A customer purchases an item from an e-commerce site. The e-commerce site must maintain proof that the data exchange took place between the site and the customer. Which feature of digital signatures is required?

Digital signatures provide three basic security services:Authenticity of digitally signed data – Digital signatures authenticate a source, proving that a certain party has seen and signed the data in question.
Integrity of digitally signed data – Digital signatures guarantee that the data has not changed from the time it was signed.
Nonrepudiation of the transaction – The recipient can take the data to a third party, and the third party accepts the digital signature as a proof that this data exchange did take place. The signing party cannot repudiate that it has signed the data.