Secure authenticated channel

Detailed description of the invention

[Technical field of invention]

The present invention relates generally to secure authentication channels, and more particularly to the calculation of session keys for establishing such channels for the protection of digital content in digital television systems and the like.

[Background of invention]

Secure authentication channels, well known in the field of cryptography, are established to allow two mutually authenticated devices (often referred to as peers) to exchange information in secret. The secure authentication channel should preferably have the following characteristics: -Mutual authentication of peers-Key verification That is, a common secret is established, and it is possible for at least one peer to prove that the secret is actually common. -Forward secrecy That is, old session keys cannot be calculated even when the private key (such as the authentication private key) is known for a long period of time.

These properties are formally mathematically provable, and the entire protocol may be broken relatively easily if there is a way around one of the above properties for a given cryptographic protocol. ..

Over the years, the crypto community has proposed a number of protocols for secure authentication channels. Only some of these channels have been shown to satisfy the above characteristics.

All protocols that provide the channel with the required characteristics are several different cryptographic primitives, namely at least one asymmetric cryptographic primitive (such as asymmetric cryptography or digital signature), a hash function, a message authentication code (MAC), and some of the above. Use other primitives such as symmetric encryption in. The problem with these protocols is that they are very resource consuming and difficult to implement in devices with limited computing power, such as portable security modules such as smart cards. Another problem is that the use of many crypto primitives makes it difficult to prove that the protocol is secure.

The present invention provides a secure access channel protocol that has the required properties and is particularly suitable for implementation in devices with limited computing power.

Throughout this description, it is assumed that the basic concept is well known, as encryption is a mature technique. For the sake of simplicity, these concepts will not be described beyond what is necessary for an understanding of the present invention.

[Outline of Invention]

In the first feature, the present invention relates to a method of calculating a session key common to the first and second devices (11, 21). The first device is the public key (g^a) And the identity (ID) that corresponds to itself_a) And a certificate (C)_a) And their corresponding identity (ID)_a), Private key (a), public key (g)^a) And know. The second device has the corresponding certificate and knowledge. The first device selects the first ephemeral private key (x) and the first ephemeral public key (g).^x) And its certificate (C_a) And the 1st ephemeral public key (g)^x) And is sent to the second device. Certificate of the first device (C_a) And the 1st ephemeral public key (g)^x) And, the second device receives the certificate of the first device (C_a), Select the 2nd ephemeral private key (y), and select the 2nd ephemeral public key (g)^y) Is calculated and the first ephemeral public key (g)^x) And the second ephemeral private key (y) and the ephemeral shared key (K)_eph) Is calculated, and the public key (g) of the first device is calculated.^a) And its own private key (b) to the permanent key (K)_perm) Is calculated and the second ephemeral public key (g)^y) And the ephemeral shared key (K)_eph) And the permanent key (K)_perm) And the identity (ID) that corresponds to itself_b) And the first value (H (g)^y, K_eph, K_perm, ID_b)) Calculate and certificate of it (C_b) And the second ephemeral public key (g^y) And the first value (H (g)^y, K_eph, K_perm, ID_b)) And is transmitted to the first device. Certificate of the second device (C_b) And the second ephemeral public key (g^y) And the first value (H (g)^y, K_eph, K_perm, ID_b)) And is received from the second device, the first device receives the certificate of the second device (C_b), And the second ephemeral public key (g)^y) And the first ephemeral private key (x) and the ephemeral shared key (K)_eph) Is calculated, and the public key (g) of the first device is calculated.^y) And its own private key (a) to the permanent key (K)_perm) Is calculated and the first value (H (g)^y, K_eph, K_perm, ID_b)) Validate the 1st ephemeral public key (g)^x) And the ephemeral shared key (K)_eph) And the permanent key (K)_perm) And the identity (ID) that corresponds to itself_a) And the second value (H (g)^x, K_eph, K_perm, ID_a)) And the second value (H (g)^x, K_eph, K_perm, ID_a)) Is sent to the second device. Second value (H (g)^x, K_eph, K_perm, ID_a)), The second device receives the second value (H (g)).^x, K_eph, K_perm, ID_a)) Validate the ephemeral shared key (K)_ephSession key (K) as a function of)_sess) Is calculated. The first device is also the ephemeral shared key (K)_ephSession key (K) as a function of)_sess) Is calculated.

In the second feature, the present invention relates to a first device (11) that participates in session key calculation together with a second device (21). The first device is the public key (g^a) And the identity (ID) that corresponds to itself_a) And the corresponding identity (ID)_a), Private key (a), public key (g)^a) And know. The first device selects the ephemeral private key (x) and the first ephemeral public key (g).^x) And the public key (g)^b) And the identity of the second device (ID)_b) And its certificate (C)_a) And the 1st ephemeral public key (g^x) And to the second device, and the second device (C)_b) Certificate and 2nd ephemeral public key (g)^y) And the second ephemeral public key (g^y), Ephemeral shared key (K_eph), Permanent key (K)_perm), And the identity (ID) corresponding to the second device_b) And the first value (H (g)^y, K_eph, K_perm, ID_b)) And is received from the second device, and the certificate of the second device (C)_b), And the second ephemeral public key (g)^y) And the ephemeral private key (x) and the ephemeral shared key (K)_eph) Is calculated, and the public key (g) of the first device is calculated.^b) And its own private key (a) to the permanent key (K)_perm) Is calculated and the first value (H (g)^y, K_eph, K_perm, ID_b)) Validate the 1st ephemeral public key (g)^x) And the ephemeral shared key (K)_eph) And the permanent key (K)_perm) And the identity (ID) that corresponds to itself_a) And the second value (H (g)^x, K_eph, K_perm, ID_a)) And the second value (H (g)^x, K_eph, K_perm, ID_a)) Is sent to the second device, and the ephemeral shared key (K)_ephSession key (K) as a function of)_sess) Has a processor (12) to calculate.

In a third feature, the present invention relates to a second device (21) that participates in the calculation of the session key together with the first device (11). The second device is the public key (g^b) And the corresponding identity (C)_b) And the corresponding identity (ID)_b), Private key (b), public key (g)^b) And know. The second device is the public key of the first device (g)^a) And identity (ID_a) And the certificate of the first device (C)_a) And the 1st ephemeral public key (g^x) And select the ephemeral private key (y) and the second ephemeral public key (g)^y) Is calculated and the first ephemeral public key (g)^x) And the ephemeral private key (y) from the ephemeral shared key (K)_eph) Is calculated, and the public key (g) of the first device is calculated.^a) And its own private key (b) to the permanent key (K)_perm) Is calculated and the second ephemeral public key (g)^y) And the ephemeral shared key (K)_eph) And the permanent key (K)_perm) And the identity (ID) that corresponds to itself_b) And the first value (H (g)^y, K_eph, K_perm, ID_b)) Calculate and certificate of it (C_b) And the second ephemeral public key (g^y) And the first value (H (g)^y, K_eph, K_perm, ID_b)) And is sent to the 1st device, and from the 1st device, the 1st ephemeral public key (g)^x) And the ephemeral shared key (K)_eph) And the permanent key (K)_perm) And the identity (ID) corresponding to the first device_a) And the second value (H (g)^x, K_eph, K_perm, ID_a)) Is received and the second value (H (g)^x, K_eph, K_perm, ID_a)) Validate the ephemeral shared key (K)_ephSession key (K) as a function of)_sess) Has a processor (22) to calculate.

[Detailed description of the invention]

FIG. 1 shows a session key exchange according to an embodiment of the present invention.

Prior to the start of this method, the first device 11 is its identity ID._aAnd its own private key a and public key g^aI know that. g^aIs g^a Abbreviation for mod p, as is well known in the art, a is the private key of the first device, g is a known generator, and p is a known prime number. The second device 21 has the corresponding ID_b, B, g^bknow. Certificates for these devices are public keys and identities, namely C_a(g^a, ID_a) And C_b(g^b, ID_b) Each. Devices 11 and 12 also have processors (CPUs) 12 and 22 configured to perform each step of the method.

In step 252, the first device 11 preferably randomly selects the first ephemeral private key x, which is its certificate C by message 254._a(g^a, ID_a) Send with ephemeral public key g^xTo calculate.

Upon receiving message 254, the second device 21 receives the certificate C of the first device 11 in step 256._a(g^a, ID_a) Is verified. If the verification is unsuccessful, the second device 21 abandons this method. However, if the validation is successful, it preferably randomly selects the second ephemeral private key y and the second ephemeral public key g.^yAnd ephemeral sharing key K_eph= g^xyAnd Diffie-Hellman permanent key K_perm= g^abIs calculated in step 258.

In step 260, the second device 21 is the second ephemeral public key g.^y, Ephemeral shared key K_eph, Diffie-Hellman Permanent Key K_permAnd its identity ID_bAnd the first hash value H (g) using an appropriate hash function, such as one of many functions known in the art.^y, K_eph, K_perm, ID_b) Is calculated. Appropriate functions other than the hash function may be used for this purpose and in the calculation of the following hash values in this embodiment. The second device 21 is then the second ephemeral public key g^y, It's certificate C_b(g^b, ID_b) And the first hash value H (g)^y, K_eph, K_perm, ID_b) Is transmitted to the first device 11 by message 262.

Upon receiving message 262, the first device 11 receives the certificate C of the second device 21 in step 264._b(g^b, ID_b) Is verified. If the verification is unsuccessful, the first device 11 abandons this method. However, if the verification is successful, the first apparatus 11 will have the ephemeral sharing key K in step 266._ephAnd Diffie-Hellman Permanent Key K_permAnd calculate. In step 268, the first device 11 verifies the first hash value using the same hash function as the second device 21 used in step 260. If the first hash value is not authenticated, the first device 11 interrupts the method, but if the first hash value is authenticated, the first device 11 in step 270, the first ephemeral public key g.^x, Ephemeral shared key K_eph, Diffie-Hellman Permanent Key K_permAnd its identity ID_a2nd hash value H (g)^x, K_eph, K_perm, ID_a) Is calculated. The first device 11 receives the message 272 and the second hash value H (g).^x, K_eph, K_perm, ID_a) Is transmitted to the second device 21.

Upon receiving message 272, the second device 21 uses the same hash function used by the first device 10 in step 270 in step 274 to use the second hash value H (g).^x, K_eph, K_perm, ID_a) Is verified. If the second hash value is not authenticated, the second device 21 interrupts the protocol, and if the second hash value is authenticated, the second device 21 performs the ephemeral sharing key K in step 276._ephSession key K by calculating the hash value of_sessTo calculate. Then it has a second hash value H (g)^x, K_eph, K_perm, ID_a) Is successfully authenticated and the session key K_sessIs sent to the first device 11 with a ready message 278 to indicate that has been calculated.

Upon receiving the ready message 278 from the second device 21, the first device 11 uses the same hash function used by the second device 21 in step 276 in step 280 to use the ephemeral shared key K._ephSame session key K by calculating the hash value of_sessTo calculate. Then the first device 11, it is also the session key K_sessA ready message 282 is sent to the second device 21 to indicate that it has calculated.

At this point, the first device 11 and the second device 21 can use the session key K to protect the information transmitted between them._sessOwn. According to the protocol of the present invention, the confidentiality of the private key is guaranteed, and authentication and key verification are reciprocal. In addition, low bustness and forward confidentiality against previous session key leaks are also guaranteed. Those skilled in the art will appreciate that the three hash functions described for steps 212, 220 and 226 may be different, identical, or two of them may be identical and the other one may be different. Will.

When this description refers to random numbers, it should be noted that these numbers are often in fact pseudo-random.

The expression "security module" refers to devices such as smart cards, PC cards (formerly known as PCMCIA cards), televisions that have processors and can be used to establish secure authentication channels according to the invention. Includes any type of portable or fixed security module, such as an integrated circuit coupled to a printed circuit board.

The above-described embodiment is particularly suitable for implementation in digital television sets and security modules. However, those skilled in the art will appreciate that the present invention is feasible and available by any type of device having the required resources, i.e., a processor and preferably a memory for storing the required information. Will do. Non-limiting examples of other devices include DVD players, computers that interact with external accessories, ATMs (Automatic Teller Machines), and bank cards.

<figref num="1">FIG. 1 shows a session key exchange according to an embodiment of the present invention.</figref>