Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
  • H04L9/30—Public key, i.e. encryption algorithm being computationally infeasible to invert or user's encryption keys not requiring secrecy
  • H04L9/3066—Public key, i.e. encryption algorithm being computationally infeasible to invert or user's encryption keys not requiring secrecy involving algebraic varieties, e.g. elliptic or hyper-elliptic curves
  • H04L9/3073—Public key, i.e. encryption algorithm being computationally infeasible to invert or user's encryption keys not requiring secrecy involving algebraic varieties, e.g. elliptic or hyper-elliptic curves involving pairings, e.g. identity based encryption [IBE], bilinear mappings or bilinear pairings, e.g. Weil or Tate pairing
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
  • H04L9/32—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols including means for verifying the identity or authority of a user of the system or for message authentication, e.g. authorization, entity authentication, data integrity or data verification, non-repudiation, key authentication or verification of credentials
  • H04L9/3247—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols including means for verifying the identity or authority of a user of the system or for message authentication, e.g. authorization, entity authentication, data integrity or data verification, non-repudiation, key authentication or verification of credentials involving digital signatures
  • H04L9/3255—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols including means for verifying the identity or authority of a user of the system or for message authentication, e.g. authorization, entity authentication, data integrity or data verification, non-repudiation, key authentication or verification of credentials involving digital signatures using group based signatures, e.g. ring or threshold signatures
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L2209/00—Additional information or applications relating to cryptographic mechanisms or cryptographic arrangements for secret or secure communication H04L9/00
  • H04L2209/46—Secure multiparty computation, e.g. millionaire problem
  • H04L2209/463—Electronic voting
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L2209/00—Additional information or applications relating to cryptographic mechanisms or cryptographic arrangements for secret or secure communication H04L9/00
  • H04L2209/76—Proxy, i.e. using intermediary entity to perform cryptographic operations

Definitions

• the present invention relates in general to cryptography and secure communication via computer networks or via other types of systems and devices, and more particularly to schemes for generating and verifying signatures of communications in systems using public key cryptography.
• each party is associated with both a private and a public key.
• the public keys are known publicly, or are available from a Certificate Authority.
• a signer uses its private key. Because the signer's private and public keys are related, a verifier may then verify the signature using the signer's public key. Because the signer's private key is known only to the signer (and perhaps to a private key generator, or PKG), third parties are not able to forge the signer's signature.
• the various embodiments of the present invention also are compatible with identity-based signature schemes.
• identity-based signature schemes are public key schemes in which the public key of an entity is derived from information associated with the entity's identity.
• the identity information may be personal information (i.e., name, address, email address, etc.), or computer information (i.e., IP address, etc.).
• identity information may include not only information that is strictly related to an entity's identity, but also widely available information such as the time or date. That is, the importance of the concept of identity information is not its strict relation to the entity's identity, but that the information is readily available to anyone who wishes to encrypt a message to the entity.
• An entity's private key in an identity-based system using public key cryptography is generated and distributed by a trusted party or logical process, typically known as a private key generator ("PKG").
• PKG uses a master secret to generate private keys.
• An entity's public key may be derived from its identity, when Bob wants to verify a signature from Alice, he does not need to retrieve Alice's public key from a database. Instead, Bob merely derives the key directly from Alice's identifying information. Databases of public keys are unnecessary. Certificate authorities (“CAs”) also are unnecessary. There is no need to "bind" Alice's identity to his public key because his identity is his public key.
• CAs Certificate authorities
• identity-based schemes involve a hierarchy of logical or actual PKGs.
• a root PKG may issue private keys to other PKGs, who in turn may issue private keys to users in particular domains.
• This enables a verifier to verify a signature from a signer without an online lookup of the signer's public key or lower-level public parameters, even if the verifier is not in the system at all, as long as the verifier obtained the public parameters of the root PKG.
• Another advantage of a hierarchical identity-based signature scheme is damage control.
• Ring signatures were recently introduced in R. Rivest, A. Shamir, Y. Tauman, How to Leak a Secret, ADVANCES IN CRYPTOLOGY— ASIACRYPT 2001 , Lecture Notes in Computer Science 2248 (2001), Spring, 552. Ring signatures enable a member of a group (not necessarily established a priori) to sign a message such that a third party can verify that some member of the group created the signature but cannot determine which member.
• efficient ring signature schemes have not been available for identity-based encryption schemes. Accordingly, there is a need for identity-based ring signature schemes.
• online signing time is more important than the total signing time.
• efficiency of the scheme can be increased by enabling more of the signature and verification algorithms to be performed offline.
• Online/offline signature schemes were proposed in A. Shamir, Y. Tauman, Improved Online/Offline Signature Schemes, ADVANCES IN CRYPTOLOGY— CRYPTO 2001, Lecture Notes in Computer Science 2139 (2001), Springer, 355-367.
• online/offline signature schemes have not been available for hierarchical identity-based systems. Accordingly, there is a need for efficient online/offline hierarchical identity-based signature schemes.
• each signer signs a subset of the messages, and the subset signed by a at least one signer is different from the subset signed by at least one other signer.
• a private key is selected for each of the signers and a message function value is generated for each of the messages using a predetermined function.
• One or more signature components are generated for each of the signers using at least the private key associated with each signer and the message function value associated with the message to be signed.
• a digital signature is generated using each of the signature components. The digital signature is then verified using at least the message function values.
• a digital signature of a digital message communicated between a signer and a verifier wherein the signer is one of a plurality of members of a set.
• the signer is associated with an identity, and the other members of the set also are associated with identities.
• First and second cyclic group of elements are generated, and a bilinear, non-degenerate pairing is selected that is capable of generating an element of the second cyclic group from two elements of the first cyclic group.
• First and second generators of the first cyclic group are selected, as is a function capable of generating an element of the first cyclic group from a string of binary digits.
• Public points are generated for each of the members of the set, and a private point is generated for the signer.
• the digital message is signed by generating the digital signature using at least the signer's private point and the public point of each of the members of the set. Using at least the public point of each of the members of the set, the signature is verified to have been generated by a member of the set.
• a proxy signer for generating and verifying a proxy signature of a digital message communicated between a proxy signer and a verifier, wherein the message is signed by the proxy signer on behalf of an original signer.
• An original signer private key and an original signer public key, both associated with the original signer, are selected.
• a proxy signer private key and a proxy signer public key, both associated with the proxy signer also are selected.
• a proxy private key is then generated using the original signer private key and the proxy signer public key.
• a message function value is generated using a predetermined function, and the proxy signature is generated using at least the message function value, the proxy private key, and the proxy signer public key.
• the proxy signature may be verified using at least the original signer public key, the proxy signer public key, and the message function value.
• a signer is t levels below a root PKG in a hierarchical system.
• the signer is associated with an ID-tuple that includes identity information associated with the signer and with each of the t— 1 lower-level PKGs in the hierarchy between the root PKG and the signer.
• a lower-level public key is generated for each of the lower-level PKGs.
• a signer private key associated with the signer is generated. In an offline mode, a random signer trapdoor secret, a random message, and a random number are selected.
• An offline signature is then generated using the random message, the random number, the trapdoor secret, the signer private key, and the lower-level public keys associated with the t-1 lower-level PKGs.
• a matching random number is determined such that the matching random number matches the offline signature with the message to be signed.
• the offline signature may then be verified using the matching random number.
• FIG. 1 shows a flow diagram illustrating a method of generating and verifying a multisignature Sig of a digital message Maccording to one presently preferred embodiment of the invention
• FIG. 2 shows a shows a flow diagram illustrating a method of generating and verifying a ring signature Sig of a digital message M communicated between a signer and a verifier according to another presently preferred embodiment of the invention
• FIG. 3 shows a flow diagram illustrating a method of generating and verifying a ring signature Sig of a digital message M communicated between a signer and a verifier in a hierarchy according to another presently preferred embodiment of the invention
• FIG. 4 shows a flow diagram illustrating a method of generating and verifying a proxy signature Sig of a digital message Maccording to another embodiment of the present invention
• FIG. 5 shows a flow diagram illustrating a method of generating and verifying a signature Sig of a digital message Maccording to another embodiment of the present invention, wherein parts of the Signature and Verification algorithms may be completed offline; and [24] FIG. 6 shows a block diagram depicting a system for implementing signature schemes according to another embodiment of the present invention.
• the present invention provides public key signature schemes. These include both identity-based schemes and non-identity-based schemes. They also include both hierarchical and non-hierarchical schemes.
• Each of the hierarchical identity-based signature schemes of the present invention requires a hierarchical structure of PKGs, including at least one root PKG and a plurality of lower-level PKGs.
• the hierarchy and the lower-level PKGs may be logical or actual. For instance, a single entity may generate both a root key generation secret and the lower-level key generation secrets from which lower-level users' encryption or signature keys are generated.
• the lower-level PKGs are not separate entities, but are merely processes or information arranged in a logical hierarchy and used to generate keys for descendent PKGs and users in the hierarchy.
• each lower-level PKG may be a separate entity.
• Another alternative involves a hybrid of actual and logical lower-level PKGs.
• the term "lower-level PKG" will be used generically to refer to any of these alternatives.
• identity-based public keys may be based on time periods. For instance, a particular signer's identity may change with each succeeding time period. Alternatively, a signer may arrange the time periods as children or descendents of itself in a hierarchy, and a verifier would use the identity of the proper time period when verifying the signature. Either way, each key may be valid for signing messages only during the associated time period.
• the hierarchical identity-based schemes of the present invention generally include five algorithms: Root Setup, Lower-level Setup, Extraction, Signing, and Verification. Three of these algorithms rely upon the identities of the relevant entities in the hierarchy.
• Each user preferably has a position in the hierarchy that may be defined by its tuple of IDs: (ID-i, . . . , ID,).
• the user's ancestors in the hierarchy are the root PKG and the users, or PKGs, whose ID-tuples are ⁇ (ID-i, . . . , ID;) : 1 ⁇ ⁇ (t-1) ⁇ .
• the ID-tuples preferably are represented as binary strings for purposes of computations.
• each lower-level PKG uses a security parameter k to generate public system parameters params and a root key generation secret.
• the system parameters include a description of the message space M. and the signature space S.
• the system parameters will be publicly available, while only the root PKG will know the root key generation secret.
• each lower-level PKG preferably generates its own lower-level key generation secret for purposes of extraction. Alternatively, a lower-level PKG may generate random one-time secrets for each extraction.
• a PKG (whether the root PKG or a lower-level PKG) generates a private key for any of its children.
• the private key is generated using the system parameters, the generating PKG's private key, and any other preferred secret information.
• the signer of a digital message signs the message Me M to generate a signature Sig e S using params and the signer's private key d.
• the verifier of the signed message verifies the signature Sig using params and the ID-tuple of the signer.
• the presently preferred signature schemes of the present invention are based on pairings, such as, for instance, the Weil or Tate pairings associated with elliptic curves or abelian varieties.
• the signature schemes also are based on the Diffie-Hellman Problem or the Bilinear Diffie- Hellman Problem.
• the schemes use two cyclic groups Gi and G 2 , preferably of the same large prime order q.
• the first group Gi preferably is a group of points on an elliptic curve or abelian variety, and the group law on i may be written additively.
• the second group G 2 preferably is a multiplicative subgroup of a finite field, and the group law on G 2 may be written multiplicatively.
• other types of groups may be used as i and G 2 consistent with the present invention.
• the methods also use a generator E 0 of the first group Gi.
• a pairing or function e : Gi x Gi -> G 2 is provided for mapping two elements of the first group i to one element of the second group G 2 .
• the function e preferably satisfies three conditions.
• the function e preferably is bilinear, such that if Q and R are in Gi and a and b are integers, then e(aQ, bR) — e(Q, R) .
• the function e preferably is non-degenerate, such that the map does not send all pairs in i x i to the identity in G 2 .
• the function e preferably is efficiently computable. A function e satisfying these three conditions is considered to be admissible.
• the security of the signature schemes of the present invention is based primarily on the difficulty of the Diffie-Hellman Problem or the Bilinear Diffie-Hellman Problem.
• the Diffie-Hellman Problem is that of finding abP given a randomly chosen P e Gi, as well as aP and bE (for unknown randomly chosen a, b, c e Z/qZ).
• the Bilinear Diffie-Hellman problem is that of finding e(P, P) abc given a randomly chosen P e Gi, as well as aP, bP, and cP (for unknown randomly chosen a, b, c e ZlqZ).
• a randomized algorithm XQ is a Bilinear Diffie-Hellman generator if XQ takes a security parameter k > 0, runs in time polynomial in k, and outputs the description of two groups Gi and G 2 , preferably of the same prime order q, and the description of an admissible pairing e G ⁇ x G-i -» G 2 .
• XQ is a Bilinear Diffie-Hellman parameter generator
• the advantage Adv ⁇ Q (B) that an algorithm B has in solving the Bilinear Diffie-Hellman problem is defined to be the probability that the algorithm B outputs e(P, P) abc when the inputs to the algorithm are G-i, G 2 , e, P, aP, bP, and cP, where (G-i, G 2 , e) is the output of XQ for a sufficiently large security parameter k, P is a random generator of Gi, and a, b, and c are random elements of Z/qZ.
• the assumption underlying the Bilinear Diffie-Hellman problem is that Ad v 1Q (B) is negligible for all efficient algorithms B.
• a similar assumption underlies the Diffie-Hellman Problem.
• a multisignature scheme is any scheme that allows several signers to sign a document (or documents) in a way that is somehow more efficient than if they each signed the document separately.
• this enhanced efficiency is in terms of signature length — i.e., the combined multisignature of n signers is shorter than n separate signatures. This is convenient for transactions that require one of the parties to acquire pre-approval from multiple sources and forward this multiple pre-approval prior to the transaction.
• the length of the multisignature is dependent at least upon the number of signers or the number of documents signed.
• the present invention provides more efficient multisignature signature schemes that enable multiple signers to sign multiple documents to generate a multisignature having a length that is independent of both the number of signers and the number of documents.
• FIG. 1 shows a flow diagram illustrating a method of generating and verifying a multisignature according to one presently preferred embodiment of the invention.
• this embodiment enables multiple signers to sign multiple documents.
• the resulting signature may be represented as a single element of a group, such as a single point on an elliptic curve, regardless of the number of different signers or the number of different documents signed.
• the multisignature scheme described with reference to FIG. 1 allows n signers to sign m digital messages and generate a single multisignature. Each signer signs a subset of the m messages. Unlike previous signature schemes, the present scheme enables different signers to sign different sets of messages without sacrificing efficiency.
• the method begins in block 102 by generating first and second cyclic groups i and G 2 of elements.
• a function e is selected, such that the function e is capable of generating an element of the second cyclic group G 2 from two elements of the first cyclic group G-i.
• the function e preferably is an admissible pairing, as described above.
• a generator P of the first cyclic group i is selected in block 106.
• a function H is selected such that H is capable of generating an element of the first cyclic group i from a first string of binary digits.
• the function H may be a hash function.
• a private key _?,- is selected for each of the n signers.
• message function values P M are generated for each of the m messages in block 112.
• Additional security measures may be useful to prevent a particular type of attack on the multisignature scheme described with reference to FIG. 1.
• the attacker modifies its public key / private key pair depending on some other party's public key so that the attacker can forge, without the other party's participation, a single-message multisignature with itself and the other party as the putative signers.
• the ultimate verifier of the multisignature may not be reassured by this approach because the verifier still has to trust that the party that collected the signature components verified those signature components correctly.
• the attack can be thwarted by requiring each signer to individually sign some message that is unique to the signer, such as, for instance, a message that contains the signer's identity information or public key, or a message that is randomly chosen for each signer.
• P M may be set to H ⁇ s.P,M ⁇ .
• the CA may require the signer to sign some "challenge" message chosen by the CA before it issues a certificate to the signer. (In fact, CAs often require this already.) In either case, the verifier is able to independently verify the multisignature without any reassurance from the party that collected the signature components.
• other methods also may be used to thwart the attack.
• Ring signatures enable a member of a group (not necessarily established a priori) to sign a message such that a third party can verify that some member of the group created the signature but cannot determine which member. For example, consider a cabinet member who wants to leak a secret to the press, but wants to maintain limited anonymity — i.e., he wants the press to know that he is a cabinet member, but not which member. Ring signatures allow a signer to choose any set containing the signer, and to prove that the signer is a member of that set without disclosing which member. Thus, a cabinet member may use his private key in combination with the public keys of the other cabinet members to create a ring signature for the cabinet. Because the cabinet-specific ring signature could only be created by a cabinet member, the press can use this signature to prove the authenticity of its anonymous source.
• Ring signatures also may be useful in contract negotiations.
• party A may wish to provide authentication but not nonrepudiation — i.e., it may want to prove to party B that the draft contract came from party A, but it may not want to give party B the ability to prove to a third party (i.e., a court) that party A signed the draft.
• party A can create a ring signature for the set ⁇ A, B).
• Party B will know that party A created the signature because party B did not create the signature itself.
• party B will not be able to convince third parties that party A created the signature because, from the perspective of third parties, party B also could have created the signature.
• FIG. 2 shows a flow diagram illustrating a method of generating and verifying a digital signature Sig of a message M communicated between a signer and a verifier according to another presently preferred embodiment of the invention.
• the signer in this case is one of t members of a set.
• the signer is associated with identity ID-i, and the other members of the set are associated with identities ID, for 2 ⁇ i ⁇ t. Note, however, that if the anonymous signer always were associated with the first listed identity, the signature would not actually be anonymous.
• the method begins in block 202 by generating first and second cyclic groups Gi and G 2 of elements.
• a function e is selected, such that the function e is capable of generating an element of the second cyclic group G 2 from two elements of the first cyclic group -i.
• the function e preferably is an admissible pairing, as described above.
• First and second generators, E and P ' , of the first group Gi are selected in block 206.
• first and second secret numbers s and s ' are selected.
• a new -. ' may be chosen for each new signature.
• a function H is selected in block 210 such that the function His capable of generating an element of the first cyclic group Gi from a first string of binary digits.
• a public point P i H ⁇ (ID.) is generated for each of the t members of the set.
• a private point sP' + s 'Ei is generated for the signer in block 214.
• the digital message Mis signed in block 216 by generating the digital signature Sig using at least the signer's private point (sP r + s 'Ei) and the public point P ⁇ of each of the t members of the set.
• the private key provided by the PKG for the first entity may be represented as (sP'+ r x P r x P)
• the private key provided by the PKG for the second entity may be represented as (-.E'-f- r 2 P 2 ,r 2 P)
• the first part of the first point in each of the private keys is sP' , which ties each of the entities to the PKG. This portion of each private key must remain constant. The remainder of each private key may be changed, however.
• an equally valid private point for the first entity is (sP'+ r ⁇ P x ,r ⁇ P ) for any r .
• This flexibility is exploited to enable the creation of a ring signature.
• a ring signature for these two entities has the form (sP'+ r P x + r P 2 , r P, r 2 P) for some r and r .
• the identities of both entities are embedded in this ring signature by the use of their public points P- ⁇ and E 2 .
• either client can produce such a ring signature.
• the first entity may produce a ring signature for both entities as follows.
• the first entity's private key be represented as (S l ,R l ) .
• This is a valid ring signature for the first and second entities.
• the first entity may choose random numbers b and r ⁇ , and compute the ring signature (S 2 - - r ⁇ P ⁇ + bP 2 , r 2 P, R 2 + bP) .
• a third entity having identity ID 3 may not create a valid ring signature for the first two entities.
• the private key provided by the PKG to the third entity may be represented as (sP'+ r 3 P 3 i r 3 P) . Because the third entity cannot remove its public point E 3 from its private key, the third entity's private key is "tainted" by its identity. This taint cannot be removed, and thus the third entity cannot forge a valid ring signature for the first two entities. Only the first two entities may create such a ring signature, essentially by adding the other entity's identity to the signing entity's private key.
• FIG. 3 shows a flow diagram illustrating a method of generating and verifying a ring signature Sig of a message M communicated between a signer and a verifier in a hierarchy according to another presently preferred embodiment of the invention.
• the method enables a signer from a ring of t ring members in a hierarchy to generate a ring signature for the t ring members.
• Each of the t entities is associated with an ID-tuple such as flD. 15 ...,ID ( . J , where /, represents the level of the respective entity in the hierarchy.
• the method begins in block 302 by generating first and second cyclic groups i and G 2 of elements.
• a function e is selected, such that the function e is capable of generating an element of the second cyclic group G 2 from two elements of the first cyclic group Gi.
• the function e preferably is an admissible pairing, as described above.
• a root generator Eo of the first cyclic group i is selected in block 306.
• a random root key generation secret so associated with and known only to the root PKG is selected.
• -? 0 is an element of the cyclic group ZlqZ.
• a root key generation parameter Q 0 SQPQ is generated in block 310.
• Qo is an element of the first cyclic group Gi.
• a first function Hi is selected such thatH is capable of generating an element of the first cyclic group Gi from a first string of binary digits.
• a second function H 2 is selected in block 314, such that H 2 also is capable of generating an element of the first cyclic group i from a first string of binary digits.
• the functions of blocks 302 through 314 are part of the Root Setup algorithm described above, and preferably are performed at about the same time. By way of example, functions such as those disclosed in Boneh-Franklin may be used as Hi and H 2 .
• next series of blocks show the functions performed as part of Lower-level Setup algorithm.
• a public element P. is generated for each of the /. — 1 ancestral lower-level PKGs associated with each of the t ring members.
• Each of the public elements, E; 7 H(lD (1 ,...,ID, ; ) for 1 ⁇ i ⁇ t and 1 ⁇ / ⁇ (I, -1 ), preferably is an element of the first cyclic group G-i.
• generation of all the public elements P d may take place over time, rather than all at once.
• a lower-level key generation secret s ⁇ t for 1 ⁇ i ⁇ t and 1 ⁇ ⁇ ( -1) is selected (block 318) for each of the — 1 ancestral lower- level PKGs associated with each of the t ring members.
• the lower-level key generation secrets s ⁇ preferably are elements of the cyclic group ZlqZ, and each lower-level key generation secret -?,, preferably is known only to its associated lower-level PKG. Again, although represented in a single block, selection of the lower-level key generation secrets -? ; may take place over time, rather than all at once.
• a lower-level secret point S (/ is generated (block 320) for each of the t * — 1 ancestral lower-level PKGs associated with each of the t ring members.
• Each lower-level secret element, S,, S l _ x) + s ⁇ P,, for 1 ⁇ i ⁇ t and 1 ⁇ / ⁇ (/; -1), preferably is an element of the first cyclic group G-i.
• generation of the secret elements S d may take place over time, rather than all at once.
• So may be defined to be the identity element of i.
• a lower-level key generation parameter Q also is generated (block 322) for each of the /, — 1 ancestral lower-level PKGs associated with each of the t ring members.
• generation of all the key generation parameters Q n may take place over time, rather than all at once.
• a ring member public point P tl associated with each of the t ring members is generated in block 324.
• a ring member secret point S a associated with each of the t ring members is then generated in block 326.
• the first function H optionally may be chosen to be an iterated function so that, for example, the public points P, may be computed as H P t(l _ X) ,TD ⁇ rather than H, (lD ⁇ . ,...ID ( .).
• the last two blocks shown in FIG. 3 represent the Signature and Verification algorithms described above.
• block 328 the message Mis signed by a signer having the ID-tuple lD jI ,...,ID j/ j to generate a ring signature Sig.
• the Signature algorithm preferably uses at least the signer's private point S and the ID-tuples lD ⁇ ...,ID (;/ J for each of the t ring members.
• the ring Signature Sig is then verified in block 330 to confirm that it was signed by one of the t ring members.
• the verification preferably uses at least the ID-tuples flD n ,...,ID j; for each of the t ring members.
• the Signature algorithm may begin by eliminating the redundancy among the public point-tuples P ⁇ k of the t ring members. There will be redundancy among these point-tuples if any of the ring members share common ancestral PKGs.
• the signer then generates the ring signature in the form [U, i, . . .
• Proxy signatures allow a designated person or group of persons, called proxy signers, to sign on behalf of the original signer.
• a proxy signature scheme should have the following properties: Strong unforqeability: The proxy signer can create valid proxy signatures for the original signer. Any other third party, including the original signer, is unable to forge a proxy signature.
• FIG. 4 shows a flow diagram illustrating a method of generating and verifying a digital proxy signature Sig of a digital message M according to another embodiment of the present invention.
• the signature Sig is signed by a proxy signer on behalf of an original signer.
• the method begins in block 402 by generating first and second cyclic groups Gi and G 2 of elements.
• a function e is selected, such that the function e is capable of generating an element of the second cyclic group G 2 from two elements of the first cyclic group Gi .
• the function e preferably is an admissible pairing, as described above.
• a generator P of the first group Gi is selected in block 406.
• a function His selected in block 408 such that the function His capable of generating an element of the first group Gi from a string of binary digits.
• a private key s or is selected for the original signer.
• a public key s 0 ,P is generated for the original signer in block 412.
• a private key s pr is selected for the proxy signer, and a public key s pr P is generated for the proxy signer in block 416.
• the total time required to sign a message is not as important as the online signing time.
• the online signing time generally is considered to be the time the signer needs to generate a signature after obtaining the message.
• Online/offline signature schemes have been proposed to reduce the time required for online signing. For instance, one such scheme employs a "trapdoor hash function" h and the "hash sign switch” paradigm.
• online/offline signature schemes have not been available for hierarchical identity-based signature systems.
• FIG. 5 shows a flow diagram illustrating a method of generating and verifying a digital signature Sig of a digital message Maccording to another embodiment of the present invention.
• This method includes a two-stage signature process in the context of a hierarchical identity-based system. The first stage of the signature process may be completed offline. This leaves only the second stage of the signature process to be completed online, thereby reducing the online signing time.
• the signer y in this hierarchical scheme is t levels below the root PKG in the hierarchy and is associated with the ID-tuple (ID ⁇ , . . . , ⁇ D yt ).
• the signer's ID-tuple includes identity information ID.,, associated with the signer, as well as identity information ID,,, associated with each of its t-1 ancestral lower-level PKGs in the hierarchy.
• the method begins in block 502 by generating first and second cyclic groups i and G 2 of elements.
• a function e is selected, such that the function e is capable of generating an element of the second cyclic group G 2 from two elements of the first cyclic group G-i.
• the function e preferably is an admissible pairing, as described above.
• a root generator Eo of the first cyclic group -i is selected in block 506.
• a random root key generation secret so associated with and known only to the root PKG is selected.
• -?o is an element of the cyclic group ZlqZ.
• Q is an element of the first cyclic group G-i.
• a first function H is selected such that H is capable of generating an element of the first cyclic group G-i from a first string of binary digits.
• a second function H 2 is selected in block 514, such thatH 2 also is capable of generating an element of the first cyclic group Gi from a first string of binary digits.
• functions such as those disclosed in Boneh-Franklin may be used asH and H 2 .
• the functions H and H 2 may be exactly the same function.
• the signer's signature may actually be a private key, which thereafter may be used to decrypt messages and forge signatures.
• blocks 502 through 514 are part of the Root Setup algorithm described above, and preferably are performed at about the same time.
• next series of blocks show the functions performed as part of the Lower-level Setup algorithm.
• a public element P yi is generated for each of the signer's t-1 ancestral lower- level PKGs.
• generation of all the public elements P yi may take place over time, rather than all at once.
• a lower-level secret element S yi is generated (block 520) for each of the signer's m ancestral lower-level PKGs.
• Each lower-level secret element, S y i — for 1 ⁇ i ⁇ t-1 preferably is an element of the first cyclic group -i.
• generation of the secret elements S yi may take place over time, rather than all at once.
• So preferably is defined to be the identity element of G-,.
• a lower-level key generation parameter Q yi also is generated (block 422) for each of the signer's t-1 ancestral lower-level PKGs.
• a signer public element P yt associated with the signer; is generated in block 524.
• the signer public element, P yt H ⁇ (ID i ID,,,), preferably is an element of the first cyclic group G-i.
• a signer secret element S yt associated with the signer y is then generated in block 526.
• the first function H optionally may be chosen to be an iterated function so that, for example, the public points P t may be computed as ID j ,,-) rather than Hi (ID-i, . . . , ID.,,-).
• the last two blocks shown in FIG. 5 represent the Signing and Verification algorithms described above.
• the two- stage signing algorithm involves the use of a trapdoor hash function h, which preferably is a discrete-log-based trapdoor hash function modified, according to methods known in the art, to apply to elliptic curves.
• a trapdoor hash function h which preferably is a discrete-log-based trapdoor hash function modified, according to methods known in the art, to apply to elliptic curves.
• a trapdoor secret s yt e Z/ 'q" is selected.
• the signer could choose s yl to be equal to s yt , the signer's lower-level secret element.
• s yl preferably should be generated anew for each signature.
• the Signing algorithm continues in block 530, as the signer selects a random message M ' and a random number r ' .
• the signer then signs the random message M ' in block 532 to generate the signature [U, ⁇ yi, • . ⁇ , Qyt, for 1 ⁇ i ⁇ t, and P M , algorithm may be
• the online portion of the Signing algorithm may be completed.
• the signer may then send r to the verifier along with the message Mand the signature Sig.
• Verification algorithm also may be broken down into online and offline stages, depending on what information the verifier has in its possession. For instance, the signer may provide the verifier with various information in advance of knowing the message M. In this way, the verifier may learn any or all of the following: (1 ) the signer's Q y t values; (2) P M , , the most recent output of the signer's trapdoor hash function H 2 ; (3) U, the signer's partial signature on the hash function output ; (4) M, the message to be signed; and/or (5) the signer's complete signature, including the value r. Using this information, the verifier may begin to verify parts of the signature, even before the message M is known or signed.
• the verifier will know the signer's Q yi values if the verifier has received a previous signature from that signer. This allows the verifier to precompute all but two pairings necessary to verify the signer's signature, regardless of how deep the signer is in the hierarchy.
• the verifier may complete the final two pairing computations after it receives P M , and U, the signer's signature on P M , .
• the system includes a number of terminals 602, 604, 606, 608, each of which may be associated with an entity that generates or verifies signatures according to the signature schemes described above.
• the system also includes one or more private key generators (PKG) 630 that generate and distribute private keys to the various terminals 602, 604, 606, and 608.
• PKG private key generators
• Each terminal includes a processor 610 in bidirectional communication with a memory 612.
• the processor 610 executes suitable program code for carrying out the procedures described above to generate or verify a digital signature.
• the processor 610 also executes suitable program code for generating information to be transmitted to other terminals. Suitable program code may be created according to methods known in the art.
• the memory 612 stores the program code, as well as intermediate results and other information used during execution of the digital signature generation and verification procedures.
• a communications network 620 is provided over which the entities 602, 604, 606, and 608 and a PKG 630 may communicate.
• the communications network 620 may be of various common forms, including, for instance, a LAN computer network, a WAN computer network, and/or a mobile telephone network provide suitable communication networks.
• the methods and systems described above are generally applicable to cryptography and secure communication via computer networks or via other types of systems and devices.
• the methods and systems are particularly useful as schemes for generating and verifying signatures of communications in systems using public key cryptography.

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