Encryption, Encryption Protocols and Digital Certificates

Encryption is used to make sure that data is unreadable if it is intercepted. An encryption algorithm converts plaintext (the readable message) into ciphertext (the scrambled message). Decryption turns the ciphertext back into plaintext.

There are two types of encryption: symmetric and asymmetric.

• Symmetric encryption uses the same key for both encryption and decryption. Symmetric encryption is faster than asymmetric encryption because it uses less complex mathematical operations. This means data can be encrypted and decrypted at a speed and volume that suits network communications.
• Asymmetric encryption uses a related set of keys: one public and one private. The keys work as a pair: one key encrypts the message and the other key decrypts it. Asymmetric encryption is slower, but it allows the sender to be authenticated.

Most computer systems today use asymmetric encryption to exchange a symmetric key safely. The symmetric key is then used to secure one limited communications session, so the key is not in use for a long time.

A major weakness of symmetric encryption, where the sender and the recipient use the same key, is the need to exchange keys. To be really secure, a key must only be used once, so a new key would have to be exchanged every time a new message is sent. When the sender and receiver are in different places, perhaps on opposite sides of the globe, key exchange is a real challenge.

This problem was solved by a system that uses asymmetric keys. In an asymmetric encryption scheme, the key used to encrypt a message cannot be used to decrypt the ciphertext. This sounds as if it could not possibly work, and it took a long time for a method to be found.

The RSA algorithm (named after Rivest, Shamir and Adleman, who devised it) generates several components. These components are mathematically linked, so a message encrypted using one set of components can be decrypted using the other set. Together the components are known as a public and private key pair. The mathematics is complex, and you do not need to understand it for A Level. It relies on the intractable problem of factorising very large numbers, which makes it computationally secure.

Public key cryptography uses an asymmetric encryption scheme, where different keys are used for encrypting and decrypting the data.

• The public key is widely shared. Anyone can have it.
• The private key is known only to its owner and must be kept secure.

Imagine that Alice (A) and Bob (B) want to exchange secure messages. They each need a public/private key pair. The public keys can be shared openly: they are of no use to anyone else. However, each person must keep their own private key absolutely secret.

When Alice wants to send a message to Bob:

1. Alice encrypts the message using Bob’s public key.
2. Bob receives the encrypted message. He decrypts it using his own private key.

As long as Bob keeps his private key absolutely secret, no one else can decrypt the message. The key pair cannot be guessed because it is computationally secure.

Public key encryption keeps a message confidential: if Alice encrypts it with Bob’s public key, only Bob can read it. But how can Bob be sure that Alice sent it (authentication), and that it has not been changed on the way (integrity)? This is what a digital signature is for.

A digital signature is like a handwritten signature that says “I am satisfied that this content is accurate and comes from me”. Here is how Alice signs a message she sends to Bob:

1. First, Alice runs a hash function on the plaintext message. This produces a hash total, also called a digest.
2. She encrypts the digest with her own private key. This encrypted digest is the digital signature for the message. Only Alice could have produced it, so it proves that the message came from her.
3. She attaches the signature to her message.
4. She then encrypts the whole thing with Bob’s public key, as before.

When Bob receives the message:

1. He decrypts it with his private key, as before. Now he has the plaintext, so he can read the message.
2. Next, he uses Alice’s public key to decrypt the signature. This gives him the digest that Alice made.
3. He runs the same hash function on the plaintext himself and checks whether the two digests match. If they do, he knows the message was not changed on the way. He has verified the digital signature, and can be sure of the message’s integrity.

If a hacker intercepts the message and changes it, the digest Bob calculates will not match the digest recovered from Alice’s signature.

The digital signature can include a timestamp, so that a false signature cannot be recreated at a later date.

You can use a digital signature to make sure that data has not been tampered with, even when the message itself is not encrypted. For example, most email clients can handle digital signatures, and your browser might use a digital signature to check that a download from a software company has not been tampered with.

However, anyone can create a digital signature and claim to be a trusted person. To guard against this, you can check a digital certificate to verify the sender’s identity.

A digital certificate is an electronic document that authenticates (verifies the identity of) a message sender or a website. It is issued by an official certificate authority (CA), such as Let’s Encrypt or DigiCert. A digital certificate includes a serial number, an expiry date, the holder’s name and the holder’s public key. It is signed with the CA’s private key, so that you can check that it is genuine.

These items are commonly found on a digital certificate:

• version number
• serial number of the certificate
• algorithm identification
• name of the certificate issuer
• validity (the start date and the expiry date of the certificate)
• public key
• issuer’s identifier
• company’s identifier
• signature algorithm used
• digital signature.

A person or an organisation gets a digital certificate from a certificate authority (CA):

1. The applicant creates a public key and a private key. The private key is kept secret.
2. The applicant sends the CA a request with their public key and proof of who they are.
3. The CA checks the applicant’s identity.
4. The CA builds the certificate. It holds the applicant’s name, their public key, the name of the CA, a serial number and the dates the certificate is valid.
5. The CA makes a digest of the certificate and encrypts it with the CA’s own private key. This is the CA’s signature on the certificate.
6. The CA sends the signed certificate to the applicant, who can now share it with anyone.

When Alice sends Bob a signed message, she attaches her certificate. Bob uses it like this:

1. Bob uses the CA’s public key, which is already stored on his computer, to check the CA’s signature on the certificate. He also checks that the certificate is still in date. If both are fine, the certificate is genuine and has not been changed.
2. Bob takes Alice’s public key from the certificate.
3. He uses Alice’s public key to decrypt her digital signature. This gives him the digest Alice made.
4. He runs the same hash function on the message he received, to make his own digest.
5. If the two digests match, the message came from Alice and was not changed on the way.

Without the certificate, anyone could send Bob a public key and claim to be Alice. The CA’s signature is what lets Bob trust that the key really belongs to her.

The protocol used to keep internet communication secure today is Transport Layer Security (TLS), which replaced the older Secure Sockets Layer (SSL).

TLS is the more modern of the two: it is based on SSL. SSL and TLS are mainly used in client/server applications. They both use standard cryptographic protocols to make sure that communication between the client and the server is secure and authenticated. Normally only the server is authenticated, and the client stays unauthenticated. Once a secure link between the server and the client is set up, the SSL or TLS protocol makes sure that no third party can eavesdrop.

Secure Sockets Layer. When a user logs on to a website, SSL encrypts the data, so only the client’s computer and the web server can make sense of what is being sent. SSL also checks the integrity of the data. A user can tell that a secure connection is being used when the web address starts with https. SSL itself is now obsolete: it has been replaced by TLS, which is what browsers use today.

TCP is used to set up a connection between the client and the server. A handshake takes place, so that communication between the client and the server can begin. One part of the SSL protocol is to agree which encryption algorithms will be used: this is essential for a secure, encrypted communication. To create an SSL connection, a web server needs an SSL digital certificate. The website owner must get this certificate so that the SSL protocols can be used.

Transport Layer Security (TLS) is a modern, more secure version of SSL. It provides encryption, authentication and data integrity more effectively. It keeps data secure and private between devices and users when they communicate over a network such as the internet. When a website and a client communicate over the internet, TLS prevents a third party from eavesdropping.

TLS is made of two main layers:

1. The record protocol holds the data being sent over the network or internet. It can be used with or without encryption.
2. The handshake protocol lets the web server and the client authenticate each other and agree the encryption algorithms to use. A secure session between the client and the server is then set up.

Note SSL is now obsolete. It was retired in 2015 and current web browsers no longer support it, so TLS is what is used today. The name lives on in the term “SSL certificate”, which is just the old name for the certificates that TLS uses.

Session caching. Opening a TLS session takes a lot of computer time, mainly because of the complex cryptographic processes involved. Session caching avoids using as much computer time for each connection: TLS can either set up a new session or try to resume an existing one. Resuming an existing session can greatly boost the performance of the system.

These are the stages when a browser (the client) opens a secure connection to a web server:

1. The client types the URL into the browser and presses the Enter key. Several steps happen before any encrypted data is sent: this is known as the handshaking stage.
2. The client’s browser requests secure pages (https) from the web server.
3. The web server sends back its SSL digital certificate, which also contains the server’s public key. The certificate is digitally signed by a third party called the certificate authority (CA).
4. When the client’s browser receives the digital certificate, it checks: the digital signature of the CA (is it one of the CAs in the browser’s trusted store? The list of trusted CAs comes as part of the browser the client downloads); that the start and end dates on the certificate are still valid; and that the domain in the certificate exactly matches the domain the client asked for.
5. Once the browser trusts the digital certificate, it creates a temporary session key, encrypts it with the server’s public key (taken from the digital certificate) and sends it to the web server.
6. The web server uses its private key to decrypt the session key. It then sends back an acknowledgement, encrypted with the same session key.
7. The browser and the web server can now encrypt all the data sent over the connection using this session key, and secure communication can take place.

Note This is asymmetric encryption used to exchange a symmetric key: the session key travels safely because it is encrypted with the server’s public key, and then the faster symmetric encryption is used for the rest of the session.

Quantum mechanics gives the fundamental laws of physics for the behaviour of particles. The particles that carry light are called photons. Photons behave like waves: each photon seems to vibrate in a particular direction, at right angles to the direction it travels in. The direction a photon vibrates in is called its polarisation, and it is drawn as a double-ended arrow.

A photon can be created with a chosen polarisation to represent the value of a bit. With four possible polarisations, there are two ways to represent a 1 and two ways to represent a 0:

The vertical and horizontal pair is called the + basis, and the two diagonal directions are the x basis. The table shows ten bits being sent: the bit value sent, the polarisation basis the sender used, the polarisation basis the receiver chose, and the bit values confirmed.

1. The sender chooses the bit pattern at random, and also chooses the polarisation basis for each bit at random.
2. The receiver chooses a polarisation basis for each bit, again at random.
3. After the transmission, the sender tells the receiver which basis was used for each bit.
4. The receiver replies saying which of its choices matched.
5. For each match, both sides now have a stored bit value. In this example (bits 4, 6, 9 and 10) the “shared secret” code 1001 has been created.

This scheme is used in Quantum Key Distribution (QKD) systems. Asymmetric encryption can be used to transfer a key that is then used for symmetric encryption. QKD offers another way to transfer the key, and the key is then used in the normal way.

The advantage of QKD is that the transfer does not send defined values, just photons. By the laws of quantum mechanics, anyone who intercepts the flow of photons to find out their polarisation will destroy the photons. A photon cannot be detected and measured and then sent on again.

The main drawback of quantum cryptography is that it cannot use standard communication media. It needs a dedicated, special-purpose quantum channel between the sender and the receiver. This is very expensive to provide, so routine use is unlikely.

There are big hopes for quantum computing, but progress is slow. QKD, a quantum technology used for communication, is one of the few areas where there has been clear success.