17.1 Encryption, Encryption Protocols and Digital certificates

**Plain text**: A The original data to be transmitted as a message
**Cipher text:** the result of applying an encryption algorithm to data.
**Encryption:** the making of cipher text from plain text.
	can be used:
	    - When transmitting data over a network.
	    - It is a routine procedure when storing data within a computing system.
**Public key:** A key that is shared between the user and sender for encryption of the data and verifying digital signatures.
**Private key:** 
	- A private key is the unpublished/secret key/never transmitted anywhere
	- It has a matching public key
	- It is used to decrypt data that was encrypted with **its** matching public key.
# Reasons for using key cryptography
- To ensure the message is authentic // came from a trusted source
- To ensure that only the intended receiver is able to understand the message
- To ensure the message has not been altered during transmission
- Non-repudiation, neither the sender or receiver can deny the transmission occurred
# Methods of key cryptography that can be used
- Symmetric
- Asymmetric
## Symmetric key encryption
when there is just one key used to encrypt and then decrypt. The sender and the receiver of a message share the secret key.
## Asymmetric encryption
### Process
- The message to be sent is encrypted using the recipient’s public key. // The message to be sent is encrypted using the sender’s private key.
- The message is decrypted using the recipient’s private key. // The message is decrypted using the sender’s public key.   
### Purpose
- To provide better security by using a public key and a private key
- One of the keys is used to encrypt the message, the matching key is used to decrypt the message.
## Differences between symmetric and asymmetric encryption
- Symmetric cryptography uses a single key to encrypt and decrypt messages, Asymmetric cryptography uses two.
- The symmetric key is shared, whereas with asymmetric, only the public key is shared (and the private key isn’t).
- … the risk of compromise is higher with symmetric encryption and asymmetric encryption is more secure.
- Symmetric cryptography is a simple process that can be carried out quickly, but asymmetric is much more complex, so slower.
- The length of the keys in symmetric encryption are (usually) shorter than those for asymmetric (128/256 bits v 2048 bits).
# Quantum cryptography
## the purpose
- to produce a virtually unbreakable encryption system / send virtually un-hackable secure messages …
- …using the laws / principles of quantum mechanics / properties of photons
- detects eavesdropping …
- …because the properties of photons change
- to protect security of data transmitted over fibre optic cables
- to enable the use of longer keys.
# Benefits
- Any eavesdropping can be identified (as the state will be changed)
- Integrity of the key once transferred can be guaranteed (cannot be copied and decrypted at a later date)
- Longer/more secure keys can be exchanged
- Provides security based on laws of physics rather than mathematical algorithms, so more secure.
- To protect the security of data transmitted over fibre optic cables.
- Virtually unhackable.
- The performance of quantum cryptography is continuously improved, making it suitable for most valuable government/industrial secrets.
- Longer keys can be used
- Eavesdropping can be detected
## Drawbacks
- Limited range
- requires dedicated fibre (optic) line and specialist hardware
- cost of dedicated fibre (optic) line and specialist hardware is expensive
- polarisation of light may be altered whilst travelling down fibre optic cables
- Lacks many vital features such as digital signature, certified mail, etc.
- High cost of purchasing / maintaining equipment required.
- Currently only works over relatively short distances.
- Error rates are relatively high as technology is still being developed.
- Polarisation of light can change during transmission.
- Allows criminals and terrorists to hide their communications.

# Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols
## Purpose
- The SSL and TLS protocols provide communications security over the
- internet / network
- they provide encryption
- They enable two parties to identify and authenticate each other and communicate with confidentiality and integrity.
## How SSL/TLS protocols are used when a client-server communication is initiated
- An SSL/TLS connection is initiated by an application which becomes the client
- The application which receives the connection becomes the server
- Every new session begins with a handshake (as defined by the (SSL/TLS) protocols)
- The client requests the digital certificate from the server // the server sends the digital certificate to the client
- The client verifies the server’s digital certificate and obtains the server’s public key
- The encryption algorithms are agreed
- The symmetric session keys are generated / defined
# Digital certification
## How a digital certificate is obtained
- enquiry made to Certificate Authority (CA)
- enquirer’s details checked by CA
- if enquirer details verified by CA then public key is agreed
- CA creates/issues certificate that includes the enquirers public key
- encrypting data sent to/by CA with the CA’s public/private key

- The organisation requests a certificate from a Certificate Authority (CA)
- The organisation may send their public key to CA
- The organisation gathers all the information required by the CA in order to obtain their certificate, which includes information to prove their identity
- The CA verifies the organisation’s identity
- The CA generates / issues the certificate including the organisation’s public key (and other information).   
## How a digital signature is produced before the message is sent
- The message is hashed with (the agreed hashing algorithm) to produce a message digest
- The message digest is then encrypted with the sender’s private key to form the digital signature
## How the digital signature can be checked
- The message together with the digital signature is decrypted using the receiver’s private key
- The digital signature received is decrypted with the sender’s public key to recover the message digest sent
- The decrypted message received is hashed with the agreed hashing algorithm to reproduce the message digest of the message received
- The two message digests are compared
- … if they are the same the message has not been altered // if they are different the message has been altered