User authentication techniques that use a companion device associated with a mobile computing device are described. The companion device receives a user authentication request from a user authentication service via the mobile computing device, displays information related to the user authentication request, receives an approval of the user authentication request, and transmits the approval of the user authentication request to the service via the mobile computing device. In one embodiment, after transmitting the approval, the companion device receives a token from the mobile computing device that includes a value obtained from the service, signs the token with a private key of a securely-stored signing key pair and provides the signed token to the service via the mobile computing device. In another embodiment, after the companion device transmits the approval to the mobile computing device, the mobile computing device provides a personal identification code from secure storage to the service.
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