# Cryptography Basics

What is Cryptography? This light-heart video will give you a general feeling about Cryptography.

Cryptography is the practice and study of hiding information, in order to achieve **privacy**, **authentication**, **data integrity**, and **non-repudiation**.

A message that is sent in its original form is called **plaintext**, even though these days it might not be text at all, but an image, for example. The secretly encoded message is called **ciphertext**, which is what results from the plaintext by applying an **encryption** algorithm, called a **cipher**. If the encryption is reversed, the process is called **decryption**.

- Symmetric Encryption

Applying a cipher typically requires one more piece of information – that is the **key**, which must be selected before applying a cipher to encrypt a message. If the same key is used to encrypt and decrypt a message, then we call the algorithm a **symmetric** encryption scheme. Sharing or distributing the key becomes a challenge. Imagine a group of people want to communicate, they have to create a secret key for each two persons, and the number of secrete keys grow exponentially as more people joins the communication.

- Asymmetric Encryption

It will be nice to have two keys, one key is used for encryption and another key is used for decryption. A message receiver can then made encryption key publicly available (**public key**), while hold the decrption key secret (**private key**). Everyone can then encrypt a plaintext with the public key and send the ciphertext to the message receiver through public network. Hackers who sniffed the ciphertext won’t be able to decrypt it, because only the intended receiver (private key holder) is able to decipher the message. Such algorithms are called **asymmetric** encryption schemes. They are also known as **Public Key Cryptography **algorithms.

- Hash

A cryptographic **hash function** is a deterministic procedure that takes an arbitrary block of data and returns a fixed-size bit string, the (cryptographic) hash value, such that an accidental or intentional change to the data will change the hash value. The data to be encoded is often called the “**message**“, and the hash value is sometimes called the **message digest **or simply digest. Cryptographic hash functions have many information security applications, notably in digital signatures, message authentication codes (MACs), and other forms of authentication.

If you want to explore further, Here is a pretty comprehensive introduction from Google University.

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