R – .Net Simple RSA encryption

encryptionintnetrsacryptoserviceprovider

I'm trying to encrypt something simple, like int or long. Simplest way I found looks like:

int num = 2;
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
byte[] numBytes = BitConverter.GetBytes(num);
byte[] encryptedBytes = rsa.Encrypt(numBytes, true);

Problem is, the encryptedBytes is 128 bytes long. How can I encrypt data (which I could still later decrypt) where the encrypted result is the same length in bytes as the input?

I know I can shorten the result if I define RSACryptoServiceProvider(512) or RSACryptoServiceProvider(384), but that's as low as it goes.

I need 4 or 8 bytes going in, and 4 or 8 bytes coming out (respectively)

Thanks!

*** Clarification:

I want to encrypt something small (ie 4 or 8 bytes) and obtain a result of a similar size. What's the simplest way to do it in C# while still using some key (with the built in libraries) rather than some mod and shift operations

Best Answer

You could use Blowfish, it has a block size of 64 bits / 8 bytes. http://www.hotpixel.net/software.html#blowfishnet

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.net – Difference between decimal, float and double in .NET

float and double are floating binary point types. In other words, they represent a number like this:

10001.10010110011

The binary number and the location of the binary point are both encoded within the value.

decimal is a floating decimal point type. In other words, they represent a number like this:

12345.65789

Again, the number and the location of the decimal point are both encoded within the value – that's what makes decimal still a floating point type instead of a fixed point type.

The important thing to note is that humans are used to representing non-integers in a decimal form, and expect exact results in decimal representations; not all decimal numbers are exactly representable in binary floating point – 0.1, for example – so if you use a binary floating point value you'll actually get an approximation to 0.1. You'll still get approximations when using a floating decimal point as well – the result of dividing 1 by 3 can't be exactly represented, for example.

As for what to use when:

For values which are "naturally exact decimals" it's good to use decimal. This is usually suitable for any concepts invented by humans: financial values are the most obvious example, but there are others too. Consider the score given to divers or ice skaters, for example.
For values which are more artefacts of nature which can't really be measured exactly anyway, float/double are more appropriate. For example, scientific data would usually be represented in this form. Here, the original values won't be "decimally accurate" to start with, so it's not important for the expected results to maintain the "decimal accuracy". Floating binary point types are much faster to work with than decimals.

.net – Encrypted data size using RSA encryption (RSACryptoServiceProvider)

You are mistaken. RSA is not a block cipher, so you cannot really talk about the block size of it.

The output of a RSA encryption will have the same length as the RSA modulus. You have not indicated any RSA key in your code, so the runtime will (as far as I recall) use a default key. That key apparently has a 1024 bit modulus, which explains the output length.

You might want to look into AES encryption instead. For many reasons you should normally only use RSA to encrypt a key and then use AES or a similar symmetric cipher algorithm to encrypt your actual text.

AES is a block cipher with block size 16 bytes, so that will (depending on which padding you use and how you transport your initialization vector) encrypt 16 bytes of plain data to 16 bytes of encrypted data.

Best Answer

Related Solutions

.net – Difference between decimal, float and double in .NET

.net – Encrypted data size using RSA encryption (RSACryptoServiceProvider)

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