Problem - Kevin Tafuro

Solution
You can’t be very confident about the security of RC4 for general-purpose use, owing
to theoretical weaknesses. However, if you’re willing to use only a very few RC4 outputs
(a limit of about 100,000 bytes of output), you can take a risk, as long as you
properly set it up.
Before using the standard initialization functions provided by your cryptographic
library, take one of the following two steps:
• Cryptographically hash the key material before using it.
• Discard the first 256 bytes of the generated keystream.
After initialization, RC4 is used just as any block cipher in a streaming mode is used.
Most libraries implement RC4, but it is so simple that we provide an implementation
in the following section.
Discussion
RC4 is a simple cipher that is really easy to use once you have it set up securely,
which is actually difficult to do! Due to this key-setup problem, RC4’s theoretical
weaknesses, and the availability of faster solutions that look more secure, we recommend
you just not use RC4. If you’re looking for a very fast solution, we recommend
SNOW 2.0.
In this recipe, we’ll start off ignoring the RC4 key-setup problem. We’ll show you
how to use RC4 properly, giving a complete implementation. Then, after all that,
we’ll discuss how to set it up securely.
As with any other symmetric encryption algorithm, it is particularly
important to use a MAC along with RC4 to ensure data integrity. We
discuss MACs extensively in Chapter 6.
RC4 requires a little bit of state, including a 256-byte buffer and two 8-bit counters.
Here’s a declaration for an RC4_CTX data type:
typedef struct {
unsigned char sbox[256];
unsigned char i, j;
} RC4_CTX;
In OpenSSL, the same sort of context is named RC4_KEY, which is a bit of a misnomer.
Throughout this recipe, we will use RC4_CTX, but our implementation is otherwise
compatible with OpenSSL’s (our functions have the same names and
parameters). You’ll only need to include the correct header file, and alias RC4_CTX to
RC4_KEY.
234 | Chapter 5: Symmetric Encryption
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