# Difference Between Block Cipher And Stream Cipher Pdf

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By Swati Tawde. In this article, Stream Cipher vs Block Cipher, both stream Cipher and block cipher are techniques used for encryption and decryption, i.

Understanding the difference between a block cipher vs stream cipher is kind of like the difference between watching a movie on DVD or via a streaming service. Sure, both will give you the entertainment you seek, but they each work differently in terms of mechanics and speed. But what is the difference between a stream cipher vs block cipher?

## Block Cipher vs Stream Cipher: What They Are & How They Work

Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time.

A block cipher encrypts one block at a time. The block may be of size one byte or more or less. That means we can also encrypt a block of one byte by help of a stream cipher as a stream. This means, when you encrypt the same plaintext block with the same key, you'll get the same result. We normally also want that the function is invertible, i. To actually encrypt or decrypt a message of any size , you don't use the block cipher directly, but put it into a mode of operation.

The simplest such mode would be electronic code book mode ECB , which simply cuts the message in blocks, applies the cipher to each block and outputs the resulting blocks. This is generally not a secure mode, though. Some early encryption schemes like the one used by Caesar could be categorized as a "block cipher with 1-character blocks in ECB-mode".

Or generally, everything that has a code book. We usually use other modes of operation, which include an initialization vector and some kind of feedback, so that every block of every message is encrypted a different way. Often the message sizes might be limited to multiples of some "block size", too, but usually with smaller blocks like whole bytes or such.

If a part of the plaintext repeats, the corresponding ciphertext usually is not the same — different parts of the message will be encrypted in different ways. Often such stream ciphers work by producing a keystream from the actual key and maybe an initialization vector and then simply XOR-ing it with the message — these are called synchronous stream ciphers.

Other stream ciphers might vary the encryption of future parts of the message depending on previous parts. You should never reuse a key and IV, if applicable of a synchronous stream cipher which includes block ciphers in streaming modes for different messages, since this can lead to compromises.

And even for the same message it will show that you repeated a message. Note that in actual usage you will also want a MAC, e.

Some schemes are broken in case of a chosen-ciphertext attack, for example, and such a MAC will prevent this if you only pass the message to the decryptor after checking the MAC. In practice, we usually also require an efficient way to compute the inverse permutation. A block cipher on its own is not very useful for practical cryptography, at least unless you just happen to need to encrypt small messages that each fit into a single block.

However, it turns out that block ciphers are extremely versatile building blocks for constructing other cryptographic tools: once you have a good block cipher, you can easily build anything from stream ciphers to hash functions, message authentication codes, key derivation functions, pseudorandom number generators, entropy pools, etc.

Not all of these applications necessarily need a block cipher; for example, many of them could be based on any pseudorandom function which need not be a permutation but, conveniently, there's a lemma that says a pseudorandom permutation will, nonetheless, work.

Also, many of the constructions are indirect; for example, you can construct a key derivation function from a message authentication code, which you can construct from a hash function, which you can — but don't have to — construct from a block cipher. But still, if you have a block cipher, you can build all the rest out of it.

Furthermore, these constructions typically come with conditional security proofs that reduce the security of the constructed functions to that of the underlying block cipher. Thus, you don't need to carry out the laborious and unreliable task of cryptanalyzing each of these functions separately — instead, you're free to concentrate all your efforts on the block cipher, knowing that any confidence you'll have on the security of the block cipher directly translates into confidence on all the functions based on it.

Obviously, all this is very convenient if you're, say, working on a small embedded platform where including efficient and secure code for lots of separate crypto primitives could be difficult and expensive.

But even if you're not on such a constrained platform, writing and analyzing low-level crypto code can be laborious due to the need to pay attention to things like side-channel attacks.

It's easier to restrict yourself to a limited number of low-level building blocks and to build everything you need out of those. Also, even on fast platforms with lots of memory, like desktop CPUs, implementing low-level crypto operations directly in hardware can be much faster than doing them in software — but it's not practical to do that for more than a few of them. Due to their versatility, block ciphers are excellent candidates for hardware implementation as in the AES instruction set for modern x86 CPUs.

There are some subtleties here; for example, most stream cipher constructions require the input to include a unique nonce value, and do not guarantee security — in the sense of indistinguishability from a truly random function — if the same nonce is used for two different inputs. Practically, we usually also require that stream ciphers, in fact, be "streaming", in the sense that arbitrarily long input bitstreams can be encrypted — and decrypted — using only constant storage and time linear in the message length.

Of course, stream ciphers are much more immediately useful than block ciphers: you can use them directly to encrypt messages of any length. However, it turns out that they're also much less useful as building blocks for other cryptographic tools: if you have a block cipher, you can easily turn it into a stream cipher , whereas turning an arbitrary stream cipher into a block cipher is difficult if not impossible.

So why do people bother designing dedicated stream ciphers at all, then, if block ciphers can do the job just as well? Mostly, the reason is speed: sometimes, you need a fast cipher to encrypt lots of data, and there are some really fast dedicated stream cipher designs out there.

However, what you gain in speed and compactness, you lose in versatility. For example, there doesn't seem to be any simple way to make a hash function out of a stream cipher , so if you need one of those and you often do, because hash functions, besides being useful on their own, are also common building blocks for other crypto tools , you'll have to implement them separately.

And, guess what, most hash functions are based on block ciphers, so if you have one, you might as well reuse the same block cipher for encryption too unless you really need the raw speed of the dedicated stream cipher. A block cipher by itself does map n bits to n bits using a key. It cannot accept longer or shorter texts. To actually encrypt a message you always need a chaining mode. ECB is one such chaining mode and a really bad one , and it's not the pure block cipher.

Even ECB consists of "add-on processing operations". These chaining modes can have quite different properties. One of the most popular chaining modes, Counter mode CTR constructs a synchronous stream cipher from a block cipher. Another mode, CFB constructs a self synchronizing stream cipher, with properties somewhere between those of CBC and a synchronous stream cipher.

So your assumption that there are no ciphers between stream and blockciphers isn't really true. Cryptographers just prefer building them from the well understood block cipher primitive, instead of creating a completely new system. It uses a 26 symbol encoding instead of a 2 symbol encoding, but that doesn't mean it's not a stream cipher.

Whereas Block Cipher takes a block of Data as input, run multiple rounds on it alongwith key mixing and produce Cipher Text. A sequential number is input to the block cipher, and its output is Xored with Plaintext to make Ciphertext. In this mode of operation only Encryption code of the block cipher is required. There is no need of decryption code, for decryption we simply input the same sequential number to block cipher, and Xored its output with Ciphertext to get Plain text.

Sometime a nounce is used along with the Counter, so input the block cipher is split in two, i. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Difference between stream cipher and block cipher Ask Question. Asked 8 years, 4 months ago. Active 2 years, 1 month ago.

Viewed k times. So, what exactly is the difference between a stream cipher and a block cipher? Improve this question. Ravindra Bagale Ravindra Bagale 1 1 gold badge 8 8 silver badges 10 10 bronze badges. They are used in the ways as such because they are convenient in discourse where commonly there are appropriate contexts to help more exact understanding.

Hence there are redundancies. I suppose a good analogy to the issue here is "a rich man" vs. You must always attribute the source of any material that you copy from external sources; see crypto. May 20 '17 at Add a comment. Active Oldest Votes. Improve this answer. Is there a difference in security? Or speed of encryption? What about stream ciphers, then? Ilmari Karonen Ilmari Karonen 42k 3 3 gold badges 93 93 silver badges bronze badges.

According to what you explained, a stream cipher is simply a special case of a block cipher, i. So I would argue for not maintaining the current distinction of terminologies. The only advantage a dedicated stream cipher has over a block cipher in an appropriate mode is performance. You can't disregard chaining modes, since nobody sane uses block ciphers without appropriate chaining.

Different applications have different performance requirements. To encrypt e. Show 1 more comments. A good chaining mode will have these properties, but bad modes still exist! It uses same key for encryption and Decryption. It uses two different keys public and private to encrypt and decrypt. Sign up or log in Sign up using Google.

## Stream cipher

Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time. A block cipher encrypts one block at a time.

## Block Cipher vs Stream Cipher: What They Are & How They Work

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### Stream Cipher vs Block Cipher

Both Block and Stream cipher are the methods of Encryptions which are primarily used for converting the plain text into cipher text directly and belong to the family of symmetric key ciphers. Nitin Sharma. Previous Page Print Page. Next Page.

A stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream keystream. In a stream cipher, each plaintext digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the ciphertext stream. Since encryption of each digit is dependent on the current state of the cipher, it is also known as state cipher. In practice, a digit is typically a bit and the combining operation is an exclusive-or XOR. The pseudorandom keystream is typically generated serially from a random seed value using digital shift registers. The seed value serves as the cryptographic key for decrypting the ciphertext stream.

Prerequisite — Block cipher modes of operation Both Block Cipher and Stream Cipher are belongs to the symmetric key cipher. These two block cipher and stream cipher are the methods used for converting the plain text into cipher text. While stream cipher Converts the plain text into cipher text by taking 1 byte of plain text at a time. Attention reader! Writing code in comment?

Learn the difference between Block Cipher and Stream Cipher. A cipher is a method of hiding words or text with encryption by replacing original letters with other letters, numbers and symbols through either substitution or transposition. A cipher often enables private communication and is often used in email, so that if an encrypted message is intercepted by an unauthorized user, the message cannot be read.

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. Basis for comparison Block cipher Stream cipher Basic Converts the plain text by taking its block at a time.

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## A textbook of fluid mechanics and hydraulic machines bansal pdf

These ciphers can be classified into two groups: stream ciphers and block ciphers.

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