What is the difference between RC4 and RSA?

What type of cipher is RC4?

I thought it was a stream cipher, but I see a lot of things that talk about CTR, XOR, etc.

Are there any benefits to using RC4 over those?

RC4 is probably a symmetric key cryptosystem and uses 128bit keys as a basis (it is usually paired with AES). It has good performance in terms of processing time and key generation which makes it an ideal choice for encryption, especially for web applications because its very fast.

There are a lot of people recommending the use of RC4 for the sole purpose of not worrying about key management or other security concerns (this is the same reason why you are now hearing a lot of discussion about using ECB over CBC. It is not entirely clear what the best practice is on RC4 usage in the web environment. It may be a good idea to implement encryption/decryption of your message before passing to the client so the data can be validated, however many sites implement encryption/decryption on the server side without giving any benefit. It's generally accepted that you don't need to worry about a message traveling over insecure networks or storing a secret decryption key in a script on the server.

In some cases you'll also use RSA-OAEP or DH-based encryption to provide additional confidentiality of messages. "How should I go about choosing between CBC and RC4 for my online applications?". This isn't just a matter of picking between a library or using one over the other, rather, it depends on the application. The most important consideration is that when choosing an encryption algorithm, you must consider how secure you want the communication to be. If the application is secure and only trusted sites, or if the sites are in a controlled environment, then you can use a single-key encryption, and the implementation will be fairly straightforward.

If you're transferring data across public networks, or between sites that aren't in a controlled environment, or if you want your site to be more robust, then you will want to add an additional key. Generally, you want your key to be strong, secure, and slow to encrypt. It's very common to generate a cryptographic key of 80 bits or 128 bits with a random number generator. It should be slow to generate because this will make it much more time consuming for an attacker to try to guess your key.

What is the difference between RC4 and RSA?

We're told that RC4 is considered to be a good-enough block cipher; and, the RSA algorithm is the only practical public-key cryptosystem.

These claims may be true, but it's important to examine them carefully.

Consider the RC4 algorithm, which is the subject of this article: It is a stream cipher with a small key size (it has 40 bytes or less). Let us assume that a message that is encoded with RC4 is of the form shown in Figure 1.

Figure 1. A message that is encoded with the RC4 algorithm Let the letters in bold be represented as digits. Assume that we have four people who encode their messages using RC4: Alice, Bob, Charlie, and Eve. The first challenge for RC4 is that the algorithm can be broken with probability greater than 1/2 with the use of a few hundred thousand computational resources. There are many techniques for doing so. The most basic is known as brute force search. When brute force search is combined with reduced dictionaries, the method is known as time/space tradeoff. And, when reduced dictionaries are combined with memory bounds, the method is known as bounded storage. If we want to know if RC4 can be broken easily with reduced dictionaries, we may refer to Wikipedia.

RSA, which is the subject of this article, was developed in the early 1990s and was quickly perceived as being the only practical public-key cryptosystem. So, is RSA really the only practical public-key cryptosystem? Well, let's examine it closely.

The RSA algorithm is a public-key algorithm. To understand the concept of a public-key algorithm, we first need to understand the concept of a private-key algorithm.

Figure 2. A private-key algorithm consists of two main components: A method for calculating an arbitrary function (such as multiplication) on a set of numbers; and, The private key consists of a value that cannot be deduced from the output of the function.

How does RC4 compare to AES?

RC4 is a stream cipher, designed for speed.

It's fast and has been used extensively in the past, including by SSL. I'm not familiar with AES, so it's difficult to compare their speeds, but I'll assume they're similarly-speedy.

For the security, I'd prefer to use SHA-1 or SHA-256 over DES, so RC4 will have an easier time cracking it. But if you don't care about the security, AES is probably better.

RC4 is a stream cipher, while AES is a block cipher. As such, they do different things. I think the first question to ask is: what are you trying to achieve? If you're just trying to encrypt a bunch of data, then AES is probably a better choice. If you want to encrypt data in real-time on your server, you're going to want to look at something faster than a block cipher.

In either case, however, you'll need a block cipher with a large key size, and a large IV size. Both RC4 and AES are quite well-suited to these requirements.

RC4 is a stream cipher, it operates on a byte stream (in practice a 16 byte buffer), and works by chaining a number of blocks together to produce a longer sequence. This sequence is encrypted using the same key as the initial blocks. The output is typically much shorter than the input, and is easily compressed using standard techniques.

It does this by maintaining a running state of various blocks of data, which are then combined to produce a new value. In its most basic form, a single block cipher can be used to perform the combining step, but if you have some input data of variable length you may be able to get better performance using two (or more) running state blocks.

Since RC4 operates in a streaming manner, it is not useful for bulk encryption, as a single stream can be attacked. It is mainly used for encrypting large amounts of data, because the key stream can be broken down into multiple smaller streams that are then combined to form the full key stream.