What is Captcha?

CAPTCHA (Completely Automated Public Turing Test to tell computers and Humans Apart), human users in online environments with automated software (bots) in order to distinguish between an authentication mechanism that is used. In particular, form submissions membership procedures, surveys, comment sections, and similar areas, malicious automated processes (eg. spam attacks, or data collection) it is preferred to increase the security of the system and prevent.

CAPTCHA for the first time the concept of Luis Von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford has been raised by (“Telling humans and computers Apart Automatically, 2003), is divided into various subtypes with different needs and in accordance with technological developments over time.

CAPTCHA Types

1. Text-based CAPTCHA (text-based CAPTCHA)

Description and application form

To the user, the various deformation techniques (bending, twisting, blur etc.) applied letters, numbers, or mixed characters, a picture is shown.

The user is located in the image, the characters recognize and is expected to enter in the appropriate field.

Advantages

• Application and integration in terms of the common and simple.
• Can work even systems that require low bandwidth.

Disadvantages

• You can create accessibility problems for people who are blind or have low vision.
• An Advanced Optical Character Recognition (OCR) technology over time, it has become easier to overcome such capable captchas.

Sample Technologies

• Standard “corrupted” text CAPTCHA applications.
• The older generation reCAPTCHA versions (reCAPTCHA v1), partially benefited from this system.

 

2. Visual (image-based) CAPTCHA (image-based CAPTCHA)

Description and application form

To the user, a particular object or concept (eg. traffic lights, bus stops, pedestrian crossing, cat, dog, etc.) contains, or does not contain the pictures shown.

In the validation step, you are prompted to select the user and recognize these images.

Advantages

• In terms of user interaction is more intuitive; you can quickly to read the text because it does not require to be grasped by many people.
• Image recognition, OCR-based bots offers different difficulty level.

Disadvantages

• Accessibility for visually impaired users constraints are available.
• Developing artificial intelligence and machine learning-based image recognition algorithms (eg. CNN-based models), these may be able to solve Captchas are becoming.

Sample Technologies

• Google reCAPTCHA (image selecting step)
• Independent developers “select object” style solutions

 

3. An audio captcha CAPTCHA audio)

Description and application form

Visual content difficulty in reading, or as an alternative to people who use a screen reader is presented.

The user noise (noise) with an attached sound file is played. This file is of numbers, letters or words are found.

Advantages

• Elderly or visually impaired users a more accessible option.
• Text-based CAPTCHA validation according to different channel presents.

Disadvantages

• Hearing or voice impaired people is not suitable for users who have restricted access to technologies.
• Automatic speech recognition (ASR) technology with the development of the voice of boots the potential for solving Captchas is increasing.

 

4. reCAPTCHA

Description and history

Enhanced by Google Captcha service. Initially digitize scanned book pages with text in order contribution of human-based validation approach was used.

Over time, it has been updated to improve the user experience and provide more effective protection against bots.

reCAPTCHA Versions

• reCAPTCHA v2: Often, “I am not a robot” box click the image or recognition steps.
• reCAPTCHA v3: the site, the user by analyzing the interaction in the background with a “risk score” results; most of the time the user without the additional step verification can be made.

Advantages

• Bot provides high accuracy through the use of advanced analysis systems.
• With user-friendly interface text, image or can help minimize the need for interaction.

Disadvantages

• Google creates dependency on infrastructure.
• Google user data to be transmitted, may be the subject of criticism in terms of privacy and data protection.

5. Mathematical or logic-based CAPTCHA

Description and application form

A simple math operation, the user (eg. “3 + 7 = ?”) a question of logic or basic (eg. “Complete the missing word in the following sentence”) directed.

The correct answer to this question by giving the user passes the test.

Advantages

• Can be applied easily and in different languages can be customized.
• Simple operations, human users, are of a nature that can be usually resolved quickly.

Disadvantages

• Automation systems (web scraping bots, artificial intelligence-based software) may become easily solvable.
• Unless diversified over time, the level of security may be reduced.

 

6. Interactive Captchas

Description and application form

The user can drag-and-drop method with an object in the correct location placement, you complete a puzzle, or similar interactive task is supposed to do.

Algorithm, a mouse or a touch screen by watching the behaviour of the bot tries to identify differences in the interaction.

Advantages

• Recognition is much more complicated than simple text or image may be difficult to imitate because of boots.
• From a user perspective, and sometimes can offer a fun experience.

Disadvantages

• Application and development costs compared to other methods is higher.
• Some users (especially persons with reduced mobility), it can be difficult to use.

 

The Boots From The People Of What Is The Difference?

1.Mouse gestures and user behavior

Mouse Movement Analysis:

How and where to move on the page when the user moves the mouse, the frequency of the clicks and coordinates are examined.

Human, mouse usage, speed changes, wait times, and the light is “zigzag” or irregular movements. Bots are generally more “straight-line” transactions or may have been progressing rapidly with the scenario, the script is hard.

Timestamps (Timing):

Filling a form with the delay between the time between clicks and keystrokes page load, scrolling behavior, such as time-based collected data.

A vaguely human pauses when filling out a form (spelling shower, spell check, etc.); they often suddenly or operate the boat at regular time intervals.

Behavioral Anomaly Detection:

The user normal user behavior on the page is what it looks like to understand the movements a script or automated, machine learning-based models are used.

For example, reCAPTCHA Invisible reCAPTCHA V3 or, as soon as you enter the page, only the users within a few hundred milliseconds, and the sender can react to form a “high-risk” you can mark it as.

2. IP address and Location Analysis

IP reputation (IP reputation Check:

The captcha provider in the background of millions of statistics of previous requests from the IP address are kept.

A specific IP address from spam, bot if it is marked for suspicious transactions or activities, high risk is evaluated with a score of new requests from the same IP.

VPN / Proxy / Tor Detection:

IP address belongs to VPN Proxy server or if this is a more challenging CAPTCHA can be shown to the user.

Some systems location (geographic IP) or time zone settings, browser language discrepancies (e.g., browser, IP localization, while Germany Brazil) also may be interpreted as abnormal behavior.

Location-Based Restrictions:

Some services already constricted or requests from specific geographical regions implementing more intensive supervision. In this case, the rate can increase reCAPTCHA tests shown or similar.

3. Browser and device data (Fingerprinting)

User-Agent Control:

The used browser (Chrome, Firefox, Safari, etc.) and version, the operating system (Windows, Mac, Linux, Android, iOS, etc.) the information is taken into account.

The software is usually fake Bot “user-agent” makes the statement; however, current systems sometimes additional confirmation CAPTCHA with JavaScript (screen resolution, language settings, font list, etc.) it makes. This “fingerprint” is considered suspicious if it shows inconsistency with the agent declared in the user profile.

Property of particles and browser tests:

CAPTCHA Modern systems, browsers, canvas, WebGL, AudioContext etc. With the API tests (for example, in a confidential manner canvas drawings) by making a unique digital fingerprint for each device (digital fingerprint) creates.

A person's browser, depending on the enhancement, by a small margin of this test is “unique” and outputs the result. The same bots running with headless browser settings if similar patterns can be detected.

4. Cookies and Session Management

Continuity and Consistency:

CAPTCHA providers, user tracking by cookies examines whether there is a similar behavior during previous visits.

The same computer and the same browser with users who are logged on a regular basis and successfully solved the captcha on the site as “trusted” ratings tend to.

Cross-Site Analysis (Large Providers):

Large providers such as Google reCAPTCHA, the Google account you use on your knowledge of the cookie on different websites or you can recognize.

So Google, “YouTube or Gmail suspicious activity on a regular basis to this user comes in and does not show the” additional data such as the risk analysis are also included.

 

5. Time and frequency Tracking (rate limiting)

Excessive Request (Brute Force/Spam) Detection:

Many requests from the same user with the same IP or if you're coming in short span of time, the system accepts it as a metric that points to the behavior of the bot.

Or completely avoid the CAPTCHA test to be shown more often (ban) could be activated in the implementation of this order.

The duration of repeated trials and the submission form:

The time of filling the form too short or too long also increases when in doubt (e.g., 1 second in 10 different fill in the form).

CAPTCHA entry system fails many times on the side “ - user is a challenge she couldn't read or really voluntary?” is analyzed in the form of.

 

6. Machine Learning-Assisted Risk Analysis

Scoring system (like reCAPTCHA v3):

reCAPTCHA v3, visible to the users instead of providing a verification box, user behavior by collecting data in real-time between 0 and 1 a “risk score” builds.

Site owner this score is based on a threshold (e.g., 0.3% for those under) additional verification steps may go the way of showing a CAPTCHA or classic.

Dynamic Difficulty Level:

If your system is considered to be low risk on you, if the simple “I am not a robot” checkbox is sufficient.

High-risk users, image recognition tests or more complex multi-stage (eg. select more than one image, in succession, several test) can be routed to validate.

 

How reliable or unreliable?

1. Bot Strategies Against The Developers

CAPTCHA Solving Services:

“Captcha Solving services” or “Captcha Farms called” captcha images shown or the services by sending a quick solution leads to real people tests. Thanks to these services, boat owners for a small fee, the questions with the help of CAPTCHA human can survive.

With this method, even very complex visual Captchas will be resolved within a specific period of time. Especially in high-volume scenarios spam or attack cost is a factor, although it is still a popular approach.

Machine learning and OCR Techniques:

Advanced artificial intelligence models, has made considerable progress in the knowledge of the writings corrupted. Text distortion to a certain extent even able to resolve Captchas.

Image-based (eg. selecting a zebra crossings or traffic lights) due to advances in the field of computer vision computer vision Captchas also partially has become resolvable. Bot developers TensorFlow/PyTorch on the frame as may invest in this type of Model.

Imitation Of Behavior (Behavior Emulation):

reCAPTCHA Invisible reCAPTCHA V3 or behavioral data such as measuring systems, advanced bots human-like mouse movements, the keyboard is trying to mimic the duration of delays or random clicks.

Although each of these imitations may seem plausible at first, captcha providers that uses multi-dimensional analysis (IP history, fingerprint scanner, etc.) still failed imitations increases your chances of catching.

 

2. The basis of security and benefits

Simple Is Effective In Blocking Bots:

Spam bots, especially at the basic level (eg. forum spam, simple login attempts) is mired in mostly CAPTCHA. Bots CAPTCHA solving services or to invest in advanced algorithms such “valuable” it may not be.

Therefore, on your site simple, automatic form fill out unwanted content attacks 90% of the proportions you can get ahead.

Risk analysis Model (Especially reCAPTCHA):

In the background of a huge data pool from services such as Google reCAPTCHA (IP reputation, trends, user behaviour, etc.) there are models and machine learning. Attackers are forced to develop more sophisticated methods to dwell on this model also.

reCAPTCHA Invisible reCAPTCHA V3 or without disturbing the user experience with behavior-oriented analysis, is forcing extra boots.

Time and cost factor:

Professional services or a boat to pass the captcha requires the use of advanced artificial intelligence models extra cost and time.

The attacker has this many “can push them to look for easier targets, or may limit the scale of the attack.

 

3. Limitations and weaknesses

User Experience / Accessibility:

Human captchas can be uncomfortable for users. Visually impaired or dyslexic problems, particularly for users with text-based Captchas it creates great difficulty. So the captcha accessibility (WCAG etc.) it is very important to adhere to the same standards.

Invisible reCAPTCHA in order to improve the user experience, etc. such as “working in background” version is preferred; however, these systems are sometimes wrong “bot” to make the determination of it is possible.

Provide 100% Protection Does Not:

As with any security system, Captchas also can be overcome. Advanced bots with CAPTCHA can be bypassed or manual human labour.

CAPTCHA, security is only one of the layers. Authentication (MFA/2FA), IP restrictions, rate-limiting, WAF (web application Firewall) must be supported by other security measures such as.

A Strain Of Machine Learning Models Evolves:

CAPTCHA providers, systems must produce constantly update and new challenges. Otherwise, once artificial intelligence on the model captured type a CAPTCHA, the bot by “solved” can be considered.

Catpcha Integration Of

1. Project Settings

Google reCAPTCHA V3 Keys:
the Google reCAPTCHA admin console (from https://www.google.com/recaptcha/admin your site Key and a secret Key is required.

Configuration:
web secret key.config or appsettings.json), such as by storing it in a configuration file directly within the code fixed (hard-coded) using, it is important to mak.

2. Form, and client-side

3. Server-side – ASP.NET MVC with C# Validation

3.1. Model class for recaptcha response

3.2. Process Validation In The Controller