Story highlights South Korean man arrested over re-tweets of "funny" North Korean tweets

Park Jung-geun also tweeted an image of himself with the North Korean flag

23-year-old is accused of spreading North Korea propaganda

Amnesty says the law is being used to stifle freedom of speech

Everyone's made a joke they thought was funny only to see it fall flat, but Park Jung-geun's attempt at humor could see him jailed for up to seven years in South Korea.

Park, a photographer by profession, re-tweeted some messages from North Korea's official twitter feed, such as reports on the late leader Kim Jong Il's travels across the country and negative tweets about South Korea.

"I found the messages ludicrous because they are so harsh and the language is so out-dated. I found them funny, so I re-tweeted them," Park told CNN.

Park, 23, also took a photo of himself holding a whiskey bottle and edited in a North Korean flag as a background for a joke. His supporters then doctored a North Korean military poster, replacing a smiling soldier with a sad photo of Park and the image of the gun with another whiskey bottle.

But Park was imprisoned in January of this year, accused of "acts that benefit the enemy" and spreading North Korean propaganda. He was released on bail in February and has another court appearance in mid-July. He's still hoping the charges against him will be dropped.

"To be honest, if you did this in North Korea, you would be severely punished," Park said. "But this is South Korea and I thought there wouldn't be a problem as I did it for fun. It certainly wasn't for propaganda purposes."

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Park is the latest person to fall foul of South Korea's controversial National Security Law (NSL). Used widely during the years of military dictatorship in the 1970s and '80s, the law was designed to protect the state against North Korean propaganda and to prosecute spies.

But human rights groups say it was also used to persecute political dissidents and stifle freedom of speech.

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Amnesty International says the law is still being abused, especially against those critical of the government's policies on North Korea.

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"The NSL has a chilling effect on freedom of expression in South Korea," said Sam Zarifi, Amnesty's Asia-Pacific director.

"It is used, not to address threats to national security, but instead to intimidate people and limit their rights to free speech. It should be reformed in line with human rights law and if the government cannot do this, it must be abolished," he said.

The United Nations has also called on Seoul to change or repeal the law.

President Lee Myung-bak's office declined to comment on the issue but the law has certainly been more widely used since the conservative president came to power in 2008. That year, just five people were prosecuted on charges of pro-North Korean online postings. Two years later that number had jumped to 82.

Some within the South Korean media are also criticizing the Lee administration for interfering in press freedom.

Journalists from four major networks went on strike earlier this year. Employees from two of those stations have since gone back to work after promises from employers their concerns would be heard, but close to one thousand journalists remain on strike.

"President Lee Myung-bak has appointed pro-government people as presidents of the media companies, what we call 'parachutes,'' said Kim Jong-wook, head of the YTN labor union which is still on strike.

"They are now trying to censor our voice and stop criticism of the government," he added.

The government denies interference but picketing journalists say they are worried they are seeing increased censorship reminiscent of the dark days of dictatorship.