Movie: A Beautiful Mind

電影中所用的加密技術似乎是 Null Cipher!

An Overview of Steganography for the Computer Forensics Examiner

Gray C. Kessler,
"An Overview of Steganography for the Computer Forensics Examiner,"
Forensic Science Communications, Volume 6, Number 3



Abstract

Steganography is the art of covered or hidden writing. The purpose of steganography is covert communication - to hide the existence of a message from a third party. This paper is intended as a high-level technical introduction to steganography for those unfamiliar with the field. It is directed at forensic computer examiners who need a practical understanding of steganography without delving into the mathematics, although references are provided to some of the ongoing research for the person who needs or wants additional detail. Although this paper provides a historical context for steganography, the emphasis is on digital applications, focusing on hiding information in online image or audio files. Examples of software tools that employ steganography to hide data inside of other files as well as software to detect such hidden files will also be presented.

這篇文章是專門為不熟悉 steganography 這個領域的人寫的, 想要一探 steganography 領域, 又不想被複雜的數學給牽絆住的人, 很適合從這篇起步。

Contents

1. Introduction
2. Null Ciphers (延伸閱讀)
3. Digital Image and Audio
4. Digital Carrier Methods
5. Steganography Examples
　a. Gif-It-Up
　b. JP Hide-&-Seek (JPHS) (延伸閱讀)
　c. S-Tools (延伸閱讀)
6. Detecting Steganography
7. Steganography Detection Tools
8. Summary and Conclusions
　
　[Original Link]
　

Null Cipher in Chinese Medicine History

From StegoRN

上個星期逛台北捷運的地下書街, 看到了這本《中醫藥典故與傳說》, 第七章的醫家軼聞, 我首先閱讀了我較為熟悉的醫聖張仲景部份, 其中有一個故事就是張仲景利用 Null Cipher 這樣的技術巧戲庸官, 故事中的方子如下:

柏子仁三錢
木瓜二錢
官桂二錢
柴胡三錢
益智二錢
附子二錢
八角二錢
人蔘一錢
台烏三錢
上党三錢
山藥二錢

Null Cipher

A null cipher hides the message according to some prearranged set of rules, such as "read every fifth word" or "look at the third character in every word."

一次世界大戰 (WWI) 期間, 一名德國間諜利用 Null Cipher 的方式, 從美國 Washington, D.C. 向位於 Berlin 總部回報了底下這段文字:

PRESIDENT'S EMBARGO RULING SHOULD HAVE IMMEDIATE NOTICE. GRAVE SITUATION AFFECTING INTERNATIONAL LAW. STATEMENT FORESHADOWS RUIN OF MANY NEUTRALS. YELLOW JOURNALS UNIFYING NATIONAL EXCITEMENT
IMMENSELY.

為求慎重, 又發了底下一段文字, 作為確認 (check) 之用:

APPARENTLY NEUTRAL'S PROTEST IS THOROUGHLY DISCOUNTED AND IGNORED. ISMAN HARD HIT. BLOCKADE ISSUE AFFECTS PRETEXT FOR EMBARGO ON BYPRODUCTS, EJECTING SUETS AND VEGETABLE OILS.

如果我們第一段訊息中, 每個字的第一個字母抽出, 或是將第二段訊息中, 每個字的第二個字母抽出, 都可以發現底下的句子:

Pershing sails from NY June 1.

John J. Pershing 是一次大戰期間, 美國遠征軍 (American Expeditionary Force) 的指揮官, 也是二次大戰期間, 美國將領們的良師(mentor)。



上述這個 Null Cipher Message 的例子, 最早我是在 IEEE Computer, February 1998 這期 Neil F. Johnson 與 Sushil Jajodia 的文章 "Exploring Steganography: Seeing the Unseen" 所讀到。

今天, 我在 WebStone 公司的 Stego Suite 網頁所提供的技術論文 "What You Can't See Can Hurt You" (2002), 又讀到了相同的例子。

不過, 上述兩篇論文都把故事發生的時間誤寫成 WWII, 發現這點是因為我後來研讀 Gray C. Kessler 的論文 "An Overview of Steganography for the Computer Forensics Examiner" [ PDF ] 時, 也讀到了這段故事, 不過故事發生的時間卻是在 WWI, 與上述兩篇所寫的 WWII 有出入。

From StegoRN

因此, 我就去翻這些論文的原始參考文獻 "THE CODE BREAKERS - The Comprehensive History of Secret Communication from Ancient Times to the Internet" (David Kahn, 1996), 終於確認時間應該是 WWI。

有趣的是書中 (P. 521) 還提到 Pershing 確實出發的日期應該是 May 28, 而不是 June 1, 顯然這位德國間諜情報有誤, 搞了一次烏龍。