There are two main types of symmetric key algorithms for encryption. One is the stream cipher, while the other is the block cipher. The stream cipher has the amiability to encrypt small portions and bits of the message at a time while block ciphers on the other hand have the capability to take more than one bit and encrypt them as a block or a gathered collective unit. The bock unit which is mostly used in the block cipher encryption is of 64 bits.
“Informally, for symmetric encryption scheme SE to be secure against chosen plaintext attack, the IND-CPA advantage of an adversary must be small, no matter what strategy the adversary tries. However, we have to be realistic in our expectations, understanding that the advantage may grow as the adversary invests more effort in its attack. Security is a measure of how large the advantage of the adversary might when compared against the adversary’s resources.” (‘Symmetric Encryption’)