Goals Of Cryptography


Goals Of Cryptography

  • Confidentiality → Authorised party can access the information
  • Integrity → Correctness and completeness of information
  • Authenticity → Source of information can be verified by the receiving party
  • Non-Repudiation → Source of information can be verified by any third party
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Tools For Concepts

Encryption [Confidentiality]
Cryptographic Hash Functions [Integrity]
SHA-2 | SHA-3
Message Authentication Codes | Authenticated Encryption [Authenticity]
HMAC and GCM block cipher mode of operation
Digital signatures [Non-Repudiation]
RSA-PSS and Ed25519
* Many more not mentioned here, this is just an example

Cryptography Goal: Confidentiality


Symmetric Encryption

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  • Property:
    • Single key K for both Encryption And Decryption
  • Constraint:
    • DEC(ENC(P,K),K)=P
  • In Practice:
    • ENC and DEC are very often the Advanced Encryption Standard (AES) block cipher
    • AES keys can be of length 128, 192, 256 bits
    • Each pair of sender and receiver needs a key and its the same key
  • Doesn’t scale well with the number of participants in the system
  • Can be used to encrypt large amount of data

Asymmetric Encryption


Hybrid Encryption (Using AES and RSA)

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  • Sender:
    • Create new random symmetric key K
    • Send data encrypted using key K
    • Send key K encrypted with Receiver’s public key KE
  • Receiver:
    • Obtain symmetric key K by using Decrypting Symmetric Key using Receiver’s Private Key KD
    • Obtain data by decrypting data using Symmetric Key K
  • Main Problem:
    • Once KD is known all symmetric keys K can be obtained and all data can be decrypted
    • Can be prevented by using the ephemeral Diffie-Hellman key exchange EDH or ECDHE