BACKGROUND [0001] Online users are typically required to maintain a set of authentication credentials (e.g., a username and password) for each service provider he or she is entitled to access. These users often face the dilemma of using different authentication credentials for each individual service provider in order to maintain a high level of security, or using the same authentication credentials for the various service providers resulting in a diminished level of security. Frequently, the latter is chosen over the former, as it is difficult to memorize and maintain numerous authentication credentials. In addition, aside from the security implications, requiring users to enter authentication credentials each time access to a service provider is necessary is a generally awkward and time consuming procedure. [0002J Various conventional technologies have been proposed to alleviate or eliminate the need to maintain multiple sets of authentication credentials that provide access to various online services. One such technology utilizes a centralized credential management that provides authentication services for participating service providers. After a user initially establishes a relationship and authenticates with the centralized credential management, the centralized credential management administers the authentication process when the user subsequently requests access to any of the participating service providers. This technology significantly reduces the complexity of having to request access of numerous service providers. The centralized credential management transparently handles the particulars of authenticating with various participating service providers, while a high level of user security is maintained. [0003] Current conventional centralized credential management technologies are not suitable for all online environments. One conventional centralized credential management technology requires a user to authenticate with an authentication server. After authentication, the authentication server issues an authentication ticket to the user. The authentication ticket is used by the user to obtain access to a server that issues service access tickets. The server will issue a service access ticket to the user if the authentication ticket is valid. The user may then use the service access ticket to gain access to a service provider. [0004] The described conventional centralized credential management technology provides secure access functionality if the service providers are centrally maintained. However, secure access to the service providers is compromised if the service providers are part of a network having numerous disparate users/entities, such as the Internet. [0005] Another conventional authentication technology uses a centralized database that contains registered users and their associated authentication credentials. Each of the registered users has a unique 64-bit ID number. This conventional authentication technology also assigns each participating service provider a unique ID. These unique IDs are also kept in a centralized database. The participating service providers agree to implement a server component that facilitates secure communication with an entity administering the centralized databases. When a registered user attempts to authenticate with a participating service provider, the user is transparently redirected to the administering entity to facilitate the authentication. The implemented secure communication path between the participating service provider and the administering entity helps to ensure the authentication request granted. [0006] The authentication technology discussed above provides secure Web-based authentication. However, the technology has not been widely adopted by the Internet community. This is mainly due to the centralized database design feature of the technology. Some service providers do not approve of the technology because central databases are used. In particular, a service provider must rely on an entity administering the centralized databases to ensure successful user authentication. If the entity experiences technical difficulties, user authentication may be disrupted. This possibility of disruption, which is not service provider controllable, may be a risk the service provider is not willing to take. Furthermore, the use of centralized databases makes the authentication technology especially prone to attacks by hackers and malware. SUMMARY [0007] Implementations described herein relate to establishing authenticated communication between a client computing device and a service provider. The communication is made possible without the use of a trusted authority that holds secrets of a client computing device and a service provider. [0008] After a registration process, a client computing device uses authentication servers to request authenticated communication with a service provider. The client computing device may use any set of a plurality of authentication servers that it is registered with, where the number of servers in the set is large than or equal to a threshold value, to request authenticated communication with a service provider. The service provider should also be registered with the authentication servers the client computing device may use to establish authenticated communication. [0009] A client computing device may send a service provider an authentication request that includes its unique identifier ID and an encrypted authentication token that includes the unique identifier ID, a network address such as IP address of the device and a nonce. The encrypted authentication token was received in an earlier communication with authentication servers. Each authentication server used a split key, obtained from the service provider the client computing device is desiring authenticated communication with, to encrypt and generate an partial authentication token. The service provider uses an undisclosed secret key to decrypt the authentication token, thereby exposing the encrypted authentication information. This information, along with the sent unique identifier ID, is used by the service provider to decide if authenticated communication will be authorized. [0010] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. BRIEF DESCRIPTION OF THE DRAWINGS [0011] Non-limiting and non-exhaustive embodiments are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. [0012] Fig. 1 illustrates an exemplary implementation of a computer network environment that includes several client computing devices that may communicate with one or more service providers. [0013] Fig. 2 illustrates an exemplary implementation where a client device requests authenticated communication with a service provider using a client device service provider access procedure. [0014] Fig. 3 illustrates an exemplary implementation where a service provider registers with an authentication server using a service provider registration procedure. [0015] Fig. 4 illustrates an exemplary implementation where a client computing device registers with an authentication server using a client device registration procedure. [0016] Fig. 5 illustrates an exemplary implementation where a client computing device authenticates with the authentication server using a client computing device registration procedure. [0017] Fig. 6 is an illustrative computing device that may be used to implement a service provider. [0018] Fig. 7 is an illustrative computing device that may be used to implement a client computing device. [0019] Fig. 8 is an illustrative computing device that may be used to implement an authentication server. DETAILED DESCRIPTION [0020] Overview Systems and methods for authenticating with a service provider are described. In the following, a broad discussion of the procedures utilized between client, service provider and authentication devices to establish authenticated communication between a client computing device and service provider device is provided. This discussion will make use of an exemplary environment illustrated in Fig. 1. Exemplary implementations of various procedures used between various devices are then described in further detail. In particular, a detailed discussion of a procedure used by a client computing device to obtain authenticated access to a service provider is provided in conjunction with Fig. 2; a detailed discussion of a service provider registration procedure with an authentication server is provided in conjunction with Fig. 3; a detailed discussion of a client computing device registration procedure with an authentication server is provided in conjunction with Fig. 4; and a detailed discussion of a client computing device authentication procedure with an authentication server is provided in conjunction with Fig. 5. Finally, exemplary implementations of a service provider, client computing device and authentication server are discussed in conjunction with Figs. 6-8, respectively. [0021] Exemplary Environment Fig. 1 illustrates an exemplary implementation of a computer network environment 100 that includes several client computing devices 102(l)-102(n) that may communicate with one or more service providers 104(l)-104(n). Bidirectional communication between the client computing devices 102(l)-102(n) and the service providers 104(l)-104(n) is facilitated with a network 120 (e.g., the Internet). Authentication servers 106(l)-106(n) are also interfaced with the network 120. One or more authentication servers 106 work together to provide authentication services to the client computing devices 102(l)-102(n) and the service providers 104(l)-104(n). Each authentication server has generally the same functionality as another authentication server. [0022] At any given time, one of the several client computing devices 102(1)-102(n), such as the client computing device 102(1), may require services provided by one of the service providers 104(l)-104(n), such as the service provider 104(1). A subset of the authentication servers 106(l)-106(n) provides technology that allows the client computing device 102(1) to properly authenticate with the service provider 104(1). The service provider 104(1) will generally not provide services to any of the client computing devices 102(l)-102(n) until proper authentication is attained. [0023] The service provider 104(1) and the client computing device 102(1) each establish a relationship with the authentication servers 106(l)-106(n) before requesting authentication services of the servers 106(l)-106(n). After the service provider 104(1) establishes initial contact with the authentication servers 106(1)-106(n), a server module 130 is provided to the service provider 104(1). This server module 130 provides the operational parameters that the service provider 104(1) will need during a relationship registration phase. The server module 130 may be stored directly in a memory, volatile or nonvolatile, of the service provider 104(1). If the service provider 104(1) comprises multiple servers (e.g. a server farm), the server module 130 may be stored on a proper one of those multiple servers. [0024] After obtaining the server module 130, the service provider 104(1) creates a secret encryption key and a corresponding secret decryption key. The secret encryption key is split to create additional keys. In one implementation, the secret key is split using a threshold scheme. However, other splitting schemes may be used as well. A split key is sent securely to each authentication server such as 106(1) with a unique ID that identifies the service provider 104(1). The authentication server 106(1) sends a success response to the service provider 104(1) after the split key and the service provider unique ID are received. The secret decryption key remains at the service provider 104(1) and is never made public to any other entity such as the authentication server 106(1). The server module 130 provides the routines for creating the secret keys and splitting the secret encryption key. [0025] The client computing device 102(1) receives a client module 140. The client module 140 may be a Web browser plug-in module that integrates with a Web browser of the client computer device 102(1). The client module 140 provides the operational parameters required during a relationship registration procedure carried out between the client computing device 102(1) and each authentication server 106(i). The client module 140 may be stored directly in a memory, volatile or nonvolatile, of the client computing device 102(1). [0026] After obtaining the client module 140, the client computing device 102(1) may proceed to register with each authentication server 106(i). This should be done before the client module 140 requests services from one or more of the service providers 104(l)-104(n). The registration process between the client module 140 and an authentication server 106(i) involves using the client module 140. A user of the client computing device 102(1) utilizes the client module 140, via a user interface such as an associated Web browser, to enter a unique username and a password. The client module 140 generates a unique client identifier from the unique username. The client module 140 also generates a client authentication key for the client to authenticate to an authentication server 106(i) from the unique username, password and an authentication server ID that was received with the client module 140. A hashing function may be used to create the client authentication key. The client module 140 sends the client authentication key and the client identifier securely to an authentication server 106(i). After receiving and retaining the client identifier and client authentication key, the server 106(i) will send a success message to the client computing device 102(1). [0027] After registering with each authentication server 106(i), the client computing device 102(1) may use a subset of the authentication servers 106(1)-106(n) to establish authenticated communication with one of the service providers 104(l)-104(n) that has already established a relationship with the authentication servers 106(l)-106(n). [0028] Before requesting authenticated communication with one of the service providers 104(l)-104(n), the client computing device 102(1) will authenticate with a subset of the authentication servers 106(l)-106(n), which will provide authentication services to the client computing device. This authentication procedure establishes session keys used by the client computing device 102(1) and the subset of authentication server 106(l)-106(n) for subsequent confidential communications. [0029] To authenticate, a user of the client computing device 102(1) sends each authentication server 106(i) in the subset an authentication request. Making the authentication request and establishing a session key are facilitated using the client module 140. Each authentication server 106(i) responds to an authentication request by sending the client computing device 102(1) a nonce. The client computing device 102(1), in response, sends the authentication server 106(i) the client identifier and the received nonce encrypted using the client authentication key to the authentication server 106(i). The encryption also includes a client device random number and a client device nonce. The client device random number and client device are generated by the client module 140. The client identifier and client authentication key were created during the registration process discussed above. [0030] The authentication server 106(i) uses the client identifier to retrieve the client authentication key that was created during the registration process. The retrieved client authentication key is used to decrypt the encrypted authentication server nonce, the client device random number and the client device nonce. If decryption is successful and the decrypted authentication server nonce matches the nonce sent to the client by the authentication server in a preceding process, the authentication server 106(i) sends the client 102(1) an encryption that includes an authentication server random number generated at the server 106, the nonce and client device nonce. At this point, both the server 106 and the client 102(1) possess the two random numbers. A hashing function is used on the two random numbers by the server 106(i) and the client 102(1) to create a session key at both ends. This session key is used to establish a secure link between the client computing device 102(1) and the authentication server 106(i) when the client computing device 102(1) uses the authentication sever 106(i) in the subset of chosen authentication servers to establish authenticated communication with one of the service providers 104(l)-104(n). [0031] After obtaining a session key from the authentication server 106(i), the client device 102(1) is ready to request authenticated communication with a service provider already registered with the authentication servers 106(l)-106(n) (e.g., the service provider 104(1)). This request begins at the point the client computing device 102(1) sends an access request to the service provider 104(1). The service provider 104(1) responds by sending its unique ID and a challenge (e.g. a service provider nonce) to the client computing device 102(1). Optionally, the service provider 104(1) may send a list of authentication servers it has already registered with to the client computing device 102(1) so that the client can request authentication services from those servers. If the client has not authenticated with the authentication servers in the list, the client authenticates with the authentication servers in the list and establishes a session key for subsequent confidential communication with each of those authentication servers. [0032] The client computing device 102(1) is now ready to contact each of the authentication servers in the subset of authentication servers, for example authentication server 106(i), to establish authenticated communication with the service provider 104(1). The client computing device 102(1) sends each authentication server 106(i) the service provider unique ID and the service provider nonce. Optionally, the client computing device 102(1) may also send its own client identifier to the authentication service provider 104(1). The authentication server 106(i) should also have the client identifier of the client computing device 102(1) and the session key retained in memory from prior communications between the two devices. [0033] The authentication server 106(i) encrypts the client identifier, a network address such as an IP address of the client computing device 102(1), and the service provider nonce using the split key received previously from the service provider 104(1). This process creates a partial authentication token that is passed on to the client computing device 102(1). The client computing device 102(1) creates an authentication token from the received partial authentication tokens and packages the authentication token with its own client identifier and sends the package to the service provider 104(1). The service provider 104(1) will attempt to decrypt the encrypted authentication token using its secret decryption key. If decryption is successful and the decrypted nonce matches the nonce sent to the client in a preceding authenticated communication, the authenticated communication is successful and access to the service provider's service is granted. The service provider 104(1) notifies the client computing device 102(1) whether authenticated communication is granted. [0034] The advantages of the discussed authentication procedure are at least the following. A service provider's secret keys are only known by the provider. Certainly, authentication servers do not know the secret keys and obtaining a service provider's secret key is generally only possible if significant collusion between various authentication servers were to occur. On a client computing device side, a user's password is never used directly during the authentication process. In fact, each entity in the authentication process, client computing devices and service providers, control their own secrets. This makes the authentication procedure described herein very attractive, as the existence of a trustworthy authority is not required. Trustworthy authorities are used with many other encryption/authentication technologies (e.g., PKI). [0035] Client Device Access to Service Provider Procedure Fig. 2 illustrates an exemplary implementation where the client device 102(1) requests authenticated communication with the service provider 104(1) using a client device service provider access procedure 200. The authenticated communication is facilitated using subset of the authentication servers 106(1)-106(n). A table I provides details related to notations used in the text that follows. Table I S A participating service provider. V A participating client device. At The i-th authentication server. UID A unique client ID for a participating U . SID A unique service provider ID for a participating service provider S. AID, An unique ID for the i-th authentication server At. Ks A secret encryption key generated by and known only to S. Ks-1 A secret decryption key corresponding to Ks and known only to S. KS The Mh partial share of Ks generated by a threshold scheme. K'v A secret client key for u to authenticate to the i-th authentication server A.. p1, p2 Two properly selected prime integers g A generator in A session key between a user U and the Mh authentication server A1. < m >k A message m encrypted by a symmetric cipher with a key k. It means mk modp where m e Zp. nx Nonce generated by entity X. rx A random number generated by entity X. x is optional in describing a protocol. Square brackets denote optional [x] parameters throughout the description [0036] The procedure begins when the client computing device 102(1) sends the service provider 104(1) an access request in a communication 201. The service provider 104(1) responds in a communication 202, [a list of/ authentication servers {Ad ,1≤/≤/}], where /is the threshold for the number of authentication servers needed to provide authentication services. Once the communication 202 is received, the client computing device 102(1) sends SID [UID} to the authentication server 106 in a communication 204. The authentication server 106 responds by sending in a communication 206, where U is the network identifier of the client computing device 102(1) such as the client's IP address (IPAddress). The contents of the communication 206 define a partial authentication token. The client computing device 102(1) uses the partial authentication tokens received in the communication 206 with the authentication servers to create an authentication token UID, U, nswhich is packaged together with UID and and sent to the service provider 104(1), where The service provider 104(1) uses its secret decryption key KS-1 to decrypt the encrypted authentication token, where [0037] Based on the above decryption and the additional information received with the authentication token, the service provider 104(1) will send an access response back to the client computing device 102(1) in a communication 210 indicating if authenticated access is granted. The optional use of a generator (g) in Z'PI during the authenticated communication provides a way to generate a session key which is used for subsequent secure communication between the client computing device 102(1) and the service provider 104(1) after the authenticated session is established. [0038] The communications 204 and 206 are secure using a session key established between the client computing device 102(1) and an authentication server 106(i). The client computing device 102(1) and the authentication server 106(i) are each in possession the session key. The specifics of creating such a session key are described later. [0039] The various discussed communications are facilitated using the client and server modules 140 and 130. However, the various communications and instructions may be carried out by any device enabled to carry out such communications and instructions, thereby providing the described client device access to service provider procedure/method. [0040] Service Provider Registration Procedure Fig. 3 illustrates an exemplary implementation where the service provider 104(1) registers with the authentication server 106(i) using a service provider registration procedure 300. The service provider registration procedure 300 is carried out before a service provider may benefit from the authentication services offered by an authentication server. The table I above provides details related to notations used in the text that follows. [0041] Before the procedure starts, the service provider 104(1) downloads and installs a server module 130. The server module 130 in this implementation provides the functionality that allows the service provider 104(1) to process authentication related requests and communications to and from the authentication server 106(i). The procedure begins when the service provider 104(1) sends the authentication server 106(i) a communication 302 requesting service provider registration. The authentication server 106(i) responds with a communication 304 that informs the service provider 104(1) that it is ready to accept registration. [0042] The service provider 104(1) uses the server module 130 to complete the service provider registration procedure illustrated in Fig. 3. The service provider 104(1) generates a secret key Ks ,1< Ks