Cryptography is the science of using mathematics to encrypt and decrypt data. Cryptography enables you to store sensitive information or transmit it across insecure networks (like the internet) so that it cannot be read by anyone except the intended recipient.
In Cryptography, we change our normal text into somewhat complex text using some techniques. A normal person who is not aware of these cryptography techniques is able to read it but he/she cannot find the meaning behind it. The person who knows these cryptography techniques can change the text to the original one and can use it.
In Cryptography, Normal text is called Plain text.
In Cryptography, Converted text is called Cipher text.
In Cryptography, Changing normal text to some complex form is called as Encryption.
In Cryptography, Changing encrypted text into the plain text again is called Decryption.
The techniques which we use for encryption and decryption are called as Alogorithems.
Here the girl converts the plain text into cipher text using a key which is “C”. The boy must have this key which he can use to convert the cipher text into the plain text.
The point here is that if you want to read the message then you must possess the key that is used for encryption.