Product Advertising API
Developer Guide (API Version 2013-08-01)

Cart Identifiers

When you create a remote shopping cart using CartCreate, Product Advertising API returns the new cart's ID (CartId). The cart ID is also an alphanumeric token that is used to identify a cart. The cart ID and HMAC values must be used in all Product Advertising API requests related to the cart (except CartCreate). HMAC (Hash-based Message Authentication Code) is a specific construction for calculating a message authentication code (MAC) involving a cryptographic hash function in combination with a secret key. You can use it to verify both the data integrity and the authenticity of a message at the same time. AWS calculates the HMAC using a standard, cryptographic hash algorithm, such as SHA-256.