In this method, encryption is done at the character level. There are two general methods for character-level encryption: substitution and transpositional.
The simplest form of character-level encryption is substitution ciphering. In monoalphabetic substitution, sometimes called the Caesar Cipher each character is replaced by another character in the set. The monoalphabetic encryption algorithm simply adds a number to the ASCII code of the character; the decryption algorithm simply subtracts the same number from the ASCII co. Ke and Kd are the same and define the added or subtracted value. To make it simple, we do not encode the space character. If the substituted character is beyond the last character (Z), we wrap it around.
Monoalphabetic substitution is very simple, but the code can be broken easily by snoopers. The reason is that the method cannot hide the natural frequencies: characters in the language being used. For example, in English, the most frequently used characters are E, T, O, and A. A snooper can easily break the code by finding which character is used the most and replace that one with the letter E. It can then find the next most frequent and replace it with T, and so on.
In polyalphabetic substitution, each occurrence of a character can have a different substitute. One polyalphabetic encryption technique is to find the position of the character in the text and use that value as the key. However, polyalphabetic substitution is not very secure either. The reason is that although "DEAR DEAR" is replaced by "EGDV JLIA", the order of characters in "EGDV" and "JLIA" is still the same; the code can easily be broken by a more experienced snooper.
An even more secure method is transpositional encryption, in which the original characters remain the same but the positions of these characters are interchanged to create the cipher text. The text is organized into a two-dimensional table, and the columns are interchanged according to a key. For example, we can organize the plaintext into an eleven-column table and then reorganize the columns according to a key that indicates the interchange rule. As you have guessed, transpositional encryption is not very secure either. The character frequencies are preserved and the snooper can find the plaintext through trial and error.
In Bit-level encryption techniques, data as text, graphics, audio, or video are divided into different blocks of bits and then each block is altered using either of the techniques: encoding/decoding, permutation, substitution, etc.