Difference Between Block Cipher and Stream Cipher

Block Cipher and Stream Cipher are the methods used for converting the plain text into cipher text directly and belong to the family of symmetric key ciphers.

The major difference between a block cipher and a stream cipher is that the block cipher encrypts and decrypts a block of the text at a time. On the other hand, stream cipher encrypts and decrypts the text by taking the one byte of the text at a time.

Content: Block Cipher Vs Stream Cipher

1. Comparison Chart
2. Definition
3. Key Differences
4. Conclusion

Comparison Chart

Basis for comparison	Block cipher	Stream cipher
Basic	Converts the plain text by taking its block at a time.	Converts the text by taking one byte of the plain text at a time.
Complexity	Simple design	Complex comparatively
No of bits used	64 Bits or more	8 Bits
Confusion and Diffusion	Uses both confusion and diffusion	Relies on confusion only
Algorithm modes used	ECB (Electronic Code Book) CBC (Cipher Block Chaining)	CFB (Cipher Feedback) OFB (Output Feedback)
Reversibility	Reversing encrypted text is hard.	It uses XOR for the encryption which can be easily reversed to the plain text.
Implementation	Feistel Cipher	Vernam Cipher

Definition of Block Cipher

Block Cipher takes a message and break it into a fixed size of blocks and converts one block of the message at an instant. For example, we have a message in plain text “STREET_BY_STREET” required to be encrypted. Using bock cipher, “STREET” must be encrypted at first, followed by “_BY_” and finally at last “STREET”.

In actual practice, communication takes place only in bits. Therefore, STREET actually means the binary equivalent of ASCII character of STREET. Subsequently, any algorithm encrypts these; the resultant bits are transformed back into their ASCII equivalent.

An evident problem regarding the usage of Block ciphers is repeating text, for which the same cipher is generated. Hence, it would give a hint to the cryptanalyst which makes it is easier to figure out the recurring strings of plain text. As a result, it can reveal the whole message.

To overcome from this problem the chaining mode is used. In this technique, the preceding block of cipher text is mixed with the current block, so as to the cipher text vague, this avoids the recurring patterns of blocks with the same content.

Definition of Stream Cipher

Stream Cipher typically encrypts one byte of the message at that moment instead of using blocks. Let’s take an example, suppose the original message (plain text) is “blue sky” in ASCII (i.e. text format). When you convert these ASCII into equivalent binary values, it will give the output in 0’s and 1’s form. Let it be translated in 010111001.

For encryption and decryption, a pseudorandom bit generator is used in which a key and plain text are loaded. A pseudorandom bit generator creates a stream of 8-bit numbers that are seemingly random known as keystream. Let the input key is 100101011. Now the key and plaintext are XORed.

The XOR logic is simple to understand. XOR produces an output of 1 when one input is 0, and the other is 1. The output is 0 if either both the inputs are 0 or both the inputs are 1.

• Confusion is a method which guarantees that a cipher text gives no clue about the original plain text.
• Diffusion is a strategy used to enhance the redundancy of the plain text by spreading it across rows and columns.

Conclusion

Block Cipher and Stream Cipher differ in the way in which plain text is encrypted and decrypted. The idea behind block cipher is to divide the plain text into blocks further encrypt those blocks. While stream cipher converts plain text bit by bit similar to stream.