Random Access vs. Parallelism modes of operation

Question

I am trying to understand some concepts regarding Random Read Access and Parallelism in mode of operations. I have written below what my concepts are regarding the two. If you can please correct where i am wrong, that would be great:

• What is the difference between random read access and parallelism?

  Ans: Random read refers to reading (decrypting) any nth block of data without calculating the previous n-1 blocks while parallelism is reading all or writing (encrypting) multiple blocks at the same time. Is that correct?

• Random read access for decryption in general is good or bad?

  Ans: It just adds convenience for the users at both ends.

• Has random read access got any security implications if its possible in a particular mode of operation. For instance, CBC that allows random read access. Is it easier for an attacker to read any specific block he wants? Or it doesnt matter because the attacker can decrypt all or one block he/she wants.

  I am not sure about this one

Answer 1

Random access (read, or write, or both) is a property of a mode of operation. Parallelism is a form of computation where multiple independent execution units can work together to complete an operation faster. They're somewhat orthogonal concepts, although random access does imply that a mode of operation is trivially parallelizable.

Random access is always a good thing because it allows more flexibility in how the mode of operation can be used (for instance, random access modes are desirable in full-disk encryption because you don't want to be waiting for your entire disk to be decrypted just to read a small text file on it). There are perfectly secure modes of operation available with either non-random-access and random-access. Random-access modes do generally tend to require a little more care with nonce/IV selection than modes without due to the constructions that tend to come up when designing for random access, but there is absolutely no reason to equate random access capability with security.

Random access has nothing to do with security; if a mode of operation that allows random access is insecure, then it is a bad mode of operation and should never be used, regardless of its random access properties. Whether an attacker does or does not leverage CBC's random access properties when decrypting has no impact on CBC's security properties (stated differently: CBC is what it is; the fact that it happens to allow random access during decryption has no bearing on its security properties).