ZeroStorageCaptcha
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Offline captcha without any file system or database storage. Offline captcha without any file system or database storage.
Dependency: Qt5. Dependency: Qt (5+).
## Inspiration ## Inspiration
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Due to this architecture, the lifetime of each captcha ranges from 1.5 to 3 minutes, after which the verification token will always show failure. Due to this architecture, the lifetime of each captcha ranges from 1.5 to 3 minutes, after which the verification token will always show failure.
To make it impossible to use one captcha twice, the used verification captcha id gets into a special cache, where it is stored for several minutes of the life cycle of TIME_BASED_SECRET_TOKEN. To make it impossible to use one captcha twice, the used verification captcha id gets into a special cache, where it is stored for several minutes of the life cycle of TIME_BASED_SECRET_TOKEN.
The captcha token is considered used after the first validation check. Storing captcha id is very cheap: the id has a weight of 8 bytes (for a 64-bit system). For example, to store a million solved captchas at one time would need less than 8 MB of RAM. The captcha token is considered used after the first validation check. Storing captcha id is very cheap: the id has a weight of 8 bytes (for a 64-bit system). For example, to store a million solved captchas at one time would need less than 8 MB of RAM. So easy!
Check `examples` folder to see C++ interface or if your project not in C++, also you can use Zero Storage Captcha as separate cross-platform local [service](https://github.com/ZeroStorageCaptcha/api-daemon). To protect the CPU from an attack where an attacker will request a lot of captchas, you should use caching (`example3.cpp`). This is a compromise between using RAM and saving CPU: it will take about 36 MB to store 4096 captchas (the default cache size). A cached captcha will be reused after <=3 minutes when its token has expired and has not been answered (correctly). Captchas that get a correct answer are immediately deleted from the cache and will not be used again.
Check `examples` or if your project not in C++ (or without Qt framework), you can use Zero Storage Captcha as separate cross-platform local [service](https://github.com/ZeroStorageCaptcha/api-daemon).