Anyone who has been given the sender's public key can in return send him an encrypted message. The advantage to this type of encryption lies in the fact that each user has only one key to encrypt and decrypt messages that they send or receive. The length of a key is determined by the number of bits of data. If a key is long and contains more bits, then it is considered more difficult to break. Stream ciphers encrypt one bit of text at a time.
Block cyphers can encrypt numerous bits of data at one time.
A block consists of 64 bits. It was designed in the 1970's and is still popular today. It is a block cipher containing a 64 bit block and has a 56 bit key. This would provide approximately 72 quadrillion different possible keys . There has been recent criticism that DES is becoming less secure.