A method for a computing system (115, 150, 160, 400) supporting a single sign-on service, the method comprising: registering a first service with the single sign-on service, the registering including: receiving a shared secret access key available to the first service; receiving a unique non-secret identifier associated with the shared secret access key; and storing the shared secret access key for the first service; receiving at least one sign-on message from the first service for a user of the first service who is attempting to sign-on via the single sign-on service, the at least one sign-on message including: sign-on information for the user that includes multiple parameters to identify the user; an identifier of the first service; and a digital signature generated using the shared secret access key of the first service and the at least one sign-on message; and verifying an identity of the first service based at least in part on the digital signature included in the at least one sign-on message, wherein the verifying includes retrieving the shared secret access key associated with the identifier included in the received at least one sign-on message; after the verifying of the identity of the first service, performing user authentication to determine whether to approve the attempted sign-on by the user based on information in a user accounts database and one or more of the parameters included in the received at least one sign-on message; if the attempted sign-on by the user is determined to be approved, providing to the first service a credential representing the user for later use in performing operations on behalf of the user; receiving at least one message from the first service to perform an indicated operation, each of the at least one message including the provided credential representing the user; and for each of the at least one message, verifying that the credential included in the message is valid for the user by sending the credential to the credential manager that has generated the credential and receiving a response from the credential manager to validate the credential, and performing the indicated operation if the credential is verified to be valid. registering a first service with the single sign-on service, the registering including: receiving a shared secret access key available to the first service; receiving a unique non-secret identifier associated with the shared secret access key; and storing the shared secret access key for the first service; receiving a shared secret access key available to the first service; receiving a unique non-secret identifier associated with the shared secret access key; and storing the shared secret access key for the first service; receiving at least one sign-on message from the first service for a user of the first service who is attempting to sign-on via the single sign-on service, the at least one sign-on message including: sign-on information for the user that includes multiple parameters to identify the user; an identifier of the first service; and a digital signature generated using the shared secret access key of the first service and the at least one sign-on message; and sign-on information for the user that includes multiple parameters to identify the user; an identifier of the first service; and a digital signature generated using the shared secret access key of the first service and the at least one sign-on message; and verifying an identity of the first service based at least in part on the digital signature included in the at least one sign-on message, wherein the verifying includes retrieving the shared secret access key associated with the identifier included in the received at least one sign-on message; after the verifying of the identity of the first service, performing user authentication to determine whether to approve the attempted sign-on by the user based on information in a user accounts database and one or more of the parameters included in the received at least one sign-on message; if the attempted sign-on by the user is determined to be approved, providing to the first service a credential representing the user for later use in performing operations on behalf of the user; receiving at least one message from the first service to perform an indicated operation, each of the at least one message including the provided credential representing the user; and for each of the at least one message, verifying that the credential included in the message is valid for the user by sending the credential to the credential manager that has generated the credential and receiving a response from the credential manager to validate the credential, and performing the indicated operation if the credential is verified to be valid.