# Patent

## Title
Method for accelerating cryptographic operations on elliptic curves

## Abstract

This invention provides a method for accelerating multiplication of an elliptic curve point Q(x,y) by a scalar k, the method comprising the steps of selecting an elliptic curve over a finite field Fq where q is a prime power such that there exists an endomorphism Ψ, where Ψ(Q)=λ.Q for all points Q(x,y) on the elliptic curve: and using smaller representations ki of the scalar k in combination with the mapping Ψ to compute the scalar multiple of the elliptic curve point Q.

## Background
This application is a continuation of International Application No. PCT/CA99/01222, filed on Dec. 23, 1999, and claims the priority of Canadian Patent Application No. 2,257,008, filed on Dec. 24, 1998, the content of all of which is incorporated herein by reference.


This invention relates to a method for performing computations in cryptographic systems utilizing elliptic curves.
BACKGROUND OF THE INVENTION
A public-key data communication system may be used to transfer information between a pair of correspondents. At least part of the information exchanged is enciphered by a predetermined mathematical operation by the sender and the recipient may perform a complementary mathematical operation to decipher the information.
Each correspondent has a private key and a public key that is mathematically related to the private key. The relationship is such that it is not feasible to determine the private key from knowledge of the public key. The keys are used in the transfer of data, either to encrypt data that is to be transferred or to attach a signature to allow verification of the authenticity of the data.
For encryption, one correspondent uses the public key of the recipient to encrypt the message and sends it to the recipient The recipient then uses her private key to decipher the message.
A common key may also be generated by combining one parties public key with the other parties private key. It is usual in such cases to generate new private and corresponding public keys for each communication session, usually referred to as session keys or ephemeral keys, to avoid the long-term keys of the parties being compromised.
The exchange of messages and generation of the public keys may therefore involve significant computation involving exponentiation when the cryptographic system utilizes in Z*p, the finite field of integers mod p where p is a prime or the analogous operation of point multiplication when the system utilizes an elliptic curve. In an elliptic curve system, an ephemeral key pair is obtained by generating a secret integer, k and performing a point multiplication in the seed point Q to provide the ephemeral public key kQ. Similarly, the generation of a common ephemeral session key will require multiplication of a public key kaQ, which is a point on the curve, with a secret integer kb of the other correspondent so that point multiplication is again required.
A similar procedure is used to sign a message except that the sender applies his private key to the message. This permits any recipient to recover and verify the message using the senders public key.
Various protocols exist for implementing such a scheme and some have been widely used. In each case, however, the sender is required to perform a computation to sign the information to be transferred and the receiver is required to perform a computation to verify the signed information.
In a typical implementation a signature component s has the form:

s=ae+k (mod n)

where; in an elliptic curve crypto system,

    
    
        P is a point on the underlying curve which is a predefined parameter of the system;
        k is a random integer selected as a short term private or session key;
        R=kP is the corresponding short term public key;
        a is the long term private key of the sender;
        Q=aP is the senders corresponding public key;
        e is a secure hash, such as the SHA-1 hash function, of a message m and the short term public key R; and
        n is the order of the curve.
    
    


The sender sends to the recipient a message including m, s, and R and the signature is verified by computing the value R1=(sP−eQ) which should correspond to R. If the computed values correspond then the signature is verified.
In order to perform the verification it is necessary to compute the point multiplications to obtain sP and eQ, each of which is computationally complex. Where the recipient has adequate computing, power this does not present a particular problem but where the recipient has limited computing power, such as in a secure token or a “Smart card” application, the computations may introduce delays in the verification process.
Key generation and signature protocols may therefore be computationally intensive. As cryptography becomes more widely used there is an increasing demand to implement cryptographic systems that are faster and that use limited computing power, such as may be found on a smart card or wireless device.
Elliptic curve cryptography (ECC) provides a solution to the computation issue. ECC permits reductions in key and certificate size that translates to smaller memory requirements, and significant cost savings. ECC can not only significantly reduce the cost, but also accelerate the deployment of smart cards in next-generation applications. Additionally, although the ECC algorithm allows for a reduction in key size, the same level of security as other algorithms with larger keys is maintained.
However, there is still a need to perform faster calculations on the keys so as to speed up the information transfer while maintaining a low cost of production of cryptographic devices.
Computing multiples of a point on an elliptic curve is one of the most frequent computations performed in elliptic curve cryptography. One method of speeding up such computations is to use tables of precomputed multiples of a point. This technique is more useful when a point is known beforehand. However, there are cases when multiples of previously unknown points are required (for example, in ECDSA verification). Thus there is a need for a system and method for facilitating point multiplications.
SUMMARY OF THE INVENTION
In general terms, the present invention represents the scalar k as a combination of components ki and an integer λ derived from an endomorphism in the underlying curve.
The method is based on the observation that, given an elliptic curve (EC) having complex multiplication mapping over a finite field, there is an λ, which is the solution to a quadratic, for which the complex multiplication mapping is equivalent to multiplying a point Q by λ. It will often be less computationally expensive to compute λQ via the complex multiplication map, compared to treating λ as a integer and performing the EC multiplication. In practice, point multiplication by other scalars (not just λ) is required. It is also shown how the multiplication mapping may be used to compute other multiples of the point.
In accordance with this invention there is provided a method for accelerating multiplication of an elliptic curve point Q(x,y) by a scalar k, the method comprising the steps of:

    selecting an elliptic curve over a finite field F such that there exists an endomorphism ψ, where ψ(Q)=λ·Q for all points Q(x,y) on the elliptic curve; and
    using smaller representation ki of the scalar k in combination with the mapping ψ to compute the scalar multiple of the elliptic curve point Q.





BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein:
FIG. 1 is a schematic diagram of a communication system;
FIG. 2 is a flow chart showing the steps of implementing a first embodiment of the present invention.
FIG. 3 is a flow chart showing the steps of providing parameters required to implement the method of FIG. 2.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
For convenience in the following description, like numerals refer to like structures in the drawings. Referring to FIG. 1, a data communication system 10 includes a pair of correspondents, designated as a sender 12, and a recipient 14, connected by a communication channel 16. Each of the correspondents 12,14 includes a cryptographic processor 18,20 respectively that may process digital information and prepare it for transmission through the channel 16 as will be described below. Each of the correspondents 12,14 also includes a computational unit 19,21 respectively to perform mathematical computations related to the cryptographic processors 18,20. The processors 18,20 may be embodied in an integrated circuit incorporated in the processor or may be implemented as instructions encoded on a data carrier to implement a predetermined protocol in conjunction with a general purpose processor. For the purpose of illustration it will be assumed that the correspondent 12 is in the form of a smart card having a dedicated processor 18 with relatively limited computing power. The processor 20 may be a central server communicating with the card by channel 16 and channel 16 may be a wireless communication channel if preferred.
The cryptographic processors 18 implement an elliptic curve cryptographic system, of ECC, and one of the functions of the cryptographic processor 18 is to perform point multiplications of the form k·Q, where k is an integer and Q a point on the underlying elliptic curve, so that they may be used as a key pair k, kQ in a cryptographic scheme. As noted above, cryptographic computations such as the multiplication of an elliptic curve point by a scalar value are computationally expensive.
A method for accelerating scalar multiplication of an elliptic curve point Q(x,y) is shown in FIG. 2 and indicated generally by the numeral 50. The subject algorithm increases the speed at which the processors 12 can for example sign and verify messages for specific classes of elliptic curves. The method is based on the observation that given the general equation for an elliptic curve E:

y2+a1xy+a3y=x3+a2x2+a4x+a6  (1)

over a finite field, exemplified as Fq (q is a prime power) and when there exists an endomorphism ψ, where ψ(Q)=λ·Q for all points Q(x,y) on the elliptic curve, then multiplication of the point Q by an integer k may be accelerated by utilizing combinations of smaller representations ki of k in combination with the mapping ψ. The mapping ψ also allows precomputation of group elements and combinations thereof, which may be used in subsequent calculation of kQ.

Referring now to FIG. 2, a flow chart of a general embodiment for accelerating point multiplication on an elliptic curve, is shown by numeral 50. The system parameters are first selected. As an initial step an underlying elliptic curve E is selected to have certain characteristics. In a first embodiment of the invention the generalized elliptic curve (1) may be expressed in the following form:

E: y2=x3+b mod p; where p is a prime.  (2)

Firstly, the modulus p can be determined such that there is a number, γ where γεFp (Fp is the field of size p consisting of all integers mod p), and γ3≡1 mod p (a cube root of unity). If for example p=7, then γ=2, since 23 mod 7=1. Such a γ does not necessarily exist for all p, and therefore this must be taken into consideration when choosing the value of p. Typically, the chosen p should be at least 160 bits in length for adequate cryptographic strength.
After the curve E has been selected, a mapping function ψ is determined. The mapping function ψ: (x, y)→(γx, y), simply maps one set of points on the curve to another set of points on the curve. There exists an integer λ such that ψ(Q)=λ·Q for all points Q(x,y) of interest on the elliptic curve, E. This integer λ may be found by noting that λ3≡1 mod n, where n is the number of points on the elliptic curve E over Fp i.e. the number of points on E(Fp). There may exist more than one solution for λ in λ3≡1 mod n, but only one of those solutions will satisfy the mapping function ψ. It is important to note that since γ3 mod p=1, both Q and ψ(Q) satisfy the equation for E. Therefore, instead of having to perform lengthy calculations to determine the results of multiplication by λ, it can be done very efficiently using the results of the mapping function so that multiplication by λ can be done very efficiently.
A seed point Q is selected and the system parameters E, p, Q, λ, ψ(Q), and γ are stored in the card 12, as indicated at 52, at manufacture time for use by the cryptographic processor 18. To implement a cryptographic procedure such as encryption, key agreement or signature it is necessary to select an integer k for use as an ephemeral private key k and generate a corresponding public key kQ.
The value of k may be expressed as:

k=(k0+k1λ) mod n  (3)

where n is the number of points on E(Fp) and k0 and k1 are integers. The point k·Q then becomes:

k·Q=(k0Q+k1λQ) mod n  (4)

For some cryptographic operations the value of k may be chosen at random and in these cases, rather than select k it is possible to select values for k0 and k1 at random, having a length of [log2 (n)]/2 not including sign bits, (i.e. the length of the k1's are chosen to be at least one half the length k) and then calculate the value for k using equation (3). Having selected the values of k0, k1 as indicated a 54 in FIG. 2, the right side of equation (4) can be calculated quickly using an algorithm analogous to the “Simultaneous Multiple Exponentiation” as described in the “Handbook of Applied Cryptography”(HAC) by Menezes et. al.(Algorithm 14.88) and indicated at 56. For convenience the algorithm is reproduced below. It may be noted that in an additive group exponentiation is analogous to addition, thus replacing the multiplication in the algorithm with addition, yields the following:










Algorithm 1 Simultaneous Multiple Addition















INPUT: group


elements g0, g1, . . . , gl−1 and non negative t-bit integers e0, e1, . . . , el−1.


OUTPUT: g0e0 + g1e1 + . . . + gl−1el−1.









step1. Precomputation. For i from 0 to (2l − 1):









Gi ← Σj=0l−1 gjij









where i = (il−1 . . . i0)2



step2. A ← 0



step3. For i from 1 to t do the following:









A ← A + A, A ← A + G1,









step4. Return (A)  where A = g0e0 + g1e1 + . . . + gl−1el−1










Applying this algorithm to equation (4) it can be seen that there are two group elements, g0, g1 namely Q and λQ, so that l=2 and two integers e0, e1 namely k0, k1. The algorithm permits precomputation of some of the values and initially Gi is precomputed. The results of precomputation of Gi with l=2 is shown in table 1.











TABLE 1





i
0
1
2
3







Gi
0
g0
g1
g0 + g1









After performing a point addition to construct the point: Q+ψ(Q). It is possible to fill in table 1 with the computed elements to yield table 2. These elements may be pre-computed and stored in memory as shown at step 58 in FIG. 2.











TABLE 2





i
0
1
2
3







Gi
0
Q
ψ(Q)
Q + ψ(Q)









Before step of the algorithm can be performed, Gi, has to be determined and accordingly I1 through It have to be found as indicated at 60. A notional matrix or combing table may be constructed using the binary representation of ki. If, for example, k0=30 and k1=10, then t has the value five since the maximum number of bits in the binary representation of ko through k1 is five and the notional matrix constructed from their binary representation is shown in Table 3. Ii is determined by the number represented in the ith column where the first row contains the least significant bit, the second row contains the next significant bit, etc. Therefore it can be seen from table 3 that I1=I12=(11)=3, I3=(01)=1, I4=3, and I5=0.















TABLE 3







i
1
2
3
4
5









k0
1
1
1
1
0



k1
0
1
0
1
0



Ii
1
3
1
3
0










All the components needed to complete the algorithm are available and the iteration of step three is performed as shown at 62.
Initially A←O and i is set to 1.
Ii=I1 which from table 3 is equal to 1. GI1 is therefore Gl which from table 2 is Q. The value of A from the iteration for I=1 is therefore O+Q=Q.
For the next iteration where i=2 the initial value of A is Q so A←Q+Q=2Q Ii=I2=3 from table 3. GI2 therefore equates to G3 from table 2 which is Q+ψ(Q).
A+GI1 therefore is computed as 2Q+Q+ψQ=3Q+Q.
The iterations continue for each value of i set out in table 4 until after the 5th iteration the value for koq=k, λQ, i.e. kQ is computed.










TABLE 4







i
A









1
Q



2
3Q + ψ(Q) 



3
7Q + 2ψ(Q)



4
15Q + 5ψ(Q) 



5
30Q + 10ψ(Q)










Each iteration requires a point doubling (A+A) and a point addition (A+GI1) although in some cases the value of GI1 may be O that will reduce the computation.
Thus it may be seen that this method will require a number of point doubles equal to max {log2(ki)}, and almost as many point additions. The number of point additions can be reduced using windowing (Alg. 14.85 HAC) and exponent recoding techniques. Since the value of i and Gi can be precomputed, the point additions are easily performed by retrieving the appropriate precomputed element Gi from table 2. Once kP has been computed, it may be used as the correspondents 12 ephemeral public key in encrypting or signing transmissions over the channel 16.
To summarize, for cryptographic operations like encryption and Diffie-Hellman, signature, an integer k is required with a corresponding public key kQ, computed. The values k0 and k1 are chosen at random, each having a length one half the length of n and the term k0Q=k1λQ generated using a suitable algorithm. When the k's are chosen in this way, the method seems to be as secure as the random generation of k itself. Of course it is possible to choose the ki's to have fewer bits in order to improve efficiency.
In the above technique, the method of writing k=k0+k1λ in conjunction with simultaneous combing achieves a speed up of the simultaneous multiple addition algorithm. The technique of writing k=k0+k1λ may also be used with the scalar multiplication techniques to advantage, namely with winding, combing ,etc.
For some mappings ψ, it is also possible to use more than two sub k's. It is possible for some ψ's to write k=k0+k1λ+k2λ2 allowing the value of k to be computed by applying the simultaneous multiple addition algorithm.
In a second embodiment of the invention a different form of the generalized elliptic curve equation (1) is used, namely:

y2=(x3−ax) mod p  (5)

Once again, p will be a prime number having at least 160 bits. For this type of curve, the properties required for y are different. It is now required to find a value such that γ2=−1 mod p. A change in the property of γ requires a different mapping function ψ′ to be used. In this embodiment the mapping takes the form ψ′: (x, y)→(−x, γy). If (x,y) is on the curve, then ψ′(x,y) is also on the curve. In this case λ4≡1 mod n (n is still the number of points on E(Fp)), and therefore λ can be calculated. The mapping ψ′(Q)=λ·Q is performed as before and once again multiplication by λ can be done very efficiently for this curve. The equation for k in this embodiment is the same as in the first embodiment and is represented by:

k=(k0+k1λ)mod n  (6)

This equation is the same as in the previous embodiment, having only two group elements. Thus using the group elements Q and Q+ψ′(Q) in the algorithm 1, the point k·Q may be calculated. This computation will require a number of point doubles equal to max {log2(ki)}, and a similar number of point additions. As described earlier the number of point additions can be reduced using windowing and exponent recoding techniques.

This method applies to other elliptic curves, so long as there exists an efficiently computable endomorphism, ψ.
The above embodiments assume that k can be chosen at random and therefore k0 and k1 can be selected instead and determine k. For cryptographic protocols, where it is not possible to choose k, it is first necessary to find k0, k1 of the desired “short” form from the given value of k such that k=(k0+k1λ) mod n. In some cases, more than two k's can be used to advantage.
As may be seen in the embodiments described above when a point is known beforehand, tables can be built to speed multiplication. However, there are cases when multiples of previously unknown points are required (for example, this can occur in ECDSA verification) and it is then necessary to take the value of k as provided and then determine suitable representations for ki.
Thus in a third embodiment, system parameters and a value k is provided, the point Q, the required multiple k, and the complex multiplication multiple λ are known. It is necessary to determine the “short” ki's from the value for k, which is predetermined. A method for doing this described as follows and illustrated in the flow chart of FIG. 3. As a pre-computation (not requiring k) we compute two relations:

a0+b0≡0 mod n

a1+b1≡0 mod n

such that ai and bi are numbers smaller than n. It is preferable that ai and bi are as small as possible, however, the present method has advantages even when ai and bi are not minimal. The pair, ai and bi, where ai and bi are both small, can be viewed as a vector, ui with a small Euclidean length. Typically the method described below produces k0 and k1 having representations one half the size of the original k.

In the present embodiment, kQ can be computed efficiently by utilizing precomputed, short vector representations to obtain an expression of the form:

k0Q+λk1 Q

This is accomplished by using precomputed vectors to derive fractions f0 and f1 that do not require knowledge of k. A vector z is generated from the combination of fractions f0 and f1 and k. The vector z is used to calculate a second vector v′ where v′=(vo′,v1′) and the value of kQ calculated as

volQ+λv1lQ  (8)

The method of achieving this solution is described below in greater detail.

To produce small a1, and bi, it is possible to make use of the L3-lattice basis reduction algorithm (HAC p. 118), which would directly result in short basis vectors. However, in this preferred embodiment the simple extended Euclidean algorithm is employed on the pair (n, λ). The extended Euclidean algorithm on (n, λ) produces linear combinations cin+d1λ=ri, where the representation of ri (e.g. bit-length) decreases and the representation of ci and di increases with i.
The two smallest values of |(d1, r1)| resulting from using the extended Euclidean algorithm are saved. The size of these vectors are measured with the squared Euclidean norm |(di, ri)|=di2+r12. The terms in these minimal relations are denoted {circumflex over (d)}0, {circumflex over (r)}0 and {circumflex over (d)}1, {circumflex over (r)}1. and will typically occur in the middle of the algorithm. Even if the minimal relations are not retained, suboptimal relations may still give the method an advantage in the calculation of point multiples.
The values of ai and bi are constructed by defining a0=−{circumflex over (r)}0, b0={circumflex over (d)}0 a b1={circumflex over (d)}0 all of which may be precomputed. The next task is to find a small representation for the multiple k.
Given the computation of a0, b0 and a1, b1, it is possible to designate the vectors u0, u1, where u0=(a0, b0) and u1=(a1, b1). These vectors satisfy ai+biλ=0 (mod n). The multiplication of the group elements Q by the vector v=(v0, v1) is defined as (v0+v1λ)Q. Since ai+biλ=0 (mod n), u0R=u1R=0 for any group element R. Hence for any integers z0 and z1, v′R=(v−z0u0−z1u1)R for any group element R.
Integers z0 and z1 may be chosen such that the vector v′=v−z0u0−z1u1 has components that are as small as possible. Again, this method will have an advantage if the components of v′ are small, but not necessarily minimally so.
The appropriate z0 and z1 are calculated by converting the basis of v into the basis {u0, u1}. The conversion between basis involves matrix multiplication. To convert the vector v=(v0, v1) from the {u0, u1} basis to the standard orthonormal basis {(1,0),(0,1)},




  
    v
    
      {
      
        
          (
          
            1
            ,
            0
          
          )
        
        ,
        
          (
          
            0
            ,
            1
          
          )
        
      
      }
    
  
  =
  
    
      
        v
        
          (
          
            
              u
              0
            
            ,
            
              u
              1
            
          
          )
        
      
      ⁢
      M
    
    =
    
      
        (
        
          
            v
            0
          
          ,
          
            v
            1
          
        
        )
      
      ⁡
      
        [
        
          
            
              
                a
                0
              
            
            
              
                b
                0
              
            
          
          
            
              
                a
                1
              
            
            
              
                b
                1
              
            
          
        
        ]
      
    
  




To convert in the other direction, from the standard orthonormal basis {(1,0),(0,1)} to the (u0, u1) basis, the multiplication is simply by the inverse of M,





  
    v
    
      (
      
        
          u
          0
        
        ,
        
          u
          1
        
      
      )
    
  
  =
  
    
      
        v
        
          {
          
            
              (
              
                1
                ,
                0
              
              )
            
            ,
            
              (
              
                0
                ,
                1
              
              )
            
          
          }
        
      
      ⁢
      
        inverse
        ⁡
        
          (
          M
          )
        
      
    
    =
    
      
        v
        
          {
          
            
              (
              
                1
                ,
                0
              
              )
            
            ,
            
              (
              
                0
                ,
                1
              
              )
            
          
          }
        
      
      ⁢
      
        
          1
          
            
              
                a
                0
              
              ⁢
              
                b
                1
              
            
            -
            
              
                a
                1
              
              ⁢
              
                b
                0
              
            
          
        
        ⁡
        
          [
          
            
              
                
                  b
                  1
                
              
              
                
                  -
                  
                    b
                    0
                  
                
              
            
            
              
                
                  -
                  
                    a
                    1
                  
                
              
              
                
                  a
                  0
                
              
            
          
          ]
        
      
    
  




Since the vector v=(k, 0) has a zero component, the bottom row of inverse(M) is not required, and therefore to convert to the {u0, u1} basis only the fractions




  
    
      f
      0
    
    =
    
      
        
          
            b
            1
          
          
            
              
                a
                0
              
              ⁢
              
                b
                1
              
            
            -
            
              
                a
                1
              
              ⁢
              
                b
                0
              
            
          
        
        ⁢
        
          
        
        ⁢
        and
        ⁢
        
          
        
        ⁢
        
          f
          1
        
      
      =
      
        
          b
          0
        
        
          
            
              a
              0
            
            ⁢
            
              b
              1
            
          
          -
          
            
              a
              1
            
            ⁢
            
              b
              0
            
          
        
      
    
  
  ⁢
  
    
  




are needed.

The fractions f0 and f1 may be precomputed to enough precision so that this operation may be effected only with multiplication. It should be noted that the computations leading to these fractions do not depend upon k, therefore they can be computed once when the elliptic curve is chosen as a system parameter, and do not need to be recalculated for each k. Similarly the vectors v, u0 and u1 may be precomputed and stored.
Once a value of k is selected or determined the value of kQ may be computed by first calculating z=(z0, z1), where z is defined as (z0, z1)=(round(kf0), round(kf1)). Other vectors near to z will also be useful, therefore rounding could be replaced with floor or ceiling functions or some other approximation.
Once a suitable z has been determined, an efficient equivalent to v (k,0) is calculated by v′=(v0′, v1′)=v−z0u0−z1u1. The phrase “efficient equivalen” implies a vector v′ such that v′P=vP and v′ has small coefficients. The value kQ is then calculated as v0′Q+v1′λQ. This value can be calculated using simultaneous point addition as described above, with enhanced efficiency obtained from the use of non-adjacent form (NAF) recoding as described above and as described in H. A. C. 14.7 at page 627. Thus, even where k is predetermined, values of k0 and kI can be computed and used with the mapping function to obtain a value of kQ and hus he key pair k, kQ.
For the case where k is to be separated into 3 portions k=k0+k1λ+k2λ2, small vectors can be obtained from L3-row-reducing




  
    [
    
      
        
          1
        
        
          0
        
        
          
            -
            
              λ
              2
            
          
        
      
      
        
          0
        
        
          1
        
        
          
            -
            λ
          
        
      
      
        
          0
        
        
          0
        
        
          
            -
            n
          
        
      
    
    ]
  
  ⁢
  
    
  
  ⁢
  
    to
    ⁢
    
      
    
    [
    
      
        
          
            u
            2
          
        
      
      
        
          
            u
            1
          
        
      
      
        
          
            u
            0
          
        
      
    
    ]
  




A small vector equivalent (three-dimensional row) can be obtained in a similar way to the two-dimensional case.
Using these methods to determine the value of k·Q greatly reduces the processing power required by the cryptographic processors 12. It also increases the speed at which these repetitive calculations can be done which, in turn, reduces the time to transfer information.
It will be appreciated that once the scalar multiple k has been represented in terms of shortened components k=k0+k1λ+k2λ2+ . . . km−λm−1, other options for efficient elliptic curve scalar multiplication may be used in place of or in conjunction with the simultaneous multiple addition algorithm. These options include windowing (fixed and sliding), combing, bit recoding and combinations of these techniques.
One particularly beneficial technique permits tables built for one component of the multiplication, say k0, to be reused for other components k1, etc. This is accomplished by transforming the computed table elements by applying the mapping γ as required.
As a further exemplification, an embodiment where k can be recast as k=k0+k1λ+k2λ2, where k has m-bits and ki have roughly m/3 bits is described below.
Once the components ki have been determined, they may be recoded from the binary representation to the signed binary representation having less non-zero bits. This recoding can take the Non-Adjacent-Form (NAF), where every 1 or −1 bit in the representation if ki is non-adjacent to another non-zero in the signed binary string. This recoding is described in H.A.C. 14.7 p. 627.
Once each ki has been recoded, a table can be constructed to aid in computing kiλiP.
A NAF windowing table precomputes certain short-bit length multiples of λiP. The width of the window determines the size of the table. As ki has been recorded to have no adjacent non zeros, odd window widths are suitable. A 3-bit wide NAF window would contain




The recoded ki values are built by concatenating these windows, and padding where necessary with zeros (H.A.C., p. 616).
The required number of additions can be reduced with use of this table, since it is necessary to add or subtract an EC point only for every window encountered instead of for every non zero bit.
Initially therefore this technique is applied to the computation of k0P.
The table built for the k0P calculation can be applied to the kiλP calculation if the table elements are mapped with the ψ mapping using the operator γ. Similarly, k2λ2P can be accelerated by using the table built for k0P, but mapping the table elements with γ2.
In applying the sliding window technique to the components, only one set of doublings need be performed.
To illustrate this example of a preferred embodiment the following example will be used:

If k=[10110101101]2+[111010101101]2λ,

then recoding





  
    
      
        
          k
          =
          
            
              [
              
                10
                -
                100
                -
                10
                -
                100
                -
                101
              
              ]
            
            +
            
              
                [
                
                  1000
                  -
                  10
                  -
                  10
                  -
                  10
                  -
                  101
                
                ]
              
              ⁢
              λ
            
          
        
        ,
      
    
  
  
    
      
        =
        
          
            k
            0
            ′
          
          +
          
            
              k
              1
              ′
            
            ⁢
            λ
          
        
      
    
  




A 3-bit window table on P is precomputed containing 1·P, [10−1]·P, [101]·P. This requires two EC additions, and two EC doublings.

After this, kP can be calculated as

kP=[10−100−10−100−101]P+[1000−10−10−10−101]·λP

by adding/subtracting elements from the table.

This can be done using an accumulator A as follows:
















A←0
;initialize



A += ψ (1 · P)
;consuming the top bit of k0r



A ← 2A
;double A



A ← 2A



A ← [10 − 1] P
;consuming the top 3 bits of k0r



A ← 24A
;



A −= [101] ψ P
;consuming a 3 bit window of k1r



A ← 2A
;double A



A −= [101] P
;consuming 3 bits of k1r



A ← 24A



A −= [101] ψ P
;consuming 3 bits of k1r



A ← 22A



A −= [10 − 1] P
;consuming the last of k0r



A += ψ P
;producing kP.










In summary, the previously described technique is as follows. Given an elliptic curve E and an endomorphism ψ, there corresponds an integer λ such that λQ=ψ(Q) for all points QεE. Select an integer m and compute an equivalent number m of “short basis vectors” b1, b2, . . . ,bm. Each such basis vector corresponds to an integer, and each such integer is divisible by the number of points n=#E(Fpm) (i.e. the number of points). Now, given an integer k, (0<k<n), we write k=Σki·λi, where the ki's are chosen to be “short”. This is done by finding the difference between a certain vector (which represents k) and a nearby vector in the lattice generated by b1, b2, . . . ,bm.
The following embodiment explicitly describes an application of the previously described technique (endomorphism and basis conversion and “Shamir's trick”) to elliptic curves defined over composite fields. In particular, we describe an application to curves E(Fpm) where p is an odd prime is described. The following embodiments exemplify techniques for such curves.
This technique is described in the case where the map ψ is the Frobenius map ψ(x,y)=(xp,yp) and E'A,B(Fpm) wherein A,BεFp.
In this case, it is known that the Frobenius map satisfies the ψ2−tψ+p=0, where t=p+1−#E(Fpm).
It follows that λ2−tλ+p=0 mod n and so λ2+I−tλ1+i+pλi=0 mod n.
Note that the vectors;



  
    (
    
      
        
          
            λ
            ⁢
            
              
            
          
          
            m
            -
            1
          
        
        ⁢
        ...
        ⁢
        
          
        
        ⁢
        
          λ
          2
        
      
      ,
      
        λ
        1
      
      ,
      
        λ
        0
      
    
    )
  




  
    
      
        
          b
          1
        
      
      
        
          (
          
            0
            ,
            0
            ,
            0
            ,
            ...
          
        
      
      
        
          
            0
            ,
            1
            ,
            
              -
              t
            
            ,
            p
          
          )
        
      
    
    
      
        
          b
          2
        
      
      
        (
      
      
        
          
            1
            ,
            
              -
              t
            
            ,
            p
            ,
            0
          
          )
        
      
    
    
      
        
          
        
      
      
        
          (
          
            1
            ,
            
              -
              t
            
            ,
            p
            ,
            0
            ,
            0
            ,
            ...
          
        
      
      
        
          
            ...
            ⁢
            
              
            
            ,
            0
          
          )
        
      
    
    
      
        
          
        
      
      
        
          (
          
            
              -
              t
            
            ,
            p
            ,
            0
            ,
            0
            ,
            ...
          
        
      
      
        
          
            ...
            ⁢
            
              
            
            ,
            0
            ,
            1
          
          )
        
      
    
    
      
        
          b
          m
        
      
      
        
          (
          
            p
            ,
            0
            ,
            0
            ,
            0
            ,
            ...
          
        
      
      
        
          
            0
            ,
            1
            ,
            
              -
              t
            
          
          )
        
      
    
  



consist of m “short” basis vectors of the vector space Qn. It follows that to compute k·Q on such a curve we can proceed using the vectors b1,b2 . . . bm and the technique described previously.

In the above embodiments it will be appreciated that k,λQ can be obtained from ψ(kQ) is the mapping is more efficient than addition.
Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.

## Claims

1. A method for multiplying an elliptic curve point Q(x,y) by a scalar k to provide a point kQ, the method comprising the steps of:
a) selecting an elliptic curve of order n over a finite field F such that there exists an endomorphism ψ where ψ (Q)=λ(Q) for all point Q (x,y) on the elliptic curve, and λ is an integer;
b) establishing a representation of said scalar k as a combination of components ki and said integer λ of the form





  
    k
    i
  
  =
  
    
      ∑
      
        i
        =
        0
      
      
        i
        =
      
    
    ⁢
    
      
    
    ⁢
    
      
        k
        i
      
      ⁢
      
        λ
        i
      
    
  



 mod n;
c) combining said representation and said point Q to form a composite representation of a multiple of the form k0Q+k1ψ(Q)+ . . . corresponding to kQ; and
d) computing a value corresponding to said point kQ from said composite representation of kQ.

2. A method according to claim 1 wherein each of said components ki is shorter than said scalar k.

3. A method according to claim 1 wherein said components k1 are initially selected and subsequently combined to provide said scalar k.

4. A method according to claim 1 wherein said components ki are selected at random.

5. A method according to claim 4 wherein said representation is of the form k0+k1λ.

6. A method according to claim 1 wherein said scalar k has a predetermined value and said components k0 and k1 are one half size of said scalar k.

7. A method according to claim 3 wherein said value of said multiple kQ is calculated using simultaneous multiple addition.

8. A method according to claim 7 wherein said simultaneous multiple addition includes precomputing a value Gi representing a grouping of like elements, said value Gi being used in said simultaneous multiple addition.

9. A method according to claim 6 wherein said components ki are obtained by obtaining short basis vectors (uo, ui) of the field F, designating a vector v as (k,0), converting v from a standard, orthonormal basis to the (uo, ui) basis, to obtain fractions f0f1 representative of the vector v, applying said fractions to k to obtain a vector z, calculating an efficient equivalent v′ in the composite representation of kQ.

10. A method of generating in an elliptic curve cryptosystem a key pair having a integer k providing a private key and a public key kQ, where Q is a point on the curve, the method comprising the steps of:
a) selecting an elliptic curve over a finite filed F such that there exists an endomorphism ψ where ψ (Q)=μQ for all points Q (x,y) on the elliptic curve, λ is an integer,
b) establishing a representation of said key k as a combination of components ki and said integer λ, of the form





  
    k
    l
  
  =
  
    
      ∑
      
        i
        =
        0
      
      
        f
        =
      
    
    ⁢
    
      
        k
        f
      
      ⁢
      
        λ
        i
      
    
  



 mod n where n is the number of points on the elliptic curve,
c) combining said representation and said point Q to form a composite representation of a multiple of the form k0+k1ψ(Q)+ . . . corresponding to the public key kQ; and
d) computing a value corresponding to said public kQ from said composite representation of kQ.

11. A method according to claim 10 wherein each of said components: ki is shorter than said scalar k.

12. A method according to claim 11 wherein said components ki are initially selected and subsequently combined to provide said scalar k.

13. A method according to claim 12 said components ki are selected at random.

14. A method according to claim 13 wherein said representation is of the form k0+k1λ.

15. A method according to claim 10 wherein said scalar k has a predetermined value and said components k0 and k1 are selected to be one half the size of said scalar k.

16. A method according to claim 12 wherein said value of said multiple kQ is calculated using simultaneous multiple addition.

17. A method according to claim 16 wherein said simultaneous multiple addition includes precomputing a value Gi representing a grouping of like elements, said value Gi being used in said simultaneous multiple addition.

18. A method according to claim 15 wherein said components ki are obtained by obtaining short basis vectors (uo, ui) of the field F, designating a vector v as (k,0), converting v from a standard, orthonormal basis to the (uo, ui) basis, to obtain fractions f0f1 representative of the vector v, applying said fractions to k to obtain a vector z, calculating an efficient equivalent v′ in the composite representation of kQ.