An authentication process using a combined code as a shared secret between a client and target service is provided. The combined code is provided out-of-band and includes data to perform two-way authentication for both the client and the target service. The target service may provide the client with a certificate to establish a secure channel. The client may use the data in the combined code to validate the target service. When the target service is validated, the client may provide credentials in the combined code to the target service for authentication. In one example implementation, the combined code includes a hash of a public key. The client may compute another hash of another public key in the certificate provided by the target service and validate the service by comparing the hash in the combined code and the computed hash.