Add the water, vinegar, sugar, salt, garlic, dried red chili flakes, and ketchup to a sauce pan. Whisk everything together very well. Bring to a boil over medium high heat. Whisking often.
Once bubbling, stir together the cornstarch and water in a measuring cup. Pour into the sauce mixture and whisk to combine.
The sauce will thicken very quickly. Turn off heat and allow to cool for 2-3 minutes.
Pour hot sauce into sterile jars and store in refrigerator for up to 4 weeks.
