Block Cipher

Introduction: – In this tutorial you will learn about another kind of encryption technique that applies deterministic algorithm along with symmetric key for encryption purpose of plain text block. So let’s dig deep into it.


What is Block cipher: – Block cipher is an encryption technique which inputs a block of bits of plain text &
processes to create block of bits of cipher text. Block size remain the same (fixed) for any specific scheme and the block size has no adverse effect on the strength of encryption. Rather the encryption’s strength and quality depends on the key length.

block ciphers

Block Size: – 
It is possible to incorporate any block size in encryption, but here are some of the points that one should keep in mind while selecting block size –

  • Try to avoid very small size of blocks. This means if you choose a size of block having n bits, then possible bit combination in plain text becomes 2 n . An encryption having large size of blocks makes dictionary attacks harder to penetrate.
  • An extremely large block size is also not a good idea to keep. This is because your cipher / or encryption mechanism will become unproductive to operate.
  • It is advisable to keep the block size a multiple of 8. This is because, it becomes easy to implement since most of the systems’ processor handles information in 8 bits’ multiple (16, 32, 64, 86 bits) so as the processors.


Padding: –
The block ciphers takes blocks having fixed sizes (let suppose 64-bits). But the plain text length will not always be a multiple of the block size you have taken. Hence, there is a process to add bits in the last block to make it a multiple of block size which is called as padding.

Various Block Ciphers: –
There are a lot of popular block ciphers that are used on various applications and systems. Many of their algorithms are known to public. The most prominent of them are listed below –

  1. DES
  2. 3DES or Triple DES
  3. Twofish: It is a 128 bits block cipher having a variable that holds the key length. It came from the updating of Blowfish algorithm which was having a 64 bits block size.
  4. AES (Will will learn about it in a separate chapter)
  5. IDEA: It is a very strong cipher also having size of block 64 and a size of key to 128 bits. The popular PGP (Pretty Good Privacy) protocol used for secured mailing and in other applications uses this encryption mechanism earlier. Since it has been patented by some organisation so, it has limited usage in applications.
  6. Serpent: is another block cipher with size of block – 128 bits but a varied key lengths of 128, 192 or 256 bits. It is slower as compared to others but one of the most secured encryption algorithm designed.

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