SYSTEM AND METHOD FOR SECURE TWO-FACTOR AUTHENTICATED ID-BASED KEY EXCHANGE AND REMOTE LOGIN USING AN INSECURE TOKEN AND SIMPLE SECOND-FACTOR SUCH AS A PIN NUMBER

Abstract

A system and method of authenticated ID-based key exchange and remote login with insecure token and PIN number can provide an authenticated key agreement protocol based on an elliptic curve bilinear type-3 pairing. A server acts as an Authentication Service to Clients and a Trusted Authority (TA) issues identity based secret numbers to Clients and Authentication Services. Included in the system and method is the capability for the Client to split their secret number into two parts, a Client selected PIN number, and the larger number, the Token.

Description

BACKGROUND

The present invention is in the field of electronic authentication and authenticated key agreement. The purpose of the invention is to provide a method of authenticated key agreement in a client to server setting that advances the concept of two-factor authentication and provides an alternative to token based schemes that are often based on expensive (and irreplaceable) smart-card tokens, and provides an alternative to password-only schemes require that username and password management in a client and server context.

SUMMARY

This method employs a Trusted Authority (TA) that issues to a Client and Authentication Service an individual Secret Number associated with their identity. While the Authentication Service stores number in whole form, each Client “splits” this number into a small user-selected PIN number, and a larger number, the Token, which can be stored anywhere, and this has the added benefit that it is not required to be secured or encrypted. The rest of the invention will refer to the simple second factor as being a simple user-selected PIN number, however, this is clearly not meant to limit the use of any form of simple easily remembered user supplied second factor which could be a simple passphrase or repeatable number biometric, etc. which could be used equally in this invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating the internal components and the external components interacting with the entire system and method in the preferred embodiment of the invention.

FIG. 2 is a block diagram that illustrates the steps taken that securely initializes the set-up phase of the Trusted Authority (TA) according to an embodiment of the invention.

FIG. 3 is a block diagram that illustrates the steps taken that securely initializes the set-up phase of the Authentication Service Layer and its interaction with the Trusted Authority (TA) according to an embodiment of the invention.

FIG. 4 is a block diagram that illustrates the steps taken that securely distributes the Secret Number to the Client from the Trusted Authority (TA) according to an embodiment of the invention.

FIG. 5 is a block diagram that illustrates the steps taken that for the Client to initialize their Secret Number into the Token and PIN form using the Initialization Program, and stores their Token.

FIG. 6 is a block diagram that illustrates the steps taken between the Client and the Authentication Service Layer to receive the Authentication Program and lookup the Client's Token.

FIG. 7 is a block diagram that illustrates the steps taken between the Client and the Authentication Service Layer to construct the authenticated key agreement protocol using the Authentication Program according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Our system and method has desirable security properties including the required use of a large identity-based secret number and a PIN number, as this gives in effect strong 2-factor authentication. We also leverage a separate TA with its own Master Secret that is not required on-line which provides an extra layer of security and limits the damage caused by the loss of Client or server long-term secrets. Another desirable feature is that a rogue client who steals another Client's long-term secret is not able to determine their PIN by performing off-line key-exchanges with themselves. That is, Clients should only be able to exchange keys with servers, not with other Clients.

While much prior art exists in the field of authenticated key agreement using bilinear pairings, the invention described contains a system and method that is the first suitable for an enterprise class production environment incorporating two-factor authentication, with the unique properties of leveraging insecure storage of the token, into a whole usable system.

The object of the invention is to provide a system and method of authenticated ID-based key exchange and remote login with insecure token and PIN number that,

1) Provides an authenticated key agreement protocol based on an elliptic curve bilinear type-3 pairing, whereby a server acts as an Authentication Service to Clients and a Trusted Authority (TA) issues identity based secret numbers to Clients and Authentication Services. Included in the system and method is the capability for the Client to split their secret number into two parts, a Client selected PIN number, and the larger number, the Token.

We describe an embodiment of the invention, how the Client is issued their secret number from the TA, and initializes their Token in a method where they input and confirm their PIN with a JavaScript program, which stores their Token in their browser's storage. With the Authentication Service providing a web server front end, the Client and Authentication Service are able to mutually authenticate each other, and derive a mutually agreed session key. We show our invention contains desirable security properties such that no eavesdropper is in the position to determine the PIN from transmitted values, irrespective of their computing power.

2) Addresses a security vulnerability in potentially similar authenticated key agreement systems with a PIN extraction protocol, enabling this system and method to withstand a Key Compromise Impersonation (KCI) attack, and ties the identities of the Clients and Authentication Service to the their individual secret number to prohibit identity spoofing, i.e., where one Client would authenticate themselves to an Authentication Service using another identity. We show that other systems and methods do not withstand the KCI attack in tandem with the inability to mitigate identity spoofing. Additionally, we describe other embodiments possessing the same security properties, demonstrating that there exists a wide range of protocols this system and method can be adapted to.

The approaches described in this section could be pursued but are not necessarily approaches that have been previously conceived or pursued when taken together as a whole system and method. Therefore, unless other wise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.

The main components of the invention include a Trusted Authority (TA), responsible for generating a Master Secret, an Authentication System that receives a secret number from the TA, and a Client that receives a secret number from the TA. The Client performs an additional step through the system and method whereby it allows the user of the Client system to create a desired PIN number, which during this additional step results in a Token being generated. This Token can then be stored insecurely, and the PIN, which in the preferred embodiment of the invention, is committed to memory by the User of the Client for later use.

The Trusted Authority (TA) is a computer process running on a computer or server that generates a Master Secret. The TA is responsible for initially enrolling participants on the system, and allotting identifiers to these entities, for example, using an email address for Clients, and a URL for Authentication Services. The TA uses the identifiers to map them using a suitable hash function to points on a particular elliptic curve.

The Authentication Service is a computer process running on a computer or server that receives a secret number from the Trusted Authority. It participates with the Client in the authenticated key agreement protocol to mutually verify each other's identity and generate a session key.

The Client is a computer process running on a computer or server that that receives a secret number from the Trusted Authority. For avoidance of doubt, there is a user of the Client, generally a human being. The Client participates with the Authentication Service in the authenticated key agreement protocol to mutually verify each other's identity and generate a session key.

As per the illustration in FIG. 1, the Trusted Authority (TA) (10) can be software running on a computer, or a dedicated hardware device, that contains a processor (11) with the appropriate computer code running the instructions. Additionally, the TA will create a Master Secret (12) and store the Master Secret is Secured Storage (13), ideally, in the preferred embodiment of the invention, storage that is tamper proof and tamper resistant. In the preferred embodiment of the invention, the Trusted Authority (10) sends to the Client (30) over a secure network connection its Secret Number (32) a computer program, i.e. the Initialization Program (14), which programmatically enables the Client (30), using the Secret Number (32), to construct its PIN (34) and store the its Token (33) in the Client's browser storage (35) or other insecure storage medium.

The As per the illustration in FIG. 1, the Authentication Service (20) can be software running on a computer, or a dedicated hardware device, that contains a processor (21) with the appropriate computer code capable running the instructions. The Authentication Service (20) is enrolled with the Trusted Authority (10) to obtain its own Secret Number (22). Ideally, in the preferred embodiment of the invention, the Secret Number (22) is stored in Secured Storage (23), storage that is tamper proof and tamper resistant. In the preferred embodiment of the invention, the Authentication Service (20) sends to the Client (30) over a secure network connection a computer program, i.e. the Authentication Program (24), which programmatically enables the Client (30) to reconstruct it's Secret Number (32) from the PIN (34) and Token (33) and engages in the authenticated key agreement protocol with the Authentication Service (20).

The As per the illustration in FIG. 1, the Client (30) can be software running on a computer, or a dedicated hardware device, that contains a processor (31) with the appropriate computer code running the instructions. The Client will also have access to Storage (35) for the purpose of storing its Token (33). In the described embodiments of the invention, this Storage (35) can be the Client's browser's storage, or a wholly separate USB flash drive. This storage need not be protected at all. The PIN (34) is not stored, but in the preferred embodiment of the invention, is manually input by the human operator of the Client (30) in the protocol to assemble the Client's Secret Number (32).

In one embodiment of the invention, as shown in FIG. 2, the TA takes the first step of finding a non-supersingular curve with low, even k value (say k=2), and for which no distortion map exists 200. As an example, k=2 non-supersingular curve over F_p, where p=512 bits, such that the curve order is c.r. and is divisible by the 160-bit prime r. It now generates its own 210 Master Secret s∈F_r. The TA chooses a 220 suitable hash function H: {0, 1}* E(F_p2), which hashes identity strings it will receive during the Authentication Service and Client enrolment process (the IDs of the Authentication Service and Client) to a point on the supersingular curve. Finally, it publishes 230 the public parameters {p,r,H} to a readily accessible location that both Clients and Authentication Services can access, such as a public facing web site.

As shown in FIG. 3, and continuing the same embodiment of the invention, the Authentication Service is uniquely identified and authenticated 300 to the Trusted Authority. The Trusted Authority takes the Authentication Service's identity 310 as input for ID_s, this is mapped to a point S on the same curve as the previous calculation 320, but this time over the extension field F_p2 using a second hash function of H₂: {0, 1}* E(F_p2), such that S=H₂(ID_s) so S is a point on the curve E(F_p2). The TA issues back to the Authentication Service over a secure network connection the Secret Number S, the curve parameters and the H₂ hash function 330 along with the location of the public parameters.

As shown in FIG. 4, and continuing the same embodiment of the invention, the Client authenticates its 400 identity to the Trusted Authority. The Trusted Authority takes the Authentication Service's identity 410 ID_a as input; the identity is hashed and mapped to a point A of large prime order on the curve 420. The Client receives from the Trusted Authority 430 over a secure network connection A and _sA, where A=c.H(ID_a) is a point of order r on the base elliptic curve E(F_p).

As shown in FIG. 5, and continuing the same embodiment of the invention, the Client receives from the Trusted Authority over the secured network connection A and _sA. In the described embodiment of the invention, a computer program, the Initialization Program (for example, in JavaScript) is served from the Trusted Authority through a web server interface to the Client, 500 which in this case is a browser, where it runs. The program takes as input the user's PIN number 510, and will calculate αA where α is the user's PIN number.

The program then subtracts the two 520 to get the number (s−α)A. Then the program then stores 530 both (s−α)A and A in the Client's browser storage, and 540 prompts the user to remember their PIN. Note that as in a simple secret sharing scheme these two halves need to be reunited to reconstruct the correct value _sA. Additionally, the Client cannot determine the Master Secret s without solving a difficult discrete logarithm problem. Note we have a simple linear relationship of _sA=(s−α)A+αA.

As shown in FIG. 6, and continuing the same embodiment of the invention, the Client and Authentication Service want to mutually authenticate each other over a secure network connection. Using the described embodiment, the Client initiates a secured connection to the Authentication Service, 600 and the Authentication Service serves the Authentication Program to the Client. The Authentication Program runs locally on the Client (for example, in the Client's browser) and prompts the user of the Client for their PIN and their identifier (i.e., email address) 610. The Authentication Program hashes (using the same hash algorithm as the Initialization Program) ID_a and looks up the key/value 620 pair to obtain the concatenated (s−a)A and A which is then used in the following section.

As shown in FIG. 7, and continuing the same embodiment of the invention, the Client and Authentication Service want to mutually authenticate each other using the system and method over a secure network connection. As we will show, our system and method is adaptable to a number of authenticated key agreement protocols over the type-3 pairing. In the preferred embodiment of the invention, the system and method is adapted to use Wang's IDAK protocol¹. ¹ Wang's IEEE 1363.3 Submission—http://grouper.ieee.org/groups/1363/IBC/submissions/idak1363.pdf

In Wang's approach we assume ID_a and ID_s are the Client's identity and the Authentication Service's identity respectively. H₁(.) is a hash function that hashes to a point of order q on the G_a and H₂(.) is a hash function that hashes to a point of order q on the G₂, and H_q(.) is a hash function that hashes to a number in the range 1 to q. Assume that the Client and Authentication Service have been issued by the TA with secrets _sA and _sS respectively, where A=H₁(ID_a), S=H₂(ID_s), and s is the TA master secret. The user of the Client then divides her secret into a token value (s−α)A and a much smaller PIN value αA.

For the Client and Authentication Service to mutually authenticate each other, the Client and Authentication Service establish a secure network connection and 700 send their respective identities ID_a and ID_s to each other. They generate values of x<q at the Client and y<q at the Authentication Service 710. Next, 720 the Client calculates S where S=H₂(ID_s) and A where A=H₁(ID_a) to achieve P_a=_xA while the Authentication Service calculates A where A=H₁(ID_a) and S where S=H₂(ID_s) to achieve P_s=_yS. The Client and the Authentication Service 730 then exchange P_a and P_s, over the secure network connection with each other. Next, the Client and Authentication Service each 740 calculate r_a=H_q(P_a|P_s) and r_s=H_q(P_s|P_a). The Client then 750 calculates k=e((x+r_a)((s−α)A+αA), r_sS+P_s) so that it may achieve K where K=H(k) for M where M=H(ID_a, ID_s, K). The Authentication Service in parallel calculates 760 k=e(r_aA+P_a, (y+r_s)sS) so that it may achieve K where K=H(k) and N=H(ID_a, ID_s, K). In this particular preferred embodiment of the invention, the next steps are crucial to get right; the Client sends M over the secure connection 770 to the Authentication Service and this must happen first, before the Authentication Service can send anything to the Client, in order to prevent the Key Compromise Impersonation attack. The Authentication Service compares N=M, which could be done through the use of a one-time nonce, and if they match 780, the Authentication Service then sends N to the Client. The Client then compares N=M, 790 and if they match, they have successfully mutually authenticated each other and have a mutually agreed upon session key.

Note that for both parties observe that k=e(A,S)^{s(x+ra)(y+rs)}. Observe (and take comfort) from the fact that the Token and PIN are recombined locally before any value calculated from them is transmitted, so no-one is in the position to determine the PIN from transmitted values, irrespective of their computing power. If the wrong PIN is entered, the Authentication Service drops the connection.

However we would emphasis that this protocol needs a very careful implementation to avoid problems. Any elaboration (like encrypting the token secret with the PIN) or omission (like the Server not confirming the key before proceeding) could lead to a compromise of the system.

Claims

1. A method comprising: at the trusted authority, finding a non-supersingular curve with an even k for which no distortion map exists;generating a Master Secret sεFr;choosing hash function H: {0,1}*→E(Fp2); andpublishing the Public Parameters.

2. The method of claim 1 comprising: identifying and authenticating an authorized server to a Trusted Authority;at Trust Authority, taking the Authentication Service's identity as input for IDs mapped to a point, S, on the curve;using the extension field Fp2 using a second hash function of H: {0,1}*→E(Fp2) such that S=H2(IDs) so S, the secret number, is a point on the curve E(Fp2); andissuing back to the Authentication Service over a secure network connection the Secret Number S, the curve parameters and the H2 hash function along with the location of the public parameters.

3. The method of claim 2, further comprising: at a client, authenticating an identity to the Trusted Authority;at the Trusted Authority, taking the Authentication Service's identity IDa as input;at the Trusted Authority, hashing the identity IDs and mapping to a point A of large prime order on the curve; andat the client, received from the Trusted Authority over a secure network connection A and sA, where A=c.H(IDa) is a point of order r on the base elliptic curve E(Fp).

4. The method of claim 3, further comprising: taking as input a PIN number, a, and calculating aA;producing the number (s−a)A; andstoring both (s−a)A and A in the Client's browser storage.

5. The method of claim 4, further comprising: at an authentication program prompting the user of the Client for their PIN and their identifier; andat the Authentication Program hashing (using the same hash algorithm as the Initialization Program) IDa and looking up a key/value pair to obtain the concatenated (s−a)A and A.

6. The method of claim 5, further comprising: between the client and Authentication Service, sending identities Ida and IDs generating values of x<q at the Client and y<q at the Authentication Service;at the Client, calculating S where S=H2(IDs) and A where A=H1(IDa) to achieve Pa=xA while, at the Authentication Service, calculating A where A=H1(IDa) and S where S=H2(IDs) to achieve Ps=ys;between the Client and Authentication Service, exchanging Pa and Ps;at the Client and the Authentication Service, calculating ra−Hq(Pa|Ps) and rs=Hq (Ps|Pa);at the client, calculating k=e((x+ra)((s−a)A+aA),rss+Ps);at the Authentication Service, calculating k=e(raA+Pa(y+rs)sS);at the client, sending M over the secure connection to the Authentication Service in order to prevent the Key Compromise Impersonation attack;at the Authentication Service, comparing N=M, and if they match, sending N to the Client; andat the client, comparing N to M, and if they match, determining that the client and Authentication Service have successfully mutually authenticated each other and have a mutually agreed upon session key N.