Do we need a random number generator to decipher (decrypt)?
encryption, random
Solution
Your confusion is because in this case the random numbers are not being used for randomness. Many cryptographic algorithms such as RC4 are based on XORing the plaintext with a cryptographically secure pseudorandom random number generator (CSPRNG) initialized with the key as a seed. (pseudorandom means the random number generator uses a algorithm to generate values) The data is then decrypted by again using the CSPRNG with the key and XORing it against the ciphertext.
- If so, why wouldn't a standard C language library be sufficient?
No, as almost all rand() implementations use an LCG, which is insecure and not suited for cryptographic purposes.
If we do need random numbers to decipher, can anyone with experience comment of the benefits/disadvantages of having hardware vs. software random numbers? Example: Is the random number portion of a decipher only 1% of the total processing and thus I would not necessarily speed things up?
As is explained, generating values using the CSPRNG takes up virtually all the computing power required for encryption; the only other step is XORing the CSPRNG with the plaintext/ciphertext. However, I don't think you need a hardware accelerated pseudorandom generator as RC4 only requires 7 cycles a byte on a Pentium and most practical cryptographic algorithms less than 100 cycles per byte. If it needs to encrypt faster than hundreds of megabytes a second, then a hardware accelerated pseudorandom generator would accelerate the encryption significantly.
Problem
I'm by far an expert on encryption and therefor am seeking out software deciphering advice. My example bellow deals with hardware, but my question is to seek software advice on whether a software solution is feasible, doable and reasonable. Background: On our product, we are considering adding encryption on our RFID keys. Our current firmware does not support this and our hardware guy now wants to upgrade the hardware to add a random number generator chip. As mentioned above, I'm no expert on encryption, but I always though that a random number generator was required for the ciphering, but not for the decipher? Additional info: We are using a low power ARM processor (don't know the model at this time). So my questions are as follows: - Do you really need random numbers to decipher data? - If so, why wouldn't a standard C language library be sufficient? And lastly, - If we do need random numbers to decipher, can anyone with experience comment of the benefits/disadvantages of having hardware vs. software random numbers? Example: Is the random number portion of a decipher only 1% of the total processing and thus I would not necessarily speed things up? Thank you in advance!