藏密學是一門關於資訊隱藏的理論。主要的目的在於避免訊息在傳遞過程中遭受竊取或破壞。這篇論文提出了一個具有高容量的藏密系統，我們採用基於MRF 模型的紋理合成演算法 [Chen2003] ，合成
出符合使用者需求大小的載體影像。一般情況下，隱藏訊息的長度受
限於載體影像的大小，而我們採用紋理合成的方法來解決這個問題。
此外，為了增加藏密的安全性，在訊息被嵌入至載體影像之前，我們
會先對其進行加密的動作，而我們所採用的加密方法是基於數論中原
根的特性。當𝑝 是一個質數，而𝑔 是 𝑝 的其中一個原根時，
{𝑔𝑥 mod 𝑝, for 1 ≤ x ≤ 𝑝 − 1} = {1,2, … , 𝑝 − 1}。我們利用這種一對一的對應關係將訊息中的字元重新排列，以達到加密的效果。最後，我們將密文中每個8-bit 的character 拆成2 個4-bit 的word，並使用LSB取代法將他們嵌入載體影像的2 個pixel 中。因此對於一張M × N的影像，它的藏密空間有(M × N)/2 個 character，相當於整個影像大小的一半。

Steganography is a theory of information hiding. It prevents secret message from being stolen and destroyed during communication. This thesis proposes a high-capacity steganographic system for data hiding. We adopt a texture synthesis algorithm based on Markov Random Field (MRF) model [Chen2003] to synthesize cover images with user-selected size. Generally, the length of a secret message to be
hidden is limited by the size of the cover image; we solved this problem by using the texture synthesis algorithm. In addition, to enhance the security, we encrypt the secret
message before embedding it into a cover image. The encryption method is based on the property of primitive root mentioned in Number Theory. When 𝑝 is a prime number, and 𝑔 is a primitive root of 𝑝 , {𝑔𝑥 mod 𝑝, for 1 ≤ x ≤ 𝑝 − 1} =
{1,2, … , 𝑝 − 1}. The relationship of one-to-one correspondence could be used to rearrange characters in message and destroy the structure between them. Finally, each
character is partitioned into two 4-bit words, and then embedded into two pixels of the selected cover image by LSB replacement method. The embedding capacity of an M × N image is (M × N)/2 characters.

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