**A little bit of terminology**

### As I wrote in my previous post, when it comes to cryptography people usually use the word **encryption** for any data manipulation algorithm.

Although this is not completely wrong it might lead (at least for beginners) to some confusions especially when we will talk about data signing.Since this is a beginners course and there is no need of any formal mambo-jambo we can assume that we are talking about digital data, so we can use the term

**bits manipulation algorithm**to refer to any of the algorithms.

**Symmetric-key algorithms**

I have talked about symmetric keys in my previous post. So obviously the bits manipulation algorithms that use symmetric keys are called symmetric-key algorithms or

Here is a list of some popular ciphers:

Usually in communication with customers you will have to decide between DES and AES, and it is always better to go with AES as it is newer and more secure algorithm.

**ciphers.**They can additionally be divided into**stream**and**block ciphers**, but we will not go that deep in this course, as for more info you can always check the appropriate wikipedia page.Here is a list of some popular ciphers:

Usually in communication with customers you will have to decide between DES and AES, and it is always better to go with AES as it is newer and more secure algorithm.

**AES**stands for**Advanced Encryption Standard**and is often (wrongly) referred to as AES-128 since it is a block chiper with block size of 128.**AES-128**however is the name of one of the three actual algorithms covered by the standard. The

**128**part of the name indicates the length (in bits) of the symmetric key used. The other two algorithms are

**AES-192**and

**AES-256**, following the same naming convention.

The key used for the algorithms does not have any structure, i.e. any bit string of a proper length can be used. Usually the security APIs provide a method for secure random number to be generated which can be used as a key.

### The bits manipulation algorithms that use asymmetric keys are called asymmetric-key algorithms, and the appropriate cryptography branch is called public-key cryptography.

**Lack of randomness**in the key might be a**security issue**.### Asymmetric-key algorithms

### The bits manipulation algorithms that use asymmetric keys are called asymmetric-key algorithms, and the appropriate cryptography branch is called public-key cryptography.

As always, the appropriate wikipedia page is a great starting point :)

The asymmetric-key algorithms provide a more complex but also more flexible type of security. It covers scenarios, as I wrote in my previous post, that are not possible to be covered by use of symmetric keys. One other importnat feature is that

Yes, this is right, we finally got to the point of using the word encryption.

We can define the

It is always best to describe things with examples, so let us use that wikipedia example here.

The two persons involved in the communication are Alice and Bob. Let's say that

You see, since no one else has the Alice's private key only she can read the data, and the only thing that the others can do is send her encrypted messages. That's why it is said that asymmetric-key algorithms don't need secure exchange of the keys, since the only key shared is the one used for encrypting the message and not the one for decrypting it.

One of the most widely used encryption algorithms today is

In the example above we have shown how Bob can send data to Alice in a way that no one else than Alice can read. But let's now say that Alice wants to send some message to Bob, and Bob needs to verify that the message sent was really from her.

In this case

We call this type of data scrambling

Signing on its own is not much different than encryption as it is just a different type of data manipulation algorithm used for different application (verifying the sender, not protecting the data). Since all data scrambling algorithms are quite complex and time consuming, usually the signing is done over a smaller amount of data sent together with the whole message.

Today

One last example that public keys can be used for is secure exchange of a symmetric key over an insecure communication channel. Once the key is securely shared it can be used to establish a secure connection with symmetric encryption of the data.

One such protocol is the Diffie–Hellman key exchange protocol shown on the image below.

Both Alice and Bob generate asymmetric key pairs and exchange the public keys between each other. Then they use a combined

This protocol is now part of the well known

**there is no need of secure initial exchange of the keys**. Of corse this last statement might look a little bit strange now, but everything should become clear by the end of this post :)**Encryption**Yes, this is right, we finally got to the point of using the word encryption.

We can define the

**encryption**as type of bits manipulation which purpose is to make the data unreadable by anyone else than the intended audience (usually one person).It is always best to describe things with examples, so let us use that wikipedia example here.

The two persons involved in the communication are Alice and Bob. Let's say that

**Bob**needs to send an**encrypted message**to**Alice**. In that case**Alice**needs to**generate a key pair**on her computer and distribute the**public key**to**Bob**.**Bob**can then use the**public key**to**encrypt**the data and send the encrypted data to**Alice**. She can use her**private key**to**decrypt**the data and use it.One of the most widely used encryption algorithms today is

**RSA.**The appropriate wikipedia page should give you more info that you need, including exact algorithm for key generation, encryption, decryption and whole bunch of security issues and analysis most important of which is the key length consideration.**Signing**In the example above we have shown how Bob can send data to Alice in a way that no one else than Alice can read. But let's now say that Alice wants to send some message to Bob, and Bob needs to verify that the message sent was really from her.

In this case

**Alice**will use her**private key**to**manipulate the bits**of the data and**Bob**can use the**public key**to**read**the original message.We call this type of data scrambling

**signing**since data is not really protected from the general audience as anyone can have the public key which is now used to read the data.Signing on its own is not much different than encryption as it is just a different type of data manipulation algorithm used for different application (verifying the sender, not protecting the data). Since all data scrambling algorithms are quite complex and time consuming, usually the signing is done over a smaller amount of data sent together with the whole message.

Today

**RSA**encryption over some checksum of the data (e.g. SHA1) is mostly used for signing.**Key exchange**One last example that public keys can be used for is secure exchange of a symmetric key over an insecure communication channel. Once the key is securely shared it can be used to establish a secure connection with symmetric encryption of the data.

One such protocol is the Diffie–Hellman key exchange protocol shown on the image below.

Both Alice and Bob generate asymmetric key pairs and exchange the public keys between each other. Then they use a combined

**signing/encryption**of the data so that the other person can receive the data encrypted and also be able to verify the sender.This protocol is now part of the well known

**SSL**protocol which we will talk about in some of my upcoming posts.
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