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07. Cryptography

2022-06-01
devfancy
Electronic-Finance

이 카테고리는 경영학부 전자금융의 이해 수업을 듣고 정리한 내용을 바탕으로 글을 작성하였습니다.

Cryptography

  • Cryptography

    • (eng) The practice and study of techniques for secure communication in the presence of adversarial behavior

    • (kor) 위험한 상황에서 메세지를 안전하게 보내기 위한 기술에 대한 연구

      • 조건1) 엿들으면 안된다.

      • 조건2) 메세지에 대한 조작이 있으면 안된다.

Cryptography - 잘못된 예시

Encryption and Decryption

  • Encryption is the process of turning a plaintext human message into cyphertext.

    • 알아볼 수 있는 문장 → 알아볼 수 없는 문장

  • Decryption is the process of turning the cyphertext back into readable plain text

    • 알아볼 수 없는 문장 → 알아볼 수 있는 문장

Symmetric Encryption

  • Suppose Alice → Bob (message)

    • Caesar cypher : They encrypt the text by shifting each letter a set number of places in the alphabet. (글자들을 정해진 칸 만큼 각각의 글자들을 옮기는 것)
  • Symmetric encryption : a type of encryption where only one key (a secret key) is used to both encrypt and decrypt electronic information
  • kor) 어떤 메세지를 encrypt 과 decrypt 하는 오로지 하나의 key가 같을 때 쓰는 encryption 유형
  • 이런 유형은 실생활에서 사용하지 않는다.
    • spot 하고 break 하기 너무 쉽다.
    • 어떤 key를 사용해야할지 동의을 위한 의사소통이 필요하다. (They have to communicate to agree what key to use for scheme)

Symmetric Encryption

Asymmetric Encryption

  • ASymmetric Encryption (Public key Cryptography) : a cryptography system that uses pairs of keys. (서로 다른 키)

    • The key used to decrypt a message is different to the key used to encrypt message.

    • You create two mathematically linked keys when you want to receive encrypted message.

      • Public key : You can share it with the world, and anyone can use it to encrypt message for you. (누구나 공유할 수 있는 키)

      • Private key : It is know only to you. (오직 혼자만 아는 키)

Asymmetric Encryption

  • Elliptic Curve Digital Signature Algorithm (ESCADA)

    • 당신은 랜덤하게 발생되어 선택된 private key를 가지고 그것으로부터 mathematically 발생된 public key를 가집니다.

      • Private key: Pick a random number between 0 and $2^{256}$ - 1 (78 digits).

      • Public key: Do some ESCADA maths on the private key.

    • 정리) 랜덤한 숫자를 추출해서 private key를 만든 후 수학적인 함수를 적용해서 public key를 만든다.

Hashes

  • Hash function : A mathematical function that converts an input value into another compressed(압축된) value

    • The input to the function is called preimage or message.

    • The output of the function is called fingerprint, digest, or hash

    • Hash functions are deterministic because the output is determined by the input.

    • Basic hash function (not used in blockchain)

      • input를 받으면 첫글자를 출력하는 것

      • 예) Hash(‘What time is it?’) ⇒ ‘W’

    • Cryptographic hash function (used in blockchains)

      • 예) MD5, SHA-256

Cryptographic Hash Functions

  • Cryptographic hash Functions는 특별하고 유용하게 만드는 몇가지 특징들이 있습니다.

  • 이상적인 Cryptographic Hash Functions 위한 5가지 특징들

    • [1] It is deterministic so the same message always results in the same hash.

    • [2] It is quick to compute the hash value for any given message (you can easily go ‘forwards’). ⇒ 계산이 빨라야한다 = 답을 간단하게 출력해야 한다.

    • [3] It is not feasible to generate a message from its hash value except by trying all possible messages (you can’t go ‘backwards’).

      • 가능한 메세지를 시도하는 것을 제외한다면, 주어진 hash value로부터 원래의 메세지를 찾는 것이 불가능하다. ex) ‘W’ (hash value) → ‘What time is it?’(Digital message)

    • [4] A small change to a message should change the hash value so extensively that the new hash value appears uncorrelated with the old hash value (a small change makes a big difference). ⇒ 조금의 변화라도 해쉬 값에 큰 차이를 가진다.

    • [5] It is not feasible to find two different messages with the same hash value (it is hard to create a hash clash). ⇒ 같은 해쉬값을 가진 두가지 다른 메세지를 찾는것이 불가능하다.

Cryptographic Hash Functions - Properties

  • Cryptographic functions are sometimes called trapdoor function (Properties 2 and 3).

    • message → hash (O) But, hash → message(X)

  • Message 가 조금만 달라도 전혀다른 hash 값이 나온다. 즉, 유추가 불가능하다.

What is Good Cryptographic Hash Functions ?

  • Is the hash function ‘Use the first character’ be good cryptographic hash functions ?

    • [1] Yes, it is deterministic

    • [2] Yes, it is quick to compute the output.

    • [3] Yes, by knowing only ‘W’ it is not feasible to guess the original sentence.

      • ex) ‘W’ → ‘What time is it?’ (X)

    • [4] No, a small change in the message doesn’t necessarily change the output. ‘What time is at¿ also hashes down to ‘W’.’

    • [5] No, we can easily create loads of inputs that will all hash down to the same output. Anything starting with ‘W’ will work.

    • => 4,5 번에 의해 좋은 cryptographic hash function 될 수 없다.

Digital Signatures

  • Digital signature : a specific type of electronic signature that requires the signer to authenticate their identity using a certified digital ID

    • It is created by taking the message you want to sign and applying a mathematical formula with your private key.

      • Message + Private key → Digital signature

    • Anyone who knows your public key can mathematically verify that this signature was indeed created by the holder of the private key.

      • Message + Digital signature + Public key → Valid / Invaild

Digital Signatures - 그림

  • Here, you encrypt data with private key and decrypt it with a public key.

Wet-ink-on-paper signature

  • There is no way of knowing if a document has been tampered after your signature is made. ⇒ 서명한 이후 조작되더라도 알 방법이 없다.

  • Your signature can easily be copied and re-used with other documents without your knowledge. ⇒ 서명이 쉽게 copy 되고, 다른 문서와 함께 재사용할 수 있다.

Digital Signatures

  • Any tampering with the message will result in the signature being invalidated. ⇒ 문서(메세지) 조작이 불가능하다

  • It is only valid for that exact piece of data, and so it cannot be copied and pasted underneath another piece of data. ⇒ 한번 사용하는 것으로 복사/붙여넣기 할 수 없다.

  • Digital signatures are used in blockchain transactions.

    • They prove account ownership.

    • No central organization (제 3자 개입 X)

Reference

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