Public Key Cryptography

之前所討論的symmetric cryptography最終都有一個無法解決的問題:
如何交換key

大概只能打扮成

然後到咖啡店

跟對方面交XDD

因為加解密都用同一把key，所以key沒有辦法透過公開網路交換

What is Public-Key (PK) Cryptography

則是一個是私人金鑰，另一個則是公開金鑰

公開金鑰可以再公開網路中傳輪

私有金鑰則自己保管

主要有兩種用途

加密資訊

數位簽章(驗證來源是否正確)

如果要同時加密資訊並驗證來源

NOTE: Does the order matter?

Sure!!

如果先用對方public key加密，再用自己private 簽名

對方收到後先驗證來源後，只能保證加密後的資料是你的

但plain text來源無法驗證

Graphic PK cryptography example

Ice cream vandor problem

What is the minimum number of vans we need?
At which intersections would we base our vans?

This problem is hard to solve:

O(n choose k) = O(n!/k!(n-k)!)

However, this solution is easy to check:

O(n)

Encrypt

Decrypt

real-world PK crypt algorithm

knapsack problem

it has been broken 😰

RSA algorithm

Performance

那既然有了 PK , 那我們還需要symmetric cryptography幹麻?

大家可以安全的交換隱密資訊了💪💪💪

What a wonderful world ~~

但事實是

非對稱的加密非常~慢

benchmark

於是美好世界

In practice

當然不是這樣世界就毀了

現實中將對稱跟非對稱一起用

用對稱加密資訊，再用非對稱加密key，這樣資訊跟key都可以在網路上傳輸啦~

同時又兼顧效能

whole picture

Coding Time~

騙你的，只要用library就好
rbnacl都做完所有該做的事了

rbnacl-pk-simple_boxlink


require 'rbnacl/libsodium'

# KEY CREATION
personA_private_key = RbNaCl::PrivateKey.generate
personA_public_key  = personA_private_key.public_key

personB_private_key = RbNaCl::PrivateKey.generate
personB_public_key  = personB_private_key.public_key

# KEY EXCHANGE
personA_box = RbNaCl::SimpleBox.from_keypair(
  personB_public_key,
  personA_private_key
)

personB_box = RbNaCl::SimpleBox.from_keypair(
  personA_public_key,
  personB_private_key
)

# SENDING
message = "secret"
ciphertext = personA_box.encrypt(message)

# RECEIVING
plaintext = personB_box.decrypt(ciphertext)

最近會儘快把前幾週的上課筆記給補起來👻👻👻

SS week6