Cryptography is the practice and study of techniques for secure communication in the presence of third parties called adversaries. Check our list of essential interview questions on cryptography and network security that will trend on Design patterns & System Architecture interviews in 2021.

- 1.
# What is a key?

# Answer:

In cryptography a

**key**is a piece of information used in combination with an algorithm (a**cipher**) to transform**plaintext**into**ciphertext**(*encryption*) and vice versa (*decryption*).A cipher can be

*reciprocal*if it is used for both encryption and decryption, or*non-reciprocal*if a transformation to the key is required when using it in reverse.*Source:*medium.com - 2.
# Explain what is Data Encryption?

# Answer:

**Data encryption**translates data into another form, or code, so that only people with access to a secret key (formally called a*decryption key*) or password can read it. Encrypted data is commonly referred to as*ciphertext*, while unencrypted data is called*plaintext*.*Source:*digitalguardian.com - 3.
# Why is the Root Certificate important?

# Answer:

A

**Root SSL certificate**is a certificate issued by a trusted certificate authority (CA).In the SSL ecosystem, anyone can generate a signing key and sign a new certificate with that signature. However, that certificate is not considered valid unless it has been directly or indirectly signed by a trusted CA.

A

**trusted certificate authority**is an entity that has been entitled to verify that someone is effectively who it declares to be. In order for this model to work, all the participants on the game must agree on a set of CA which they trust. All operating systems and most of web browsers ship with a set of trusted CAs.*Source:*dnsimple.com - 4.
# What is Symmetric Encryption?

# Answer:

**Symmetric Encryption**is the simplest kind of encryption that involves only one secret key to cipher and decipher information. Symmetric encryption is an old and best-known technique. It uses a secret key that can either be a number, a word or a string of random letters. It is a blended with the plain text of a message to change the content in a particular way.**The sender and the recipient should know the secret key**that is used to encrypt and decrypt all the messages. Blowfish, AES, RC4, DES, RC5, and RC6 are examples of symmetric encryption. The most widely used symmetric algorithm is AES-128, AES-192, and AES-256.The main disadvantage of the symmetric key encryption is that all parties involved have to exchange the key used to encrypt the data before they can decrypt it.

*Source:*www.ssl2buy.com - 5.
# What are Confusion and Diffusion in Cryptography?

# Answer:

**Confusion**means that each binary digit (bit) of the ciphertext should depend on several parts of the key, obscuring the connections between the two. The property of confusion hides the relationship between the ciphertext and the key. This property makes it difficult to find the key from the ciphertext and if a single bit in a key is changed, the calculation of the values of most or all of the bits in the ciphertext will be affected. Confusion increases the ambiguity of ciphertext and it is used by both block and stream ciphers.**Diffusion**means that if we change a single bit of the plaintext, then (statistically) half of the bits in the ciphertext should change, and similarly, if we change one bit of the ciphertext, then approximately one half of the plaintext bits should change.2 Since a bit can have only two states, when they are all re-evaluated and changed from one seemingly random position to another, half of the bits will have changed state. The idea of diffusion is to hide the relationship between the ciphertext and the plain text. This will make it hard for an attacker who tries to find out the plain text and it increases the redundancy of plain text by spreading it across the rows and columns; it is achieved through transposition of algorithm and it is used by block ciphers only.

These two properties help to ensure that extracting the key from plaintext-ciphertext pairs is difficult or infeasible.

*Source:*en.wikipedia.org - 6.
# Explain difference between Hashing and Encryption algorithms

# Answer:

**Hash Functions**provide a mapping between an arbitrary length input, and a (usually) fixed length (or smaller length) output. A hash function could be considered the same as baking a loaf of bread. You start out with inputs (flour, water, yeast, etc...) and after applying the hash function (mixing + baking), you end up with an output: a loaf of bread.Going the other way is extraordinarily difficult - you can't really separate the bread back into flour, water, yeast - some of that was lost during the baking process, and you can never tell exactly how much water or flour or yeast was used for a particular loaf, because that information was destroyed by the hashing function (aka the oven).

Many different variants of inputs will theoretically produce identical loaves (e.g. 2 cups of water and 1 tsbp of yeast produce exactly the same loaf as 2.1 cups of water and 0.9tsbp of yeast), but given one of those loaves, you can't tell exactly what combo of inputs produced it.

**Encryption Functions**provide a 1:1 mapping between an arbitrary length input and output. And they are always reversible. The important thing to note is that it's reversible using some method. And it's always 1:1 for a given key. Encryption could be viewed as a safe deposit box. Whatever you put in there comes back out, as long as you possess the key with which it was locked up in the first place. It's a symmetric operation. Given a key and some input, you get a certain output. Given that output, and the same key, you'll get back the original input. It's a 1:1 mapping.

*Source:*stackoverflow.com