本文是计算机专业的留学生作业辅导范例，题目是“Steganography And Visual Cryptography In Computer Forensics Computer Science Essay（计算机取证中的隐写术和视觉密码学）”，近年来，隐写术和视觉密码学领域提出了许多新的算法，以提高安全性、可靠性和效率。本文对这两种方法进行了讨论和比较。除了讨论各自的一些最著名的算法外，还提出了一些相似和不同之处。最后，提出了一种结合隐写术和视觉密码术的可能算法。有几种方法可以将数据隐藏在不同格式的文件中，从而留下隐藏数据的各种迹象。
Abstract 摘要
Recently, numerous novel algorithms have been proposed in the fields of steganography and visual cryptography with the goals of improving security, reliability, and efficiency. This paper discusses and compares the two methodologies. Some similarities and differences are presented, in addition to discussing some of the best known algorithms for each. Lastly, an idea for a possible algorithm which combines the use of both steganography and visual cryptography is suggested. There are several ways of hiding data in files of different formats, leaving various signs of hidden data.
Can data hidden in an original image be detected after it undergoes visual cryptography? Would that be a scenario which computer forensic investigators and forensic software developers have to account for? Keywords-Visual Cryptography, Steganography, Computer Forensics, Anti-forensics, Data Hiding, Secrecy, Novel Visual Cryptographic and Steganographic Methods, Forensic Investigation.
隐藏在原始图像中的数据经过视觉加密后能被检测出来吗?这是计算机取证调查人员和取证软件开发者必须考虑的情况吗?关键词:视觉密码学，隐写术，计算机取证，反取证，数据隐藏，保密，新的视觉密码学和隐写方法，取证调查。
1.History 历史
Throughout history, a multiple of methods and variations have been used to hide information. David Kahn’s The Codebreakers provides an excellent accounting of this history. Bruce Norman recounts numerous tales of cryptography and steganography during times of war in Secret Warfare: The Battle of Codes and Ciphers.
纵观历史，有多种方法和变体被用来隐藏信息。大卫·卡恩的《密码破译员》为这段历史提供了精彩的叙述。布鲁斯诺曼在《秘密战争:密码与密码之战》中讲述了许多战争时期的密码学和隐写术的故事。
In ancient Greece, text was written on wax covered tablets. In one story Demeratus wanted to notify Sparta that Xerxes intended to invade Greece. To avoid capture, he scraped the wax off of the tablets and wrote a message on the underlying wood. He then covered the tablets with wax again. The tablets appeared to be blank and unused so they passed inspection by sentries without question.
Another ingenious method was to shave the head of a messenger and tattoo a message or image on the messengers head. After allowing his hair to grow, the message would be undetected until the head was shaved again.
Another common form of invisible writing is through the use of Invisible inks. Such inks were used with much success as recently as WWII. An innocent letter may contain a very different message written between the lines. Early in WWII steganographic technology consisted almost exclusively of invisible inks. Common sources for invisible inks are milk, vinegar, fruit juices and urine. All of these darken when heated Null ciphers (unencrypted messages) were also used. An example of a message containing such a null cipher from is:
Fishing freshwater bends and saltwater
Coasts rewards anyone feeling stressed.
Resourceful anglers usually find masterful
Leapers fun and admit swordfish rank
Overwhelming anyday.
By taking the third letter in each word, the following message emerges:
Send Lawyers, Guns, and Money.
The Germans developed microdot technology which FBI Director J. Edgar Hoover referred to as “the enemy’s masterpiece of espionage.” Microdots are photographs the size of a printed period having the clarity of standard-sized typewritten pages. The first microdots were discovered masquerading as a period on a typed envelope carried by a German agent in 1941. The message was not hidden, nor encrypted. It was just so small as to not draw attention to itself (for a while).Besides being so small, microdots permitted the transmission of large amounts of data including drawings and photographs.
2.Introduction 引言
Steganography is the art and science of writing hidden messages in such a way that no one apart from the intended recipient knows of the existence of the message; this is in contrast to cryptography, where the existence of the message itself is not disguised, but the content is obscured. The word “Steganography” is of Greek origin and means “covered or hidden writing”. Generally, a steganographic message will appear to be something else: a picture, an article, a or some other message – the cover text.The advantage of steganography over cryptography alone is that messages do not attract attention to themselves, to messengers, or to recipients.
隐写术是一种书写隐藏信息的艺术和科学，这种隐藏信息的书写方式使接收者不知道信息的存在;这与密码学相反，密码学不隐藏信息本身的存在，但内容是模糊的。“隐写术”这个词起源于希腊语，意思是“隐藏的或隐藏的文字”。一般来说，隐写术的信息会显示为其他内容:图片、文章、或其他信息——封面文字。与单独的密码学相比，密码学的优势在于，信息本身不会引起注意，也不会引起信使或接收者的注意。
An unhidden coded message, no matter how unbreakable it is, will arouse suspicion and may in itself be incriminating. In some countries encryption is illegal. Steganography uses in electronic communication include steganographic coding inside of a transport layer, such as an MP3 file, or a protocol, such as UDP.Today, computer and network technologies provide easy-to-use communication channels for steganography. Essentially, the information-hiding process in a steganographic system starts by identifying a cover medium’s redundant bits 1 the embedding process creates a stegomedium by replacing these redundant bits with data from the hidden message.
Modern steganography’s goal is to keep its mere presence undetectable, but steganographic systems-because of their invasive nature-leave behind detectable traces in the cover medium. Even if secret content is not revealed, the existence of it is: modifying the cover medium changes its statistical properties, so eavesdroppers can detect the distortions in the resulting stego medium’s statistical properties. The process of finding these distortions is called statistical steganalysis. This article discusses existing steganographic systems and presents recent research in detecting them via statistical steganalysis. Other surveys focus on the general usage of information hiding.
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3.How Steganography works? 隐写术是如何运作的吗?
When using steganography on a computer, you actually hide a message within another file. That resulting file is called a “stego file.” The trick to computer steganography is to choose a file capable of hiding a message.
在计算机上使用隐写术时，实际上是将消息隐藏在另一个文件中。生成的文件称为“stego文件”。计算机隐写术的诀窍是选择一个能够隐藏信息的文件。
A picture, audio, or video file is ideal for several reasons: These types of files are already compressed by an algorithm. For example, .jpeg, .mp3, .mp4, and .wav formats are all examples of compression algorithms. These files tend to be large, making it easier to find spots capable of hiding some text. These files make excellent destructors. That is, few people expect a text message to be hidden within a picture or an audio clip. If the steganographic utility does its job well, a user shouldn’t notice a difference in the quality of the image or sound, even though some of the bits have been changed in order to make room for the hidden message.
4.Uses of Steganography 使用隐写术
Steganography is a means of storing information in a way that hides that information’s existence. Paired with existing communication methods, steganography can be used to carry out hidden exchanges. Governments are interested in two types of hidden communications: those that support national security and those that do not. Digital steganography provides vast potential for both types. Businesses may have similar concerns regarding trade secrets or new product information.
隐写术是一种以隐藏信息存在的方式存储信息的方法。结合现有的通信方法，可以使用隐写术进行隐藏交换。政府对两种类型的隐藏通信感兴趣:支持国家安全的通信和不支持国家安全的通信。数字隐写术为这两种类型提供了巨大的潜力。对于商业秘密或新产品信息，企业可能有类似的担忧。
Avoiding communication in well-known forms greatly reduces the risk of information being leaked in transit. Businesses have increasingly taken advantage of another form of steganography, called watermarking. Watermarking is used primarily for identification and entails embedding a unique piece of information within a medium without noticeably altering the medium. (For details, see Katzenbeisser and Petitcolas1 and Barán et al.2) For example, if I create a digital image, I can embed in the image a watermark that identifies me as the image’s creator.
I would achieve this by manipulating the image data using steganography, such that the result contains data representing my name without noticeably altering the image itself. Others who obtain my digital image cannot visibly determine that any extra information is hidden within it. If someone attempts to use my image without permission, I can prove it is mine by extracting the watermark. Watermarking is commonly used to protect copyrighted digital media, such as Web page art and audio files. Steganography can also enhance inpidual privacy.
Although not a substitute for encryption, digital steganography provides a means of communicating privately. Of course, this is effective only if the hidden communication is not detected. If a person simply wants to communicate without being subjected to his or her employer’s monitoring syst then digital steganography is a good solution – the most private communication is the one that never existed!
5.Modern Steganography 现代隐写术
Accordingly, with the development and release of tools to steganographically hide infomation within images, various tools have also been developed and released to detect steganographic content. Most of these tools use statistical analysis to detect steganographic content. Once an image is suspected to have information hidden within it, the majority of tools launch a dictionary attack to determine the passphrase that was used to encrypt the hidden information.
因此，随着隐写术在图像中隐藏信息的工具的开发和发布，各种工具也被开发和发布来检测隐写术的内容。这些工具大多使用统计分析来检测隐写内容。一旦怀疑图像中隐藏了信息，大多数工具都会发起字典攻击，以确定用于加密隐藏信息的密码短语。
Although most of the initially available tools generated output that could be easily defeated by simple statistical analysis, various tools have appeared recently with more sophisticated information hiding and encryption algorithms that can escape simple forms of statistical analysis. For example, content encoded using the latest version of Outguess, a freely available tool on the Internet, is not detectable using most available tools to detect steganographic content. Similar to encryption technologies, new encoding techniques are being developed at the same rate as techniques to detect them.
The left one contains steganographic information, in this case the first page of this document in ASCII format. . The information was encoded into the left image using JPHide, a freeware steganography tool available for Windows. Approximately 4KB of information is hidden within the image on the left. It took approximately 1 minute to hide the information and write out the new JPEG file using the tool’s point and click user interface.
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6.STEGANOGRAPHY GOES MAINSTREAM 隐写术成为主流
Even with easily accessible means to steganographically hide information within an image, one does not necessarily need sophisticated methods to encode information. Historic use of steganography has shown that low technology solutions have been highly effective. In late September 2001, several posters appeared in Bangladesh and in Pakistan that raised the eyebrows of people familiar with the television show, “Sesame Street.” Bert, one of the characters on “Sesame Street” could be seen in one small corner of the poster.
即使使用易于访问的方法在图像中隐写隐藏信息，也不一定需要复杂的方法来编码信息。历史上对隐写术的使用表明，低技术的解决方案是非常有效的。2001年9月下旬，孟加拉国和巴基斯坦出现了几张海报，让熟悉电视节目“芝麻街”(Sesame Street)的人感到惊讶。海报的一个小角落里可以看到《芝麻街》(Sesame Street)中的一个角色Bert。
At first, various “experts” on terrorism claimed that the image of Bert was deliberately planted as a hidden message to sleeper terrorist cells in the United States. A Bangladesh entrepreneur subsequently claimed that he had created the poster by piecing together random images of Osama bin Laden he had found off the Internet.
Strangely, the appearance of this poster in photos taken at Pakistan protests occurring on the same day as the protests in Bangladesh were never explained. One of the fallouts of this incident, along with other similar perceived threats of encoded messages from unreviewed video transmissions from Osama bin Laden, caused the United States government to request that United States media refrain from showing unreviewed video originating from the Middle East.
Steganography and visual cryptography have many similarities and differences, and thus have various uses in the digital and real worlds. Different algorithms for steganography and visual cryptography have different advantages and power, as well as disadvantages and weaknesses. So we notice that certain methods are easier to detect than others. But generally, the job of forensic and security investigators is not easy. When steganography and visual cryptography detection tools are used exclusively, it is almost impossible for investigators to uncover hidden or encrypted data. On the other hand, if these detection tools are used in conjunction with other tools and factors that narrow down the search to a somewhat smaller data set, then it makes the lives of investigators much easier and gives them a better chance of detecting suspicious data. It would be very interesting to learn how detectable data is after applying visual cryptography with perfect reconstruction to an image with hidden data. Also, an interesting detection question is whether we can reconstruct a set of ‘n’ shares into a meaningful image that is different than the image used to create those shares by omitting some of the n original shares and by including an additional share specifically constructed for such purpose. Basically this is a question about the uniqueness of the shares created by different visual cryptography algorithms.
隐写术和视觉密码学有许多相似和不同之处，因此在数字和现实世界中有各种各样的用途。不同的隐写和视觉密码学算法有不同的优点和能力，也有缺点和弱点。所以我们注意到某些方法比其他方法更容易被检测到。但一般来说，法医和安全调查人员的工作并不容易。当只使用隐写术和可视密码术检测工具时，调查人员几乎不可能发现隐藏或加密的数据。另一方面，如果这些检测工具与其他工具和因素结合使用，将搜索范围缩小到一个较小的数据集，那么就会使调查人员的生活更容易，并使他们有更好的机会发现可疑数据。在对隐藏数据的图像进行完美重建后，了解如何检测数据将是非常有趣的。此外，一个有趣的检测问题是，我们是否可以通过省略n个原始共享中的一些，并通过包含一个专门为此目的构建的额外共享，将一组“n”共享重构为一个有意义的图像，该图像与用于创建这些共享的图像不同。基本上，这是一个关于由不同的视觉密码算法创建的共享的唯一性的问题。
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