To improve reliability S3 calculates an MD5 hash of the object, this process can be validated from both ends; the customer and AWS. This mechanism will give customers the guarantee that the object hasn't been compromised on transit when it is created, and that the object remains the same when reading it.

Internally AWS constantly compares objects with the MD5 database to validate consistency. When inconsistencies are detected the object is repaired automatically. Buckets can also be versioned, providing data integrity by keeping multiple versions of every object as they are changed through time.