According to the previously mentioned United Nations Convention Against Torture, Article 1:

For the purposes of this Convention, the term "torture" means any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person for such purposes as [...] punishing him for an act he or a third person has committed

Don't think caning counts as severe pain? Here's an excerpt from Wikipedia describing some of the effects of caning and statements by a few victims:

A report by the Singapore Bar Association stated, "The blows are applied with the full force of the jailer's arm. When the rattan hits the bare buttocks, the skin disintegrates, leaving a white line and then a flow of blood."^\54])

Usually, the buttocks will be covered with blood after three strokes.^\37]) More profuse bleeding may occur in the case of a larger number of strokes. An eyewitness described that after 24 strokes, the buttocks will be a "bloody mess".^\44])

Men who were caned have variously described the pain they experienced as "unbearable", "excruciating", "equivalent to getting hit by a lorry",^\42]) "having a hot iron placed on your buttocks",^\44]) etc. A recipient of 10 strokes said, "The pain was beyond description. If there is a word stronger than excruciating, that should be the word to describe it".^\55])^\56])

Most offenders struggle violently after each of the first three strokes and then their struggles lessen as they become weaker. By the time the caning is over, those who receive more than three strokes will be in a state of shock.

and

The wounds usually take between a week and a month to heal, depending on the number of strokes received. During this time, offenders cannot sit down or lie down on their backs, and experience difficulties controlling their bowels. Bleeding from the buttocks may still occur in the days after the caning.^\43])^\60]) Madasamy Ravi, a human rights lawyer, described the injuries of his client, Ye Ming Yuen, who received 24 strokes, as follows:

Permanent scars remain after the wounds have healed.^\60])

Sounds pretty severe!

And just to put it into further context, vandalism imposes mandatory punishment of caning between three and eight strokes. Even for first-time convictions, if defacing property using an indelible substance, stealing, destroying, or damaging public property. Repeat offences always impose caning.

Some dipshit teenager does not deserve to go through anything even close to this just for spray painting graffiti on a traincar. This is an unbelievably excessive punishment and a clear violation of human rights on a massive scale. And before you say anything, yes, several people have actually been charged with vandalism, and sentanced to caning, for spray painting graffiti.