[网络安全] Week 1 Note 3

Cryptography

Why？

prevent third parties from reading private messages

How？

encrypt messages before transmission;

decrypt messages upon receipt;

what:

confidentiality（保密） — secrecy

integrity（完整） — accuracy

authentication（身份验证） — ascription（归属）

non-repudiation（不可抵赖） — liability

which：

Transiposition

Scramble input to produce output

Key: the permutation of input permutation：排列组合

Substitution

Substitute one input for another

Key: the substitution( cookbook )

（！）破解一个密码就可以解密所有的通话。

OTP: One-Time Pad

Key: a secret bit string s of length n

messgae m of length n , the ciphertext c as:

for all i = 1 to n:  ci = mi⊕si 

DES: Data Encrytion Standard

• Block cipher
• 64-bit plaintext/ciphertext
• 56-bit key

3DES/TDES: Triple DES

AES: Advanced Encryption Standard

KeyExpansion: Rijndael’s key schedule derive a 128-bit key for each round

Initial round key addition AddRoundKey: state XOR round key

9 rounds (assume 10 rounds needed) SubBytes, ShiftRows, MixColumns, AddRoundKey

Final round without MixColumns

RSA

Homomorphic Cryptography

Proxy Re-encryption

Forward Secrecy

• Session keys will not be compromised even if the private key of the server is compromised
• By generating a unique session key for every session a user initiates, the compromise of a single session key will not affect any data other than that exchanged in the specific session protected by that particular key