Principle of Cybersecurity

用户8279

2025年12月11日修改

Classic Crypto

Symmetric Key Cryptography

Kerckhoff's Principle: The key(s) of a cryptosystem should be hidden, but the mechanisms should be public.​

•
Experience has shown that secret algorithms tend to be weak when exposed​

•
Secret algorithms never remain secret​

•
Better to find weakness beforehand​

Symmetric Key Cryptography: a single key (k) is used for E and D

common.docs_name - LarkCCM_Docs_Menu_Image

Simple Substitution

•
shift by n (n is the key)​

Caesar's cipher: shift by 3

Attack I: Exhaustive key search-try them all. Only 25 keys possible.

•
General case: any permutation of letters, 26! variations in total 每个letter都可能对应一个不一样的letter​

Attack II: Cannot try 26! combinations, we can use the frequency of English letters​

Again, Principle: Cryptosystem is secure if the best-known attack is to try all keys (exhaustive key search). Cryptosystem is insecure if any shortcut attack is known.​

One-Time Pad (OTP)

XOR: 相同为0，不同为1（0和任何数异或都是那个数自己）

•
Encryption: plaintext XOR key = ciphertext​

•
Decryption: ciphertext XOR key = plaintext​

Requirements:

•
Pad (key) must be random, used only once​

•
Pad must be the same size as message​

OTP is provably secure: ciphertext gives no information about plaintext. Assume value of each bit is equally likely, then all ciphertexts are also equally likely.​

假设pad被使用两次：C1=M1 XOR K    C2=M2 XOR k，那么攻击者可以得到C1 XOR C2=M1 XOR M2，也就得到了M1和M2之间的关系，也就有了破译密文的可能。​

Codebook Cipher

A book filled with codewords, with everyword in this book has a code

Additive：在普通的codebook cipher中，同一个词会被映射为同一个code，容易被攻击者按照频率pattern破译。配合一个additive book，每次encrypt的时候，选出一页（一个additive position），将code与这一页的数字逐位相加。每次发送时，发送cyphertext和additive position。additive position本身不是secret，其内容是需要保密的。收件人收到内容后用减法还原原始code。这样，attacker看到的同一个词的cyphertext都不一样（same word encodes into different representations），因此难以破译。​

Symmetric Key Crypto

Modern Symmetric Key Crypto

Stream cipher:

•
generalize one-time pad​

•
Key is relatively short, it works on one bit/byte a time 一次加密一个bit/byte，连续加密​

•
Key is stretched into a long key stream​

•
Keystream is used just like a one-time pad​

Block cipher:

•
works on large chunks of data (block) at a time 一次加密一个数据块​

•
can combine blocks for additional security​

Stream Cipher

A5/1: Shift Registers

Principle of Cybersecurity​

Principle of Cybersecurity