I still remember the night when I struggled to receive a picture of the Chinese President from an interviewee on WeChat because the Chinese app repeatedly blocked it.

Key points: WeChat scans a picture's coding imprint and matches it to blacklisted photos

WeChat scans a picture's coding imprint and matches it to blacklisted photos The Chinese Government censors anything deemed relevant to "national security"

The Chinese Government censors anything deemed relevant to "national security" WeChat is so central to life in China that it is difficult for many to leave

It was last month while working on a story about China's crackdown on religion and the burning of crosses when I asked a pastor to send me a picture.

Three times he told me he sent it, three times I failed to receive it.

Then he sent me a screenshot to prove the picture had been sent successfully.

When I realised what was happening — that the picture was being censored — I immediately asked the pastor not to try again.

The feeling of having my private communications invaded, by a government determined to censor my messages, helped me to decide that I needed to step back from an app that I have so heavily relied on in so many aspects of my life.

You may think quitting WeChat is almost like quitting Facebook, but the app so permeates the daily life of Chinese people that it is a far bigger deal than just deleting your Facebook account.

WeChat is a "super app" produced by China's tech giant Tencent, that is a digital fusion of messaging, payment and social services.

The picture of Xi Jinping's portrait in a church was blocked by WeChat instantly while I was waiting to receive it. ( ABC News: Bang Xiao )

As President Xi Jinping continues to reinforce internet censorship with his Cyberspace Administration, Chinese apps like WeChat are constantly refining their technology to monitor and censor content from more than 1 billion active users.

With almost all of your family, friends and colleagues on the app, people use WeChat for communications, planning and financial transactions like booking flights or hotels.

It has become an intrinsic part of life in China, where popular foreign apps like Facebook, Line and WhatsApp are completely blocked by authorities.

WeChat is a 'super app' with multiple facets. ( ABC News: Jarrod Fankhauser )

How did the censors block the photo?

Jeffrey Knockel, an expert from Canada's Citizen Lab — that recently published an analysis into the image filtering on WeChat moments — used technology to explain how the image of Mr Xi was filtered on my WeChat.

"We found that on WeChat Moments many images are filtered related to Falun Gong, lampoons of Xi Jinping, the '709 Crackdown', Liu Xiaobo, and Hong Kong independence," Knockel told the ABC.

For years, Chinese activists and human rights campaigners have used text-based images to get around censors who were only looking at text, but images are increasingly being scrutinised.

According to Knockel, his research found that WeChat compares the cryptographic hash of images sent in chat to images that have been blacklisted.

He said the data of the image sent from the pastor must have matched a blocked one and triggered the censor.

The photo that Chinese censors blocked me from receiving. ( Supplied: Liang Zhang )

"WeChat does not provide end-to-end encryption, meaning that any message sent over the platform may be read by Tencent and anyone else Tencent provides access to," Knockel said.

The ABC contacted Tencent for comment concerning image filtering in private chats, but has received no reply.

'The Government is getting smart'

This is not the first time that people have raised concerns about privacy on WeChat.

Earlier this year, staff of the Australian Defence Department were banned from using the app by management for security issues.

Despite knowing about these security concerns, I still decided to add the pastor on WeChat when trying to contact him for my story last month.

This is because without WeChat, many people are almost unreachable.

Professor Jyh-An Lee, a Chinese censorship law expert from the Chinese University of Hong Kong, said WeChat is part of the infrastructure which enables more government control of a user's personal information under China's Cyber Security Laws.

"Nowadays I think the Government is getting smart, so it is not completely using the law to regulate human behaviour, but using technologies developed by private parties," Professor Lee told the ABC.

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He said that while in democratic countries like Australia, governments had to get a subpoena or warrant granted by the court in order to get access to private data, in China there are no similar checks and balances, because all internet businesses are required to store user log information for at least six months by law.

"That might include all these private messages that you sent to your friends," Professor Lee said.

In the past, Tencent made a number of announcements, claiming that the company never monitors private messages and never enabled any kind of surveillance of users.

"But no-one believes that," Professor Lee said.

"It's very easy for Tencent to argue that I need to block your image transmission because I have some national security or criminal concerns."

The Great Firewall first came into effect in the mid 1990s. ( AP: Eranga Jayawardena )

In the name of national security

In China, WeChat's censorship protocols have been legally "justified" by a host of new laws granting the Government sweeping powers over any matter considered relevant to China's national security.

For example, in WeChat's chat groups it is compulsory to nominate one group member as the admin of the group, with groups limited to a maximum of 500 members.

Group owners must manage the group's content, such as banning political sensitive messages, according to China's Cyberspace Administration.

Failure to control groups in line with government wishes means the group owner can be held legally responsible.

China defines the concept of cyber security in a way that is very different from the rest of the world.

"According to the Chinese Government, it also includes the ideological threat, not only the technical threats," Professor Lee said.

"The political leaders, or the party, try to maintain its permanent control over the regime and the internet is of course one of its very important tools."

So for now, I have decided that it is best to not post anything or chat with friends, family, or sources on WeChat, in order to protect my own privacy and theirs.

At this stage, I will keep the app to follow friends and family, but as a silent observer and nothing more.