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I am trying to learn PGP at CTF.
I want to encrypt messages with GPG (Linux command)

https://id0-rsa.pub/problem/19/
It is around the end of the site.

$echo "hello id0!" | gpg -e --armor
I don't know what to do after typing

How can I move on to the next problem?

Error message

$gpg --import [file]
(Import PGP PUBLIC KEY)

$echo "hello id0!" | gpg -e --armor

You did not specify a user ID. (you may use "-r")

Current recipients:

Enter the user ID. End with an empty line:

came out.
I tried to enter my user ID many times, but it didn't work.
I don't understand the meaning of the last "End with an empty line".

Applicable source code

root @ A: ~ # echo "hello id0!" | gpg -e --armor
You did not specify a user ID. (You may use "-r")

Current recipients:

Enter the user ID. End with an empty line: id0-rsa.pub

gpg: 8EA95EFADDF97CC6: There is no assurance this key belongs to the named user

sub rsa2048/8EA95EFADDF97CC6 2015-11-19 id0-rsa.pub (http://id0-rsa.pub)
Primary key fingerprint: AB3A 59D4 3681 1F98 5B9B F6EA 2503 D0F1 A81B 09D4
Subkey fingerprint: F7CC 71CA 795D 166B 1E1B 3E6C 8EA9 5EFA DDF9 7CC6

It is NOT certain that the key belongs to the person named
in the user ID.If i * really * know what i am doing,
you may answer the next question with yes.

Use this key anyway? (y/N) y

Even if I entered the other party's userID, I couldn't understand English, so I entered various things that seemed to be related, such as email addresses, but all died with the same result.

Supplemental information (FW/tool version etc.)

VMware-KaliLinux

https://id0-rsa.pub/problem/19/

http://pgp.mit.edu/