If they say they're using regular AES-GCM from a library that's been decently vetted, the main things you have to worry about are 1. intentional backdoors 2.

If they say they're using regular AES-GCM from a library that's been decently vetted, the main things you have to worry about are 1. intentional backdoors 2. someone using the library improperly (like using the same IV for every encryption) If they're rolling their own encryption library, you have to worry about 1. intentional backdoors 2. someone using the library improperly 3. the quality of their implementation of crypto primitives Getting #2 right is definitely doable for a typical programmer who takes a little time to understand the basic dos and donts. IMO #3 is not doable for a typical programmer, and certainly isn't trivial even for people that are experts (at least for a relatively complicated pair of algorithms like AES and GCM). So even though #3 is just one more thing that can go wrong, it's way more likely to be a problem than #2