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.
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?
Gur nafjre vf bs yratgu svir
Weet bksa, co vhyudt, mu qhu qbb cqhhyut, unsufj oek qdt jxqj tkbb rhejxuh mxe xqhtbo sxqdwui qj qbb!