hm...

@fgrieu regarding crypto.stackexchange.com/questions/52525/… I think your answer obscures the main point: It was never used with secret keys only.

@Elias Might be me, but isn’t that what his answer's first line – “RSA never was extensively used as a symmetric/secret key cryptosystem.” – states?

@e-sushi yes, but it should end there

@Elias I see your point; I have reformatted and polished my answer to make the main point immediately apparent.

@fgrieu I'm still not sure what you are even trying to say with the rest of your answer. ;)

Well, the last paragraph is fine. The middle one though.

@fgrieu That Pohlig-Hellman system sounds weird. They have e and d but use only e for encryption and decryption?

@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.

@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

@Elias pohlig hellman allows for a nice, secure three-pass protocol

@EllaRose I like pornu's idea of "there's no point in comparing 'unbreakable' with even 'more unbreakable'" for this sort of discussion

that pretty much sums it up exactly