# What is public-key cryptography?

In this series, we will learn about blockchain and Cryptography, which you need to understand when trading or investing. We will focus on the necessity and function rather than the technical principles of encryption.

Before we get into the subject, let's think about why people have the inconvenience of using an 'address' that is difficult to recognize when trading cryptocurrency, and why it can't be managed at a level that can be easily memorized like a phone number. Perhaps you may have wanted to use a blockchain service (usually through a wallet), but ran into difficulty from the beginning of the account creation or felt lost due to unfamiliar terms that are difficult to understand. These reasons occur because cryptocurrencies and blockchain are made up of several cryptographic systems.

Cryptography is a technology that is widely used in our daily life. It is also used when entering and leaving the house, opening and locking the car door, using a computer or smartphone, and logging in to use online services such as e-mail or Internet banking. Cryptography is used even when sending and receiving documents via e-mail, it is also used in the act of encrypting documents and sharing passwords so that only limited people can access documents.

One important piece of information emerges from this example. That's the encryption key. In all cases where ciphers are applied, you need to know the cipher key to use them properly. A commonly used password is also a type of encryption key. In addition, an algorithm suitable for the encryption key is also required. Just like a lock and a key, a cryptographic system consists of a `cryptographic algorithm`

and a `cipher key`

.

Cryptographic systems that we use in various situations can be broadly classified into two categories. One is `public-key cryptography`

and the other is `symmetric-key cryptography`

. From the name, it can be inferred that public-key cryptography will have a public encryption key, and symmetric-key cryptography will have a pair of symmetric encryption keys.

### Symmetric key cryptography

Symmetric key cryptography is a system that is often used for encryption that you have already used. When A and B send and receive information, the process of encrypting and decrypting the information is required so that no one else can look at the information. For this, A and B use the same encryption key, it's like sharing a password to open an encrypted document.

Symmetric key cryptography is a system in which the encryption key is operated by the encryption key shared by both the encrypter and the decryptor, so it can be used for information encryption, but it is a method in which two or more people necessarily know the encryption key. Therefore, there is no uniqueness.

Why do we need uniqueness? This is the same principle as signing when you write a contract. It is a testament to the uniqueness of the only action that no one else can do. To express this digitally, you need an encryption key that no one can know and only you know. Therefore, a system with a unique encryption key is also required.

### public-key cryptography

For this uniqueness, public-key cryptography is used. Public-key cryptography, as the name suggests, is a system that one key can be made public while the other key must never be disclosed. That's why we call each encryption key a `Public Key`

and a `Private Key or Secret Key`

. Since the private key is information only known to me, an action done through the private key has the ability to prove that it is an action done by me. Conversely in the digital world, anyone that holds the private key will have the authority to perform an action. Therefore, the private key is very sensitive information that must be managed secretly.

In public-key cryptography, it is possible to create a signature using a unique private key. In this process, the other party must be able to verify the actions performed with the private key. The public key that can be disclosed is required so that anyone can verify it. (Public-key cryptography can also be used for encryption, but it is not relevant to the current focus, so we will skip it.)

In summary, it can be seen that symmetric key cryptography is used to encrypt information, and public-key cryptography is used for actions such as creating signatures that require uniqueness. In the next article, we will dive into the subject of digital signature, which is the most widely used use case of public-key cryptography.

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