Top 27 Cryptography Interview Questions And Answers

Question 1

What is a _key_?

Accepted 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.

Question 2

Explain what is _Data Encryption_?

Accepted 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_.

Question 3

Explain difference between Hashing and Encryption algorithms

Accepted 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.

Question 4

What are Confusion and Diffusion in Cryptography?

Accepted 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.

![](https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSPBu-5vl6TedvkeSY8CvFyK6jRFLLZ7GBjyA&usqp=CAU)

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

Question 5

What is Symmetric Encryption?

Accepted 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.

![](https://www.ssl2buy.com/wiki/wp-content/uploads/2015/12/Symmetric-Encryption.png)

Question 6

Why is the Root Certificate important?

Accepted 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.

Question 7

What is the difference between encryption, encoding, and hashing?

Accepted Answer

Encryption, encoding and hashing are techniques used for converting the format of data. 
* **Encryption** is used for changing plain text into cipher text so that only authorized entities can understand it. Encryption deals with keys which are used to encrypt and decrypt the data. These keys are used to transform a simple text into a cypher text and the vice versa. 
* **Encoding** is used for changing the data into a special format which makes it usable by external processes. Unlike encryption, the intention of encoding is not related to security. The message is encoded by using an algorithm or scheme. 
* In **hashing**, the data is converted to a message digest or hash, which is usually a number generated from a string of text. Hashing is not reversible as encryption and encoding.  The data is converted to a message digest or hash, which is usually a number generated from a string of text. These digests are important as one can easily match the hash of sent and received messages to ensure that both are the same and no tempering is done with the data.

Question 8

What is PKI?

Accepted Answer

A **public key infrastructure (PKI)** supports the distribution and identification of public encryption keys, enabling users and computers to both securely exchange data over networks such as the Internet and verify the _identity_ of the other party.

Without PKI, sensitive information can still be _encrypted_ (ensuring confidentiality) and exchanged, but there would be no assurance of the _identity_ (authentication) of the other party. Any form of sensitive data exchanged over the Internet is reliant on PKI for security.

Question 9

What is Asymmetric Encryption?

Accepted Answer

**Asymmetric encryption** is also known as _public key cryptography_, which is a relatively new method, compared to symmetric encryption. Asymmetric encryption uses two keys to encrypt a plain text.

* Firstly, a **public key** must be made public in order to encrypt the data. 
* Secondly, a **private key** used to decrypt the data.

The public key and the private key are not the same thing but they are related. You create your message then encrypt it with the recipient’s public key. After that, if the recipient wants to decrypt your message he/she would have to do it with his/her private key.

![](https://www.ssl2buy.com/wiki/wp-content/uploads/2015/12/Asymmetric-Encryption.png)

Asymmetric encryption is mostly used in day-to-day communication channels, especially over the Internet. Popular asymmetric key encryption algorithm includes EIGamal, RSA, DSA, Elliptic curve techniques, PKCS.

Question 10

Provide an example on _Reciprocal cipher_

Accepted Answer

**ROT13 (rotate 13)** is a specific implementation of the *Caesar cipher* where the shift is 13 places. Due to the basic Latin alphabet being 26 characters long this means that the direction of the shift does not matter, the result is the same in either direction. They key (13) can be used without transformation.

![](https://miro.medium.com/max/1540/1*LJndVQFjI3FeT67aeCf5Zw.png)

Top 27 Cryptography Interview Questions

What is a key?

Explain what is Data Encryption?

What are Confusion and Diffusion in Cryptography?

Why is the Root Certificate important?

Explain difference between Hashing and Encryption algorithms

What is Symmetric Encryption?

What is the difference Between Block Cipher and Stream Cipher?

Why not use symmetric encryption?

What are the differences between MD5, SHA and RSA?

Explain the role of Digital Certificates in Asymmetric Encryption process

What are pros and cons of Public Key Cryptography?

What is Asymmetric Encryption?

Provide an example on Reciprocal cipher

Provide an example of non-reciprocal cipher

What is PKI?

Name the elements of PKI

What is the difference between encryption, encoding, and hashing?

Is it possible to decrypt MD5 hashes? Explain.

How does SSL/TLS work ?

How to ensure that a file can only be decrypted after a specific date?

How is it possible that people observing an HTTPS connection being established wouldn't know how to decrypt it?

What does “key with length of x bits” mean?

What would happen had we not invented asymmetric encryption?

Explain why the length of the key does matter?

What is the difference between a Hash Function and a Cryptographic Hash Function?

What is a Salt and How Does It Make Password Hashing More Secure?

Explain types of Resistance any Cryptographic Hash Function shall have?