Block ciphers do not use public cryptography (private and public keys). Block ciphers is a type of symmetric-key encryption algorithm that transforms a fixed-size block of plaintext (unencrypted text) data into a block of ciphertext (encrypted text) data of the same length. They are appropriate for software implementations and can operate internally as a stream. See more info below about DES in Output Feedback Mode (OFB), which makes use internally of a stream cipher.
The output feedback (OFB) mode makes a block cipher into a synchronous stream cipher. It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext. Just as with other stream ciphers, flipping a bit in the ciphertext produces a flipped bit in the plaintext at the same location. This property allows many error correcting codes to function normally even when applied before encryption.
Reference(s) used for this question: Wikipedia on Block Cipher mode at: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation and http://www.itl.nist.gov/fipspubs/fip81.htm