Soy sauce or fish sauce can be substituted for shrimp paste. Shrimp paste is made from salted shrimp, traditionally dried in the sun.
Because it is made from salted shrimp, shrimp paste is very salty, which is why soy sauce is a good substitute, though it does not have the same depth of flavor. Fish sauce is made in a similar way to shrimp paste; that is, it is made from salted fish (Thailand) or shrimp (Vietnam), but it is not as fishy as shrimp paste. Shrimp paste is also very thick, while both soy sauce and fish sauce are liquids, so they can change the consistency of a recipe.