28. As noted in the Mouse Fingerprinting section, mouse movements are another biometric profiling vector and disabling JavaScript is therefore recommended. High accuracy is achieved in limited situations, such as active authentication during log-on.
  29. This does not clear EU false positive requirements however, so they recommend it is combined with keystroke dynamics for extra confirmation, see: User re-authentication via mouse movements, On Using Mouse Movements as a Biometric and http://www.cs.wm.edu/~hnw/paper/ccs11.pdf
  30. For instance, stylometry works with less data (final text only) and in concert with keystroke fingerprinting is completely effective. An adversary can compare statistics about a user's typing over clearnet, then compare it to texts composed over Tor in real-time.
  31. For example, launch KWrite: Start menu button -> Applications -> Utilities -> Text Editor (KWrite). Once KWrite is open, click on Tools -> Automatic spell checking. Misspelled words will be underlined with a red color.
  32. https://trac.torproject.org/projects/tor/ticket/19186
  33. User Behavior

    While somewhat outside the scope of browser fingerprinting, for completeness it is important to mention that users themselves theoretically might be fingerprinted through their behavior while interacting with a website. This behavior includes e.g. keystrokes, mouse movements, click speed, and writing style. Basic vectors such as keystroke and mouse usage fingerprinting can be mitigated by altering Javascript's notion of time. More advanced issues like writing style fingerprinting are the domain of other tools.

  34. This is a variation of an older attack perfected during the Cold War where recorded typewriter sounds allowed discovery of what was typed. See: https://freedom-to-tinker.com/2005/09/09/acoustic-snooping-typed-information/ and https://www.schneier.com/blog/archives/2016/10/eavesdropping_o_6.html
  35. This deanonymization technique works by playing a unique sound inaudible to human ears which is picked up by the microphones of untrusted devices. Watermarked audible sounds are equally dangerous, which means that hardware incapable of ultrasound is an ineffective protection.
  36. https://www.schneier.com/blog/archives/2015/11/ads_surreptitio.html
  37. https://arstechnica.com/information-technology/2018/08/researchers-find-way-to-spy-on-remote-screens-through-the-webcam-mic/
  38. https://www.newscientist.com/article/2110762-your-homes-online-gadgets-could-be-hacked-by-ultrasound/
  39. https://trac.torproject.org/projects/tor/ticket/20214
  40. Stealing Keys from PCs using a Radio: Cheap Electromagnetic Attacks on Windowed Exponentiation: Extraction of secret decryption keys from laptop computers, by non-intrusively measuring electromagnetic emanations for a few seconds from a distance of 50 cm. The attack can be executed using cheap and readily-available equipment: a consumer-grade radio receiver or a Software Defined Radio USB dongle.
  41. Another attack involves measuring acoustic emanations: RSA Key Extraction via Low-Bandwidth Acoustic Cryptanalysis.
  42. A poor man's implementation of TEMPEST attacks (recovering cryptographic keys by measuring electromagnetic emissions) using $3000 worth of equipment was proven possible from an adjacent room across a 15cm wall. These attacks were only possible for adversaries with nation-state resources for the past 50 years. See: CDH Key-Extraction via Low-Bandwidth Electromagnetic Attacks on PCs
  43. Keystroke Recognition Using WiFi Signals
  44. An attack variant using USRP (cellphone radio ranges) has performed poorly because of background energy interference.
  45. http://scraping.pro/no-captcha-recaptcha-challenge/
  46. CAPTCHAS also directly enhance the strike capabilities of military drones, see: https://joeyh.name/blog/entry/prove_you_are_not_an_Evil_corporate_person/
  47. Such as unique camera IDs and often GPS coordinates in the case of photographs.
  48. https://www.schneier.com/blog/archives/2018/07/recovering_keyb.html
  49. https://bugs.debian.org/726578

License[edit]

Gratitude is expressed to JonDos for permission to use material from their website. (w) (w) [1] The Surfing, Posting, Blogging page contains content from the JonDonym documentation Surfing and Blogging page.


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