For the raita: Place the cucumber and tomatoes in a sieve for about 15 minutes, to drain off any excess liquid. Add to a bowl with the yogurt and buttermilk and stir well to combine. Season with salt.
For the soup: Put the coriander and cumin seeds into a small skillet and toast over low heat until they just begin to smoke and are very fragrant, about 15 seconds. Grind in a mortar and pestle.
Put the raita into a large glass bowl and whisk in the toasted spices, vegetable broth and salt. Place in the fridge until completely chilled, about 2 hours. Ladle the soup into bowls and garnish with a pinch of paprika and chopped fresh mint.