It is probably from ISA/KR. Here is the relevant part from Annex D of PCT Applicant’s Guide (
http://www.wipo.int/pct/guide/en/gdvol1/annexes/annexd/ax_d_kr.pdf):
“Applicants that provide an e-mail authorization in their international application will be sent an encrypted copy of the International Search Report (PCT/ISA/210) by e-mail. It can be opened using a password which appears at the bottom of the Notification of Receipt of Search Copy (PCT/ISA/202).”
You can send an email directly to ISA/KR: isa.kipo@korea.kr for any further questions
Best regards