Conventional encryption are cryptographic system which uses same key used by sender to encrypt message and by receiver to decrypt message. It was only type of encryption in use prior to development of publickey encryption.
It is still much preferred of the two types of encryption systems due to its simplicity. It is a relatively fast process since it uses a single key for both encryption and decryption In this encryption model, the sender encrypts plaintext using receiver’s secret key, which can be later used by receiver to decrypt ciphertext. Below is figure that illustrate this concept.
Suppose A wants to send a message to B, that message is called plaintext. Now, to avoid hackers to read plaintext, plaintext is encrypted using algorithm and a secret key (at 1). This encrypted plaintext is called ciphertext. Using same secret key and encryption algorithm run in reverse(at 2), B can get plaintext of A and thus message is read and security is maintained.
The idea that uses in this technique is very old and that’s why this model is called conventional encryption.
A conventional encryption has mainly 5 ingredients :

Plain text –
It is the original data that is given to the algorithm as a input 
Encryption algorithm –
This encryption algorithm performs various transformations on plain text to convert it into cipher text. 
Secret key –
Secret key is also an input to the algorithm. The encryption algorithm will produce different output based on the keys used at that time. 
Cipher text –
It contains encrypted information because it contains a form of original plaintext that is unreadable by a human or computer without proper cipher to decrypt it. It is output from algorithm. 
Decryption algorithm –
This is used to run encryption algorithm in reverse. Cipher text and Secret key is input here and it produces plain text as output.
Requirements for secure use of conventional encryption :
 We need a strong encryption algorithm.
 Sender and Receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure.
Advantages of Conventional Encryption :

Simple –
This type of encryption is easy to carry out. 
Uses less computer resources –
Conventional encryption does not require a lot of computer resources when compared to public key encryption. 
Fast –
Conventional encryption is much faster than asymmetric key encryption.  Origin and authenticity of message cannot be guaranteed, since both sender and receiver use the same key, messages cannot be verified to have come from a particular user.
 It isn’t much secured when compared to public key encryption.
 If the receiver lost the key, he/she cant decrypt the message and thus making the whole process useless.
 This scheme does not scale well to a large number of users because both the sender and the receiver have to agree on a secret key before transmission.
Disadvantages of conventional encryption :
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