Same Riddle Different Answer: I am both lock and key - who am I?

NOTE:
Though the body is identical to I am both lock and key - who am I?, a different answer is expected, and thus this is not a duplicate.
Cyrnfr qb abg pybfr guvf dhrfgvba.
Thanks!

Enter! Small or large, as the case may be

I'll assist you - it's easy! - I'm both lock and key

Who am I?

HINTS:

1

Gur nafjre vf bs yratgu svir

2

Weet bksa, co vhyudt, mu qhu qbb cqhhyut, unsufj oek qdt jxqj tkbb rhejxuh mxe xqhtbo sxqdwui qj qbb!

• Can anyone please tell me why is this downvoted? It is my original idea of "Same riddle Different Answer" Question. Thanks! Dec 23, 2018 at 11:27
• I upvoted to counter the downvote ;) Apr 2, 2019 at 0:47
• thanks @user477343 Apr 2, 2019 at 1:11

3 Answers

All in all, @Shahriar and @rhsquared, had it all. I just joined their answer and added an explanation for hints.

I think the answer is

ROT13

Enter! Small or large, as the case may be

ROT13 can work on both "small" and "large" case

I'll assist you - it's easy! - I'm both lock and key

ROT13 is both lock and key in the sense that a text "encrypted" with ROT13 can be decrypted by applying a new ROT13 operation.

Hint 1

ROT13 has 5 characters

Hint 2

"Good luck, my friend, we are all married, except you and that dull brother who hardly changes at all!" All ROTxx is "maried" with ROT[26-xx] except for ROT13 married with himself, and ROT0, who doesn't change the text at all.

• Well done! Wanna upvote but couldn't, reached daily upvote limit... Will do so tmr! Dec 24, 2018 at 10:30
• sorry for this super late +1, here you go ;) Feb 17, 2019 at 15:27

The answer is-

ROT13

Because-

It's both the encryption method and decryption method for hiding plaintext.

• You havent explained the first line and the second line of the body. Keep it up! Dec 23, 2018 at 9:55

Considering that you used:

both Rot13 and Rot10 in you puzzle and hints I would say the answer is Rot16 in order to match the requirement for 5 letter length.

Enter! Small or large, as the case may be

The Rotxx encryption works on both caps (large) and small letters.

I'll assist you - it's easy! - I'm both lock and key

With the ROT encryption/decryption you can both encode and decode the text. We only need to keep in mind that only Rot13 is 'symetrical'. In all the rest the lock/key pair the sum of the Rotxx + Rotyy is 26. Considering that Hint 2 can be decoded using Rot10, I think the answer is its missing counterpart Rot16.

• on the right track, but who in the lot am i specifically asking for? by the way, you already have all info needed. keep it up! Dec 23, 2018 at 23:40
• edited hint 2 pls check Dec 24, 2018 at 7:04