A device, such as a smartcard, may be externally-connected to a host platform and may be used to enhance or extend security services provided by the host platform's Trusted Platform Module (TPM). The device and the platform exchange keys in order to facilitate reliable identification of the platform by the device and vice versa, and to support cryptographic tunneling. A proxy component on the host device tunnels information between the platform and the device, and also provides the device with access to the TPM's services such as sealing and attestation. The device can provide secure services to the platform, and may condition provision of these services on conditions such as confirming the platform's identity through the exchanged keys, or platform state measurements reported by the TPM.