影像驗證碼技術之研究

Study on Image-based CAPTCHA

巫愛雯(Ai-Wen Wu)

指導教授 ： 黃國峰

國立臺中科技大學/設計學群/多媒體設計系碩士班/碩士(2011年)

摘要

驗證碼(CAPTCHA)，是利用電腦自動判別遠端使用者是電腦程式還是人類，主要用來阻擋自動化程序(bots)對網路服務的濫用。目前使用較為廣泛的驗證碼是文字型的驗證碼，但隨著光學文字辨識(OCR)技術的演進，文字驗證碼的安全性受到極大的考驗，因此近年來驗證碼逐漸朝向影像驗證碼、語音驗證碼等形式發展。
本篇文章提出以拼圖為設計概念的影像驗證碼技術，稱之為Puzzle CAPTCHA。其產生影像驗證碼的方式是利用影像切割技術，隨機將影像切割成數個尺寸不等之正方形的影像區塊，並將區塊進行隨機旋轉0°、90°、180°、270°，最後加入適當的干擾線。當使用者利用滑鼠點擊正方形影像區塊，區塊便會進行順時針90°的旋轉，直到將所有影像區塊旋轉拼回原圖便可通過驗證。
本研究進行使用者測試，根據實驗結果顯示通過機率達八成，平均花費時間為十秒以內。利用邊緣偵測技術亦無法有效得知區塊的位置，故可提升驗證碼的安全性。Puzzle CAPTCHA不受國別、語言及文化的隔閡進行使用者驗證。

關鍵字

驗證碼 ； 影像處理 ； 資訊安全

並列摘要

CAPTCHAs (Completely Automated Public Test to tell Computers and Humans Apart) are computer-generated tests designed to differentiate humans from automated programs. The developments of CAPTCHAs are focusing on issues such as the protection of internet resource from being abused by bots. Traditionally, most CAPTCHAs are text-based. But currently, the security of text-based CAPTCHAs encounters a hard challenge from the improvements in OCR (Optical Character Recognition) technology. Therefore, in recent years, various different types of CAPTCHAs are developed with different approaches such as like image-based or audio-based CAPTCHAs.
This paper proposed an image-based CAPTCHA named Puzzle CAPTCHA. Puzzle CAPTCHA utilized image segment technology. In this approach, an image will be embedded with several tetragon cutout blocks with different sizes. The puzzle is built by rotating these original tetragon cutouts by 0, 90, 180, or 270 degrees randomly. Finally, the establishment of Puzzle CAPTCHA is finished by randomly adding interfering lines on the image.
When users face the image of Puzzle CAPTCHA, they click mouse on each tetragon block to make a 90° clockwise rotation of the block. Users should put all tetragon blocks to the right angle to reveal the original image to pass the CAPTCHA test. The preliminary user’s test of Puzzle CAPTCHA shows an 80.4% accuracy rate. Most users can solve the puzzle in less than 10 seconds. Furthermore, the location of tetragon blocks cannot be resolved by current edge detection technology. This could also boost the safety of Puzzle CAPTCHA. The other merit of Puzzle CAPTCHA lies on the universality that it can be implemented without the consideration of language and culture.

並列關鍵字

CAPTCHA ； Image Processing ； Information Security

參考文獻

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