A client device comprising: one or more authenticators for authenticating a user of the client device with a relying party, each authenticator comprising a plurality of authentication components (1101, 1103), each of the authentication components (1101, 1103) within the client device performing a different function within the context of the authenticator within which it is used; and component authentication logic on the client device to attest to a model and/or integrity of at least one of the plurality of authentication components (1101, 1103) to one or more of the other authentication components (1101, 1103) prior to allowing the authentication components (1101, 1103) to be combined on the client device to form the authenticator, wherein authenticating the user with the relying party comprises: reading biometric authentication data from the user and determining whether to successfully authenticate the user based on a comparison with biometric reference data; establishing communication with the relying party; performing an attestation transaction with the relying party to attest to the model and/or integrity of a biometric device to the relying party by receiving a challenge from the relying party, signing the challenge using an attestation key to generate a signature, and sending the signature to the relying party, wherein the relying party verifies that the signature is valid using a key of the relying party. one or more authenticators for authenticating a user of the client device with a relying party, each authenticator comprising a plurality of authentication components (1101, 1103), each of the authentication components (1101, 1103) within the client device performing a different function within the context of the authenticator within which it is used; and component authentication logic on the client device to attest to a model and/or integrity of at least one of the plurality of authentication components (1101, 1103) to one or more of the other authentication components (1101, 1103) prior to allowing the authentication components (1101, 1103) to be combined on the client device to form the authenticator, wherein authenticating the user with the relying party comprises: reading biometric authentication data from the user and determining whether to successfully authenticate the user based on a comparison with biometric reference data; establishing communication with the relying party; performing an attestation transaction with the relying party to attest to the model and/or integrity of a biometric device to the relying party by receiving a challenge from the relying party, signing the challenge using an attestation key to generate a signature, and sending the signature to the relying party, wherein the relying party verifies that the signature is valid using a key of the relying party. reading biometric authentication data from the user and determining whether to successfully authenticate the user based on a comparison with biometric reference data; establishing communication with the relying party; performing an attestation transaction with the relying party to attest to the model and/or integrity of a biometric device to the relying party by receiving a challenge from the relying party, signing the challenge using an attestation key to generate a signature, and sending the signature to the relying party, wherein the relying party verifies that the signature is valid using a key of the relying party.