The material was allegedly stored behind password protection on his phone(s) but also on Mega and on Telegram, where Herrera is said to have "created his own public Telegram group to store his CSAM." He also joined "multiple CSAM-related Enigma groups" and frequented dark websites with taglines like "The Only Child Porn Site you need!"

My guess would honestly be Telegram. For starters, they aren't end-to-end encrypted by default, you have to turn it on. The only end-to-end encryption that Telegram offers is their "secret chats" which are only available between two users. Groups are not encrypted.

The Ars article seems to suggest that they were able to crack his phones pretty easily

Android uses data at rest encryption, which isn't really useful without a lockscreen PIN/password since data gets decrypted after you unlock your screen the first time after each boot

Although it doesn't appear he was actually using any encryption apps to store material; rather, he used a fake calculator app as password protection. Obviously not the brightest bulb in the drawer.

Agreed, he probably felt safe enough "hiding" the files. Definitely not the sharpest tool in the shed, which is great because fuck this guy

I honestly don't think he really had any opsec apart from those few applications, look at what tools he was using, what a joke. Fake calculator app to store files are great to protect from your parents, not the FBI.

He was clearly using Android and I bet he was using the stock rom, kyc sim card, and not even a vpn behind tor.

Don’t get me wrong, I’m very happy and relieved he was caught, but if he had done serious research and did a better opsec, it wouldn’t have been so easy for the authorities to get him

Phones are horseshit for OPSec, always. Every darknet admin and their dog know that.