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## Goals Of Cryptography

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**Confidentiality â†’**Authorised party can access the information

**Integrity â†’**Correctness and completeness of information

**Authenticity â†’**Source of information can be verified by the receiving party

**Non-Repudiation â†’**Source of information can be verified by any third party

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## Tools For Concepts

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Concept | Tool |

Encryption [Confidentiality] | AES | RSA |

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 |

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* Many more not mentioned here, this is just an example

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## Cryptography Goal: Confidentiality

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### 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

- Doesnâ€™t scale well with the number of participants in the system

- Can be used to encrypt large amount of data

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**NIST â†’**

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### Asymmetric Encryption

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**Properties:**- Public Key KE to encrypt
- Private Key KD to derypt

**Constraint:**- DEC(ENC(P,KE),KD) = P

**In Practice:**- Often RSA with keys â‰¥ 2048 bits
- Requires 1 key-pair for each participant

- Scales well

- Size of data that can be encrypted is almost equal to the size of the key.

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**WIKI â†’**

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### 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 Receiverâ€™s public key KE

**Receiver:**- Obtain symmetric key K by using Decrypting Symmetric Key using Receiverâ€™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

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