@Elias Pohlig-Hellman is symmetric. Knowledge of e and d is equivalent, since p is public. One way to look at it is that there is an encryption key e and a decryption key d. Another way is that the key is e, used directly for encryption, and first turned into d for decryption.
Hi guys, I was wondering, in symmetric-key cryptography algorithms, like DES or AES, if we increased the number of rounds (16 in DES, 10 in AES) if it noticeably increased the security of the message? (without factoring in the slower execution time)
@hitter In general that is true, though it is possible to construct example where it not (i.e. a cipher that uses only xor/rotate is doomed no matter how many rounds are applied). You really have to define what you mean by "security of the message": if the message is sufficiently secure with the default round count, then no, increasing the round count does not increase security in any meaningful way. A lot of cryptography is about finding the sweet spot between efficiency and security.
If the cipher in question was designed by someone knowledgeable, they will have already calculated how much resistance to a given attack is required and have tuned their design accordingly to acquire that resistance
