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Difference between Block Cipher and Stream Cipher

Last Updated : 05 Apr, 2025
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Block Cipher and Stream Cipher are the types of symmetric key cipher. These two block ciphers are used to transform plain text into ciphertext. The difference between a Block cipher and a Stream cipher is that the block cipher transforms the plain text into cipher text by taking the plain text block by block. On the other hand, a stream cipher produces cipher text from plain text by taking one byte of plain text at a time. In this article, we will see the difference between Block Cipher and Stream Cipher in detail.

What is Block Cipher?

A block cipher encrypts data in fixed-size blocks usually 64 or 128 bits at a time. The encryption algorithm processes each block of data separately using the cryptographic key to transform the plaintext into the ciphertext. Block ciphers function on complex mathematical computation and permutation to ensure that the data encrypted is safe. The choice of block size does not directly affect the strength of the encryption scheme.

The strength of the cipher depends upon the key length. However, any size of the block is acceptable. The following aspects can be kept in mind while selecting the size of a block: Avoid very small block sizes, Do not have very large block sizes, and in multiples of 8-bit.

Block-cipher
Block Cipher

Key Features of Block Ciphers

  • Fixed Block Size: The Data is encrypted in a fixed-size block.
  • Complex Operations: In block ciphers, substitution combined with permutation forms the operation to achieve encryption.
  • Modes of Operation: Block ciphers employ several modes such as ECB (Electronic Codebook) and CBC (Cipher Block Chaining) for enhanced security.

Examples: AES (Advanced Encryption Standard), DES (Data Encryption Standard) and Blowfish.

What is Stream Cipher?

A stream cipher encrypts data one bit or one byte at a time rather than in fixed-size blocks in block cipher. It generates a keystream that is combined with the plaintext to the produce ciphertext. Stream ciphers are made for the scenarios where data needs to be encrypted in the continuous stream making them suitable for the real-time applications.

It can be categorized into the synchronous, self-synchronizing and one-time pad types. The Synchronous encryption requires independently generated keystream from both the plaintext and the ciphertext. They have to be in the same state, with the same key, in order to decode the data properly.

Stream Cipher

Key Features of Stream Ciphers

  • Continuous Encryption: The data is encrypted in a stream that runs continuously, a bit or byte at a time
  • Keystream Generation: To create encryption keys, the Stream ciphers use a pseudorandom keystream generator.
  • Efficiency: Stream ciphers are generally more efficient for encrypting data of variable length and in the streaming applications.

Examples: RC4, Salsa20, and ChaCha20.

Difference Between Block Cipher and Stream Cipher

Block CipherStream Cipher
Block Cipher Converts the plain text into cipher text by taking plain text's block at a time.Stream Cipher Converts the plain text into cipher text by taking 1 bit plain text at a time.
Block cipher uses either 64 bits or more than 64 bits.While stream cipher uses 8 bits.
The complexity of block cipher is simple.While stream cipher is more complex.
Block cipher uses confusion as well as diffusion.While stream cipher uses only confusion.
In block cipher, reverse encrypted text is hard.While in-stream cipher, reverse encrypted text is easy.
The algorithm modes which are used in block cipher are ECB (Electronic Code Book) and CBC (Cipher Block Chaining).The algorithm modes which are used in stream cipher are CFB (Cipher Feedback) and OFB (Output Feedback).
Block cipher works on transposition techniques like rail-fence technique, columnar transposition technique, etc.While stream cipher works on substitution techniques like Caesar cipher, polygram substitution cipher, etc.
Block cipher is slow as compared to a stream cipher.While stream cipher is fast in comparison to block cipher.
Suitable for applications that require strong encryption, such as file storage and internet communications.Suitable for applications that require strong encryption, such as file storage and internet communications.
More secure than stream ciphers when the same key is used multiple times.Less secure than block ciphers when the same key is used multiple times.
Key length is typically 128 or 256 bits.Key length is typically 128 or 256 bits.
Operates on fixed-length blocks of data.Encrypts data one bit at a time.

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