A method, performed by a server system, for providing a request for sensitive information and authorization to perform a transaction using the sensitive information to a client device over a network, the method comprises: receiving, by a signature service executing on the server system, a first request from a merchant system, the first request requesting that a second request for sensitive information and user input to serve as authorization to perform a transaction using the sensitive information be sent to the client device, the first request including contact information for a user of the client device, the contact information comprising one of a phone number or an email address of the user; generating, by the signature service, the second request, the second request including user interface code for rendering one or more user interfaces on the client device, the one or more user interfaces including one or more fields (201a, 201b) for receiving the user input to serve as authorization, the one or more fields being configured to cause the signature service to receive the user input to serve as authorization; sending, by the signature service, a request to a secure overlay service executing on the server system in a secure environment, the request including instructions for causing one or more overlaid fields of a separate user interface to be combined with the one or more user interfaces such that the one or more overlaid fields are displayed overtop of and as part of the one or more user interfaces, the one or more overlaid fields being configured to cause the secure overlay service to receive the sensitive information from the user of the client device; sending, by the signature service, a text message or email to the user of the client device using the contact information received from the merchant system, the text message or email including a link for receiving the second request; in response to the user selecting the link while the text message or email is displayed on the client device, sending, by the signature service, the second request to the client device thereby causing the one or more user interfaces to be displayed on the client device including displaying the one or more overlaid fields overtop of and as part of the one or more user interfaces; in response to the user providing user input to serve as authorization into the one or more fields, receiving, by the signature service, the user input to serve as authorization from the client device; in response to the user providing sensitive information into the one or more overlaid fields, receiving, by the secure overlay service, the sensitive information; and causing a transaction to be performed using the sensitive information, the user input serving as evidence that the user has authorized the transaction. receiving, by a signature service executing on the server system, a first request from a merchant system, the first request requesting that a second request for sensitive information and user input to serve as authorization to perform a transaction using the sensitive information be sent to the client device, the first request including contact information for a user of the client device, the contact information comprising one of a phone number or an email address of the user; generating, by the signature service, the second request, the second request including user interface code for rendering one or more user interfaces on the client device, the one or more user interfaces including one or more fields (201a, 201b) for receiving the user input to serve as authorization, the one or more fields being configured to cause the signature service to receive the user input to serve as authorization; sending, by the signature service, a request to a secure overlay service executing on the server system in a secure environment, the request including instructions for causing one or more overlaid fields of a separate user interface to be combined with the one or more user interfaces such that the one or more overlaid fields are displayed overtop of and as part of the one or more user interfaces, the one or more overlaid fields being configured to cause the secure overlay service to receive the sensitive information from the user of the client device; sending, by the signature service, a text message or email to the user of the client device using the contact information received from the merchant system, the text message or email including a link for receiving the second request; in response to the user selecting the link while the text message or email is displayed on the client device, sending, by the signature service, the second request to the client device thereby causing the one or more user interfaces to be displayed on the client device including displaying the one or more overlaid fields overtop of and as part of the one or more user interfaces; in response to the user providing user input to serve as authorization into the one or more fields, receiving, by the signature service, the user input to serve as authorization from the client device; in response to the user providing sensitive information into the one or more overlaid fields, receiving, by the secure overlay service, the sensitive information; and causing a transaction to be performed using the sensitive information, the user input serving as evidence that the user has authorized the transaction.