It give us a message with eml extension

rom [email protected] Tue Nov 30 19:07:09 2021
Date: Tue, 30 Nov 2021 14:09:11 -0500
From: Pep Sparkles <[email protected]>
To: Tiny Jingles <[email protected]>
Subject: Rudolf's Location
Mime-Version: 1.0
Content-Type: multipart/mixed; boundary="----=_Part_5028_7368284.1115579351471"

------=_Part_5028_7368284.1115579351471
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
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From: Pep Sparkles <[email protected]>
Date: Nov 30, 2021 1:17 PM
Subject: Rudolf's Location
To: [email protected]

Hello Mr Jingles,

We got the reindeer as you requested. There is a problem though. Its nose is so red and bright and makes it very hard to hide him anywhere near north pole. We have moved to a secret location far away. I have encrypted this information with your public key in case you know who is watching.

------=_Part_5028_7368284.1115579351471
Content-Type: application/text/plain; name*=secret.enc
Content-Transfer-Encoding: base64
Content-Disposition: attachment
Ci95oTkIL85VWrJLVhns1O2vyBeCd0weKp9o3dSY7hQl7CyiIB/D3HaXQ619k0+4FxkVEksPL6j3wLp8HMJAPxeA321RZexR9qwswQv2S6xQ3QFJi6sgvxkN0YnXtLKRYHQ3te1Nzo53gDnbvuR6zWV8fdlOcBoHtKXlVlsqODku2GvkTQ/06x8zOAWgQCKj78V2mkPiSSXf2/qfDp+FEalbOJlILsZMe3NdgjvohpJHN3O5hLfBPdod2v6iSeNxl7eVcpNtwjkhjzUx35SScJDzKuvAv+6DupMrVSLUfcWyvYUyd/l4v01w+8wvPH9l
------=_Part_5028_7368284.1115579351471
Content-Type: application/octet-stream; name*=pubkey.der
Content-Transfer-Encoding: base64
Content-Disposition: attachment
-----BEGIN PUBLIC KEY-----
MIIBIDANBgkqhkiG9w0BAQEFAAOCAQ0AMIIBCAKCAQEA5iOXKISx9NcivdXuW+uE
y4R2DC7Q/6/ZPNYDD7INeTCQO9FzHcdMlUojB1MD39cbiFzWbphb91ntF6mF9+fY
N8hXvTGhR9dNomFJKFj6X8+4kjCHjvT//P+S/CkpiTJkVK+1G7erJT/v1bNXv4Om
OfFTIEr8Vijz4CAixpSdwjyxnS/WObbVmHrDMqAd0jtDemd3u5Z/gOUi6UHl+XIW
Cu1Vbbc5ORmAZCKuGn3JsZmW/beykUFHLWgD3/QqcT21esB4/KSNGmhhQj3joS7Z
z6+4MeXWm5LXGWPQIyKMJhLqM0plLEYSH1BdG1pVEiTGn8gjnP4Qk95oCV9xUxWW
ZwIBAw==
-----END PUBLIC KEY-----
------=_Part_5028_7368284.1115579351471--

It's a secret message that have encrypted using public key. If we want to decrypt the message, we should have the private key that paired with that public key. But we didn't have it, so let's crack it 😁

The public key aren't too long, we can crack it using RsaCtfTool from github

https://github.com/Ganapati/RsaCtfTool

Im using this command to decode the encrypted message.

python3 RsaCtfTool/RsaCtfTool.py --publickey pubkey --uncipherfile encrypt --attack cube_root

pubkey are the file that contain public key from the message. And encrypt are the encrypted text that have been decoded from base64 format.

FLAG = HTB{w34k_3xp0n3n7_ffc896}