Key agreement and transport protocol

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KEY AGREEMENT AND TRANSPORT PROTOCOL

This application is a continuation-in-part of United States Application No.

08/426,090.

The present invention relates to key agreement protocols for transfer and authentication of encryption keys.

To retain privacy during the exchange of information it is well known to encrypt data using a key. The key must be chosen so that the correspondents are able to encrypt and decrypt messages but such that an interceptor cannot determine the contents of the message.

In a secret key cryptographic protocol, the correspondents share a common key that is secret to them. This requires the key to be agreed upon between the correspondents and for provision to be made to maintain the secrecy of the key and provide for change of the key should the underlying security be compromised.

Public key cryptographic protocols were first proposed in 1976 by DiffieHellman and utilized a public key made available to all potential correspondents and a private key known only to the intended recipient. The public and private keys are related such that a message encrypted with the public key of a recipient can be readily decrypted with the private key but the private key cannot be derived from the knowledge of the plaintext, ciphertext and public key.

Key establishment is the process by which two (or more) parties establish a shared secret key, called the session key. The session key is subsequently used to achieve some cryptographic goal, such as privacy. There are two kinds of key agreement protocol; key transport protocols in which a key is created by one party and securely transmitted to the second party; and key agreement protocols, in which both parties contribute information which jointly establish the shared secret key. The number of message exchanges required between the parties is called the number of passes. A key establishment protocol is said to provide implicit key authentication (or simply key authentication) if one party is assured that no other party aside from a specially identified second party may learn the value of the session key. The property of implicit key authentication does not necessarily mean that the second party actually

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PCT/CA03/00317 possesses the session key. A key establishment protocol is said to provide key confirmation if one party is assured that a specially identified second party actually has possession of a particular session key. If the authentication is provided to both parties involved in the protocol, then the key authentication is said to be mutual if provided to only one party, the authentication is said to be unilateral.

There are various prior proposals which claim to provide implicit key authentication.

Examples include the Nyberg-Rueppel one-pass protocol and the MatsumotoTakashima-Imai (MTI) and the Goss and Yacobi two-pass protocols for key agreement.

The prior proposals ensure that transmissions between correspondents to establish a common key are secure and that an interloper cannot retrieve the session key and decrypt the ciphertext. In this way security for sensitive transactions such as transfer of funds is provided.

For example, the MTI/AO key agreement protocol establishes a shared secret K, known to the two correspondents, in the following manner:1. During initial, one-time setup, key generation and publication is undertaken by selecting and publishing an appropriate system prime p and generator a e Z* in a manner guaranteeing authenticity. Correspondent A selects as a longterm private key a random integer a,l<a<p-2, and computes a long-term public key z_A - c? mod p. B generates analogous keys b, ζθ. A and B have access to authenticated copies of each other's long-term public key.

2. The protocol requires the exchange of the following messages.

A —» B: c/ mod p (1)

A <- B: c/ modp (2)

The values of x and y remain secure during such transmissions as it is impractical to determine the exponent even when the value of a and the exponentiation is known provided of course that p is chosen sufficiently large.

3. To implement the protocol the following steps are performed each time a shared key is required.

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PCT/CA03/00317 (a) A chooses a random integer x, l<x<p-2, and sends B message (1) i.e. ¢/ mod p.

(b) B chooses a random integer y,l<y<p-2, and sends A message (2) i.e. cf mod p.

(c) A computes the key K = (o^y)^az_B^x mod p.

(d) B computes the key K = (c/)^bz_A^y mod p.

(e) Both share the key K - c?^x+ay.

In order to compute the key K, A must use his secret key a and the random integer x, both of which are known only to him. Similarly B must use her secret key b and random integer y to compute the session key K. Provided the secret keys a,b remain uncompromised, an interloper cannot generate a session key identical to the other correspondent. Accordingly, any ciphertext will not be decipherable by both correspondents.

As such this and related protocols have been considered satisfactory for key establishment and resistant to conventional eavesdropping or man-in-the-middle attacks.

In some circumstances it may be advantageous for an adversary to mislead one correspondent as to the true identity of the other correspondent.

In such an attack an active adversary or interloper E modifies messages exchanged between A and B, with the result that B believes that he shares a key K with E while A believes that she shares the same key K with B. Even though E does not learn the value of K the misinformation as to the identity of the correspondents may be useful.

A practical scenario where such an attack may be launched successfully is the following. Suppose that B is a bank branch and A is an account holder. Certificates are issued by the bank headquarters and within the certificate is the account information of the holder. Suppose that the protocol for electronic deposit of funds is to exchange a key with a bank branch via a mutually authenticated key agreement. Once B has authenticated the transmitting entity, encrypted funds are deposited to the account number in the certificate. If no further authentication is done in the encrypted

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PCT/CA03/00317 deposit message (which might be the case to save bandwidth) then the deposit will be made to E's account.

It is therefore an object of the present invention to provide a protocol in which the above disadvantages are obviated or mitigated.

According therefore to the present invention there is provided a method of authenticating a pair of correspondents A,B to permit exchange of information therebetween, each of said correspondents having a respective private key a,b and a public key p_A,pe derived from a generator a and respective ones of said private keys a,b, said method including the steps of

i) a first of said correspondents A selecting a first random integer x and exponentiating a function f(a) including said generator to a power g^w to provide a first exponentiated function f(a)^s(x);

ii) said first correspondent A forwarding to a second correspondent B a message including said first exponentiated function f(o)^gW;

iii) said correspondent B selecting a second random integer y and exponentiating a function f (a) including said generator to a power g^(y) to provide a second exponentiated function f (a)^g(y);

iv) said second correspondent B constructing a session key K from information made public by said first correspondent A and information that is private to said second correspondent B, said session key also being constructible by said first correspondent A for information made public by B and information that is private to said first correspondent A;

v) said second correspondent B generating a value h of a function F[ô,K] where F[ô,K] denotes a cryptographic function applied conjointly to δ and K and where ό is a subset of the public information provided by B thereby to bind the values of δ and K;

vi) said second of said correspondents B forwarding a message to said first correspondent A including said second exponential function f (a)^g(y) and said value h of said cryptographic function F[ô,K];

vii) said first correspondent receiving said message and computing a session key

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K' from information made public by said second correspondent B and private to said first correspondent A;

viii) said first correspondent A computing a value h' of a cryptographic function h,h' F[0,K']; and ix) comparing said values obtained from said cryptographic functions F to confirm their correspondence.

As the session key K can only be generated using information that is private to either A or B, the binding of K with δ with the cryptographic function h prevents E from extracting K or interjecting a new value function that will correspond to that obtained by A.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which.

Figure 1 is a schematic representation of a data communication system.

Figures 2 through 7 are schematic representations of implementations of different protocols.

Referring therefore to Figure 1, a pair of correspondents, 10,12, denoted as correspondent A and correspondent B, exchange information over a communication channel 14. A cryptographic unit 16,18 is interposed between each of the correspondents 10,12 and the channel 14. A key 20 is associated with each of the cryptographic units 16,18 to convert plaintext carried between each unit 16,18 and its respective correspondent 10,12 into ciphertext carried on the channel 14.

In operation, a message generated by correspondent A, 10, is encrypted by the unit 16 with the key 20 and transmitted as ciphertext over channel 14 to the unit 18.

The key 20 operates upon the ciphertext in the unit 18 to generate a plaintext message for the correspondent B, 12. Provided the keys 20 correspond, the message received by the correspondent 12 will be that sent by the correspondent 10.

In order for the system shown in Figure 1 to operate it is necessary for the keys 20 to be identical and therefore a key agreement protocol is established that allows the transfer of information in a public manner to establish the identical keys. Implementations are shown schematically in figures 2 through 7.

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Referring to figure 2, a mutual public key authenticated key agreement protocol is complemented between a correspondent A shown on the left hand side of the figure and a correspondent B shown on the right hand side. Correspondent A has a public-private key pair Pa,Sa respectively and similarly correspondent B has a public private Key pair Pb,S_b.

As a first step, correspondent A generates a session private key as a random number RND_A and computes a corresponding public session key Ga =Fa (RND_a). The function Fa is a cryptographic one way function, typically an exponention by the group generator, such as a point multiplication in an elliptic curve cryptosystem.

The public session key Ga is forwarded to correspondent B who generates corresponding parameters of a session private key RND_B and a public session key GbThe correspondent B computes a session key K as a function of A’s public information Ga,Pa AND B’s private information RND_b,Sb. A corresponding key K’ can be computed by A using the private information of A and the public information of B namely f(RNDA,GB,SA,PB)·

After correspondent B has generated the key K, he compiles a string (Ga H G_b H Id a) where Id_A is a string that identifies A. The concatenated string is hashed with a cryptographic function hk which is a keyed hash function that uses the key K to yield a string hashe.

The string hashu is forwarded to correspondent A together with Ma and G_B.

Upon receipt of the message from B, correspondent A computes the key K’ as described above. Correspondent A also computes a hash, hashverify_B from the string (G_b//Ga//M_a) using the hash function keyed by the key K’. Correspondent A checks that the hashes verify to confirm the identity of the keys K,K’.

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Correspondent A then computes a hash hie using the key K’ on the string (Ga//Gb//Wb) and forwards that together with Ids of correspondent B. Correspondent

B similarly computes a hashverifyA using the keyed hash function hie using the key K on the same string and verifies that hashA =hashverifyAA similar protocol is shown in figure 3 to implement a mutual entity authentication protocol. In this protocol the correspondents share a key K obtained over a secure channel. The correspondents A.B, each generate a random integer which is used as the session public key of A and B respectively. Thereafter the exchange of information and verification proceeds as above with respect to figure 2 with the shared secret key being utilised in the keyed hash functions.

A full mutual public key authenticated key exchange protocol is shown in figure 4. An initial exchange of the public keys Ρα,Ρβ is performed over an authenticated channel followed by the exchange of information as shown in the protocol of figure 4. In this case the correspondent A sends Ga computed as described above with respect to figure 2, together with a string x₂ that A wants confirmation of receipt by B. Correspondent B computes the key K as in figure 2 and also generates a pair of strings yi,y₂ which B wants to have authenticated by A and receipt confirmed by A respectively. The strings are sent to A with the hash hashp and identity Id_A..The hash kasha is performed on a string including the message x₂ and the string yi to be authenticated.

Correspondent A computes the key K and verifies the hash as before. This also confirms receipt of x₂ by B.

Correspondent A in turn generates strings zi,z₂ where Zj is a string that A wants authenticated by B and z₂ is a string that may be used in a subsequent execution of the protocol described below. The strings, zi and y₂ together with the identifying

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PCT/CA03/00317 information of B, Ids, are included in the string that is hashed with the key K to provide the string hash_A. This is sent together with the identity of B and the strings Z],Z2 to the correspondent B who can verify the hashes as before, thereby confirming receipt of y₂ and authentication of zi by the correspondent A.

Thus information is exchanged in an authenticated manner and a common key obtained that allows subsequent exchange of correspondence on a secure channel.

With the protocol described in figure 4 it is possible to implement a mutual public key authenticated key agreement protocol by letting the strings X2,yi,y2,zi,Z2 all be empty strings. Alternatively, a mutual public key authenticated key agreement protocol with implicit key agreement can be implemented by using x₂ as a string that is assumed to represent Eic(k), the result of applying an encryption function E with key K on the value of k. Correspondent B can compute the value of K and hence retrieve the notional value of k from the string. He can use this as his shared session key with the correspondent A. The value of yi may be used to represent E_K(k₂i) and Z] as E_K(k]2) where k₂i and kj₂ are different keys for communication or other secret information to be shared between the correspondents. In this case y₂ and Z2 are empty strings. In this way there is a key agreement on a shared key Kab together with authenticated key transport of the keys k21 and k]₂ between the correspondents and authenticated key agreement on k. Moreover, if additional information is provided in the strings x₂ and y₂ then confirmation of proper receipt is also obtained.

The protocol of figure 4 may also be used to increase efficiency in successive sessions by using the string Z2 to pass the information exchanged in the first pass of the next session. Thus as shown in figure 5, the string Ga,X2 is sent as Z2 in the previous session. The protocol then proceeds from correspondent B as before. As seen in Figure 5, the third transmission may be optionally omitted. Correspondent B may also take advantage of this facility by including the information Gb,Yi for the next session in the exchange as y₂.

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The mutual public key authenticated key agreement protocol may also be adapted for entity authentication implementations as shown in figure 6. In this case, as in figure 3 above, the key generation is omitted as the correspondents have a shared key obtained over a secure channel.

Similarly, the protocol of figure 6 may be modified as illustrated in figure 7 to take advantage of the exchange of information in a previous session, similar to that of figure 5.

It will be seen therefore that a number of versatile and flexible protocols can be developed from the general protocol to meet particular needs. These protocols may implement elliptic curve cryptography or operate in Zp as preferred.

It can be readily seen that the message flows of the public-key authenticated key agreement protocol depicted in Figure 3 and those of the entity authentication protocol depicted in Figure 2 have identical structure. Moreover, the computation of the hash values hashA and hashB by correspondent A and B respectively, as well as the verification thereof, take strings with an identical structure as input. In fact, both protocols only differ in the way the key K used in the protocol is derived. Thus, a combined implementation of both protocols may take advantage of a common handling of telecommunication flows, including messaging, error handling and thelike, and may take advantage of a common handling of the key confirmation steps (i.e., generation and processing of hash values).

A similar reasoning holds for the message flows and processing steps of the public-key authenticated key agreement protocol depicted in Figure 4 and the version thereof depicted in Figure 5. It will be appreciated that the latter consists of executing only part of the former. A similar remark holds for the entity authentication protocol depicted in Figure 6 and the one depicted in Figure 7. It shouls also be noted that the

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PCT/CA03/00317 augmented public-key authenticated key agreement protocol depicted in Figure 4 can be used to implement the one depicted in Figure 3 and that, similarly, the augmented entity authentication protocol depicted in Figure 6 can be used to implement the one depicted in Figure 2. Thus, all the protocols described can be implemented with largely common routines to handle telecommunication and message flows and with a large degree of commonality of the implementation of the protocol steps of each and every one of the protocols.

It will be appreciated that although the invention has been described with reference public key based agreement protocols and entity authentication protocols, it may equally be utilized on symmetric key agreement protocols.