void RC4_set_key(RC4_KEY *key, int len, const unsigned char *data);
void RC4(RC4_KEY *key, unsigned long len, const unsigned char *indata, unsigned char *outdata);
RC4 is a stream cipher with variable key length. Typically, 128 bit (16 byte) keys are used for strong encryption, but shorter insecure key sizes have been widely used due to export restrictions.
RC4 consists of a key setup phase and the actual encryption or decryption phase.
RC4_set_key() sets up the RC4_KEY key using the len bytes long key at data.
RC4() encrypts or decrypts the len bytes of data at indata using key and places the result at outdata. Repeated RC4() calls with the same key yield a continuous key stream.
Since RC4 is a stream cipher (the input is XORed with a pseudo-random key stream to produce the output), decryption uses the same function calls as encryption.
Applications should use the higher level functions EVP_EncryptInit(3) etc. instead of calling the RC4 functions directly.