1. Giới thiệuNgày nay chúng ta thường nghe nói đến Steganography. Đó là một hình thức che dấu sự tồn tại của thông điệp. Khác với Cryptography thì Steganography ẩn đi các thông tin cần giữ bí mật trong các dữ liệu vô hại để đối phương không thể biết được sự hiện diện của thông điệp. Thực chất Steganography đã được con người biết tới từ rất lâu rồi. Năm 440 trước công nguyên, người Herodotus (Người Hy Lạp xa xưa) đã cạo trọc đầu các nô lệ tin cậy rồi xăm lên đó các thông điệp và chờ tóc mọc lại. Mục đích của việc này là nhằm gửi tin đi trong cuộc chiến tranh giữa người Herodotus và Persians (Người Ba Tư). Trong cuộc cách mạng của Mỹ, mực không màu cũng được sử dụng để trao đổi thông điệp giữa người Mỹ và người Anh. Trong các cuộc chiến tranh thế giới, Steganography cũng được sử dụng. Người Đức đã sử dụng mực không màu để viết các dấu chấm nhỏ lên phía trên và dưới các chữ cái bằng cách thay đổi chiều cao các chữ trong đoạn văn bản. Trong chiến tranh thế giới thứ nhất, các tù nhân cũng sử dụng mã Morse để gửi thư về nhà bằng cách viết các dấu chấm và gạch ngang lên các chữ cái i,j,t,f. Trong chiến tranh thế giới thứ 2, các gián điệp của Đức đã sử dụng đoạn văn bản sau:
“Apparently neutral’s protest is thoroughly discounted and ignored. Isman hard hit. Blockade issue affects for pretext embargo on by-products, ejecting suets and vegetable oils.”Bằng cách đọc những chữ thứ 2 của các từ ta sẽ được đoạn thông điệp sau:
“Pershing sails for NY June 1”.Ngày nay trên mạng có rất nhiều đoạn nhạc hay các bức hình tưởng chừng như bình thường nhưng nó lại chứa các thông tin hoàn toàn bí mật. Điều này càng làm cho Steganography càng trở nên phổ biến. Dưới đây là hình ảnh minh họa cho điều này:
2. Quá trình mã hóa và giải mã của StegranographyTiếp theo ta sẽ cùng tìm hiểu về ẩn thông tin trong Steganography. Đầu tiên, thông điệp và bức ảnh ban đầu sẽ được đưa vào trong bộ mã hóa (encoder). Trong bộ mã hóa, một bộ giao thức sẽ được thực thi để nhúng thông điệp vào trong bức ảnh. Kiểu của giao thức sẽ phụ thuộc vào kiểu của thông điệp và cách ta nhúng. Ví dụ hình dưới đây sẽ cho ta thấy giao thức để ẩn thông điệp trong các bức ảnh.
3. Một số cơ chế trong Stegranography3.1 Cơ chế ẩn thông điệp trong văn bản hoặc đoạn ký tự Đối với các tài liệu giấy thì khi bạn copy, bạn có thể sẽ không vi phạm bản quyền vì chất lượng của các tài liệu này khác nhau. Tuy nhiên đối với các tài liệu trên máy tính thì các bản copy sẽ giống hệt các bản gốc nên bạn rất dễ vi phạm bản quyền. Sở dĩ có thể biết được là do trong các tài liệu này đã được chèn thêm vào một số đoạn text để đánh dấu. Những đoạn này là ẩn với người dùng và bằng mắt thường khó có thể nhận biết được. Chúng ta chỉ có thể giải mã nhờ máy tính thì mới có thể nhìn thấy được. http://www.darkside.com.au/snow/index.html. 3.1.4 ẨN THÔNG ĐIỆP TRONG NỘI DUNG CỦA CÁC ĐOẠN VĂN BẢN KHÁC Một trong những cách khác để ẩn thông điệp là ta thay đổi cấu trúc của văn bản ban đầu sao cho nghĩa không bị thay đổi để mà ta có thể lưu trữ được thông điệp. TextHide là một chương trình sử dụng cơ chế này. Bạn có thể truy nhập vào link sau và tìm hiểu thêm: http://www.compris.com/TextHide/en/. Ngoài ra ta có thể ẩn thông điệp trong các đoạn email spam. Điểu này có thể thực hiện bởi chương trình SpamMimic theo link sau: http://www.spammimic.com. Ở đây mình sẽ thêm chuỗi “Hi ! How are you ?” vào email spam:
Khi đó ta được email sau:
Dear Friend , Especially for you – this breath-taking news ! If you no longer wish to receive our publications simply reply with a Subject: of “REMOVE” and you will immediately be removed from our database . This mail is being sent in compliance with Senate bill 2116 , Title 8 ; Section 309 . This is not multi-level marketing . Why work for somebody else when you can become rich within 65 weeks ! Have you ever noticed how many people you know are on the Internet & society seems to be moving faster and faster . Well, now is your chance to capitalize on this ! We will help you turn your business into an E-BUSINESS and decrease perceived waiting time by 150% ! You are guaranteed to succeed because we take all the risk ! But don’t believe us . Mr Ames of Iowa tried us and says “Now I’m rich, Rich, RICH” . This offer is 100% legal ! We beseech you – act now ! Sign up a friend and you get half off ! God Bless ! Dear Friend ; This letter was specially selected to be sent to you . This is a one time mailing there is no need to request removal if you won’t want any more . This mail is being sent in compliance with Senate bill 1620 , Title 9 , Section 302 . THIS IS NOT MULTI-LEVEL MARKETING . Why work for somebody else when you can become rich in 41 DAYS . Have you ever noticed people love convenience plus most everyone has a cellphone . Well, now is your chance to capitalize on this . We will help you deliver goods right to the customer’s doorstep & turn your business into an E-BUSINESS . The best thing about our system is that it is absolutely risk free for you ! But don’t believe us . Ms Anderson of Hawaii tried us and says “I was skeptical but it worked for me” ! We are licensed to operate in all states ! We BESEECH you – act now ! Sign up a friend and you get half off ! God Bless .
3.2 Cơ chế ẩn thông điệp trong tệp tin âm thanhĐịnh dạng MP3 là định dạng phổ biến của các file nhạc.Do đó ta có thể sử dụng chúng để ẩn các đoạn text trong đó.Có nhiều công cụ có thể giúp bạn ẩn thông tin trong file .MP3 và MP3Stego là một chương trình trong số đó. Dưới đây là giao diện của chương trình này:
Còn đây là giao diện khi bạn muốn giải mã:
- Steganography and Digital Watermarking
- Một số tài liệu khác trên Internet.
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