Abstract:
An apparatus arranged to accept digital data as an input and to process the data according to one of either the Secure Hash Algorithm (SHA-1) or Message Digest (MD5) algorithm to produce a fixed length output word. The apparatus includes a plurality of rotational registers for storing data, one of the registers arranged to receive the input data, and data stores for initialization of some of the plurality of registers according to whether the SHA-1 or MD5 algorithm is used. The data stores include fixed data relating to SHA-1 and MD5 operation. Also included is a plurality of dedicated combinatorial logic circuits arranged to perform logic operations on data stored in selected ones of the plurality of registers.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an efficient hardware implementation of the Secure Hash Algorithm (SHA-1) and Message Digest Algorithm (MD5). 
   2. Description of the Related Art 
   Hash algorithms and message digests are frequently used in applications such as digital signatures, where it is desirable to verify the authenticity of a document or file. Techniques for producing message digests are beneficial as they reduce the amount of data processing needed to a manageable and consistent level. 
   The Secure Hash algorithm (SHA-1) is specified in Secure Hash Standard (FIPS PUB 180-1) and is an algorithm that operates on an input data file to produce a condensed representation of that file. Specifically, a message of arbitrary length is processed to produce a message digest consisting of exactly 160 bits. 
   The Message Digest Algorithm (MD5), developed by Professor Ronald Rivest, has a similar function. It accepts inputs of arbitrary length and produces an output message digest consisting of exactly 128 bits. 
   Both algorithms may be used as a constituent part of a digital signature application. Both algorithms are computationally intensive and, when implemented in software, as is the norm in prior systems, can take a great number of processor clock cycles to complete. 
   BRIEF SUMMARY OF THE INVENTION 
   The disclosed embodiments of the present invention therefore aim to overcome problems with the implementation of these systems, particularly in relation to speed of operation and power consumption. 
   In a first broad form of one embodiment of the present invention, an apparatus arranged to accept digital data as an input and to process the data according to one of either the Secure Hash Algorithm (SHA-1) or Message Digest (MD5) algorithm to produce a fixed length output word is provided. The apparatus includes a plurality of rotational registers for storing data, one of the registers being arranged to receive the input data, and data stores for initialization of some of the plurality of registers according to whether the SHA-1 or MD5 algorithm is used, the data stores including fixed data relating to SHA-1 and MD5 operation, and a plurality of dedicated combinatorial logic circuits arranged to perform logic operations on data stored in selected ones of the plurality of registers. 
   Preferably, the register that receives the input data is arranged to receive the input data serially. 
   Preferably, the registers and combinatorial logic circuits are interconnected for communication via a pair of data busses. It is particularly preferable if the registers and combinatorial logic circuits are connected to write to a respective bus via respective tristate buffers. 
   Preferably, the apparatus includes a control circuit arranged to generate individually gated clock signals for each register. This results in lower power consumption as only active registers need to be clocked. 
   Preferably, the control circuit is further arranged to generate individual enabling signals to control the tristate buffers. The control circuit may be implemented as a dedicated state machine or by another means such as a microcontroller. 
   Preferably, the rotational registers are arranged to be multiplexed prior to connection to a tristate buffer. This results in a lower bus loading. 
   Preferably, the combinatorial logic circuits include a copy circuit, a shift left circuit, a NOT circuit, an ADD circuit, an OR circuit, an AND circuit and an XOR circuit. Each circuit is dedicated to its particular task, avoiding redundancy. 
   Preferably, the apparatus is implemented as an integrated circuit, typically of the ASIC type. The apparatus may be incorporated with other apparatus, typically digital signature apparatus. 
   Embodiments of the present invention utilize the fact that both algorithms may be broken down into a series of individual steps. Prior approaches to implementing the algorithms in software do not utilize any specialized hardware components, which results in a relatively slow process. However, embodiments of the invention identify, where possible, the common elements between the MD5 and SHA-1 algorithms and provide specialized hardware components to achieve the required functionality. Hardware is selected to allow for the maximum sharing of components and hence the minimum overall component count. 
   Embodiments of the present invention allow a relatively small number of dedicated components to be used in a circuit to efficiently calculate either MD5 or SHA-1 message digests. Since the operations involved in both algorithms are similar, the circuit can be optimized such that components that are common to both algorithms are provided only once. Allowing either of the algorithms to be used in calculating a message digest is advantageous as there are several digital signature systems operational that make use of one or other of the SHA-1 or MD5 algorithms. Systems that utilize an embodiment of the invention will benefit from increased flexibility and speed. 
   In accordance with another embodiment of the invention, a circuit is provided that includes a plurality of data storage registers for storing data to be processed, a plurality of shift registers for temporary data storage, a plurality of logic circuits for performing operations on data, and a control circuit configured to control the data storage registers, the circular shift registers, and the logic circuits to selectively perform MD5 and SHA-1 operations on data. 
   In accordance with another aspect of the foregoing embodiment, a plurality of initialization storage registers are provided to store and output initialization data. 
   In accordance with a further embodiment of the invention, a dual hash algorithm circuit is provided that includes a first bank and a second bank of data storage registers, a first bank and a second bank of circular shift registers, including at least one register to receive a data stream as input to the circuit, a bank of initialization data registers, a bank of temporary data registers, a plurality of combinatorial logic circuits, a read bus and a write bus, a control system for selectively coupling and uncoupling the first bank and second bank of data storage registers, the first bank and second bank of circular shift registers, the bank of initialization data registers, the bank of temporary data registers, and the plurality of combinatorial logic circuits to the read bus and the write bus to selectively perform MD5 and SHA-1 operations on the data to output data of a fixed length in accordance with the selected MD5 and SHA-1 operations. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     For a better understanding of the present invention and to understand how the same may be brought into effect, the invention will now be described by way of example only, with reference to the appended drawings in which: 
       FIG. 1  shows a view of the architecture of the combined SHA-1 and MD5 processor; and 
       FIG. 2  shows a view of the control circuit used to control the architecture of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a customized architecture which is arranged to receive a data input  150 , process it using the shown elements, and produce a data output  155 . The hardware shown is able to perform either SHA-1 or MD5 processing on the input data, and has been optimized in order to minimize the amount of hardware needed to perform either one of the algorithms. 
   The circuit includes a plurality of registers for storing data. There are ten registers provided in two banks  110 ,  115  for storing part of the data being processed. In addition, two temporary registers  120 ,  135  are provided for intermediate processing and temporary storage. Also provided are two banks  125 ,  130  of circular shift registers W 15 [31:0]−W 0 [31:0]. Register W 15  of bank  125  is arranged to receive the input data  150 . Any data held in W 15  at that time is shifted to W 14 ; the data in W 14  is shifted to W 13  and so on, until the data held in W 0  is lost. The outputs of banks  125  and  130  are multiplexed before being attached to the read bus  140  by a tristate buffer in order to reduce bus loading. 
   The registers are mutually interconnected for communication via a read bus  140  and a write bus  145 . 
   The read bus  140  is connected to a range of logic circuits which provide combinatorial functions. These functions are: Copy (CP)  200 , Shift Left multiple positions (SL*)  205 , NOT  210 , ADD  215 , OR  220 , AND  225 , XOR  230  and Shift Left one position (SL 1 )  235 . Functions  200 ,  205 ,  210  require only a single input variable and receive it directly from the read bus  140 . The other functions  215 ,  220 ,  225  and  230  require two input variables and receive one from the read bus  140  and the other from the temporary register (ACCU[31:0])  135 . Register  135  also provides the input for shift register  235 . 
   Also connected to the read bus via a multiplexer and a tristate logic gate is a bank  160  of registers including fixed constants used in the initialization of the circuit for either SHA-1 or MD5 mode calculations. K[t] is provided for initialization of SHA-1, and T[i] is provided for initialization of MD5. In total, approximately seventy five constants each having a length of 32 bits are required, and grouping them together in this fashion allows them to be conveniently accessed. The synthesis tool which places the gates in the finished custom device is then able to optimize the logic, resulting in a smaller gate count, and thus a smaller area of silicon is required. 
   Calculation of either SHA-1 or MD5 requires the use of selected ones of the provided registers and combinatorial functions. In particular, calculation of the SHA-1 algorithm uses all the registers of bank  110  and of bank  115 . Calculation of MD5 requires only the use of four registers (H 0 -H 3 ) of bank  110  and four registers (A-D) of bank  115 . This allows the unused registers to be used for temporary storage if required. However, when the result of the calculation  155  is unloaded from register H 0  of bank  110 , all five registers are read since they are implemented as shift registers, and this ensures that their contents are unchanged. 
   All devices that can output data to the read bus  140  are connected to the bus via a tristate buffer. Each buffer is individually enabled via a control signal created by the control circuit shown in  FIG. 2 . Likewise, the combinatorial functions  200 - 235  which can write data onto the write bus  145  are connected to the write bus  145  via individually controllable tristate buffers. 
   The group of clock signals  345  to individual registers are created from a master clock signal  340 . The master clock signal is ANDed with a control signal to create a gated clock signal for the appropriate register. In this way, the energy consumption of the complete circuit is reduced because only active registers need to be clocked. 
     FIG. 2  shows a top level view of the control circuit  400  which generates the various control signals for the circuit of  FIG. 1 . In particular, it generates, from a master clock signal  340 , a series  345  of gated individual clock signals that are used to clock the various registers of  FIG. 1 . It also generates individual enable signals for each of the tristate buffers shown in  FIG. 1 . The control circuit may take the form of a finite state machine including associated controlling circuits. 
   The following pseudo-code represents the steps performed in calculating a message digest according to the SHA-1 algorithm on an input data word of arbitrary length. 
   The high level algorithm details in broad terms the steps taken in performing a calculation according to the SHA-1 algorithm. The following more detailed code provides step by step instructions on performing the individual instructions needed to calculate the message digest. 
   
     
       
             
           
             
             
           
         
             
                 
             
             
               SHA-1 Algorithm 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               // High Level Algorithm 
             
             
                 
               initialize SHA-1 internal registers (H0,H1,H2,H3,H4) 
             
             
                 
               for each Mi, block of 512 bits of M do 
             
             
                 
               load Mi into data registers W[0] to W[15] 
             
             
                 
               start core SHA-1 
             
             
                 
               end 
             
             
                 
               unload H0,H1,H2,H3,H4 
             
             
                 
               //Detailed steps 
             
             
                 
               SHA-1 initialization 
             
             
                 
               H0 = 67452301 
             
             
                 
               H1 = EFCDAB89 
             
             
                 
               H2 = 98BADCFE 
             
             
                 
               H3 = 10325476 
             
             
                 
               H4 = C3D2E1F0 
             
             
                 
               Core SHA-1 
             
             
                 
               A=H0, B=H1, C=H2, D=H3, E=H4 
             
             
                 
               MASK=0000000F 
             
             
                 
               for t=0 to 79 do 
             
             
                 
               s = t and MASK; 
             
             
                 
               if (t&gt;=16) W[s] = SL1 ( W[(s + 13) and MASK] xor 
             
             
                 
               W [(s + 8) and MASK] xor 
             
             
                 
               W [(s + 2) and MASK] xor W[s]); 
             
             
                 
               end if 
             
             
                 
               TEMP = SL5(A)+ ft(B,C,D)+E +W[s]+K[t] 
             
             
                 
               E=D, D=C, C=SL30(B), B=A, A=TEMP 
             
             
                 
               end for 
             
             
                 
               H0 =H0+A, H1 =H1+B, H2 =H2+C, H3=H3+D, H4=H4+E 
             
             
                 
               // The functions SL1, SL5 and SL30 are circular left rotation 
             
             
                 
               // of the 32 bit operand by 1 bit, 5 bits and 30 bit 
             
             
                 
               // respectively. 
             
             
                 
               // The constants Kt are defined by the following: 
             
             
                 
               Kt = 5A82 7999 ( 0 &lt;= t &lt;= 19) 
             
             
                 
               Kt = 6ED9 EBA1 (20 &lt;= t &lt;= 39) 
             
             
                 
               Kt = 8F1B BCDC (40 &lt;= t &lt;= 59) 
             
             
                 
               Kt = CA62 C1D6 (60 &lt;= t &lt;= 79). 
             
             
                 
               // The functions ft(B,C,D) is defined by the following: 
             
             
                 
               ft (B,C,D) = (B and C) or ((not B) and D) (0 &lt;= t &lt;= 19) 
             
             
                 
               ft (B,C,D) = B xor C xor D (20 &lt;= t &lt;= 39) 
             
             
                 
               ft (B,C,D) = (B and C) or (B and D) or (C and D) (40 &lt;= t &lt;= 59) 
             
             
                 
               ft (B,C,D) = B xor C xor D (60 &lt;= t &lt;= 79). 
             
             
                 
                 
             
           
        
       
     
   
   The following pseudo-code represents the steps performed in calculating a message digest according to the MD5 algorithm on an input data word of arbitrary length. 
   
     
       
             
           
             
           
         
             
                 
             
             
               N05 Pseudo Algorithm 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               // Here, the four auxiliary functions that each take as input 
             
             
               // three 32-bit words and produce as output one 32-bit word 
             
             
               // are defined: 
             
             
               F (X, Y, Z) = (X and Y) or (not (X) and Z) 
             
             
               G(X,Y,Z) = (X and Z) or (Y and not(Z)) 
             
             
               H(X,Y,Z) = X xor Y xor Z 
             
             
               ┌(X,Y,Z) = Y xor (X or not(Z)) 
             
             
               // A 64-element table T[1 ... 64] constructed from the sine 
             
             
               // function is defined. Let T[i] denote the i-th element of 
             
             
               // the table, which is equal to the integer part of 4294967296 
             
             
               // times abs(sin(i)), where i is in radians. 
             
             
               High Level Algorithm 
             
             
               initialize MD5 internal registers (H0,H1,H2,H3) 
             
             
               for each Mi, block of 512 bit of M do 
             
             
               load Mi into data registers W[0] to W[15] 
             
             
               start core MD5 
             
             
               end 
             
             
               unload H0, H1, H2, H3 
             
             
               MD5 initialization 
             
             
               H0 = 67 45 23 01 
             
             
               H1 = of cd ab 89 
             
             
               H2 = 98 ba dc fe 
             
             
               H3 = 10 32 54 76 
             
             
               Core MD5 
             
             
               A=H0, B=H1, C=H2, D=H3 
             
             
               // Round 1. 
             
             
               // Let (abcd k s i) denote the operation 
             
             
               // a = b + ((a + F(b,c,d) + W[k] + T[i]) &lt;&lt;&lt; s). 
             
             
               // Do the following 16 operations. 
             
             
               [ABCD 0 7 1] [DABC 1 12 2] [CDAB 2 17 3] [BCDA 3 22 4] 
             
             
               [ABCD 4 7 5] [DABC 5 12 6] [CDAB 6 17 7] [BCDA 7 22 8] 
             
             
               [ABCD 8 7 9] [DABC 9 12 10] [CDAB 10 17 11] [BCDA 11 22 12] 
             
             
               [ABCD 12 7 13] [DABC 13 12 14] [CDAB 14 17 15] [BCDA 15 22 16] 
             
             
               // Round 2. 
             
             
               // Let (abcd k s i] denote the operation 
             
             
               // a = b + ((a + G(b,c,d) + W[k] + T[i]) &lt;&lt;&lt; s). 
             
             
               // Do the following 16 operations. 
             
             
               [ABCD 1 5 17] [DABC 6 9 18] [CDAB 11 14 19] [BCDA 0 20 20] 
             
             
               [ABCD 5 5 21] [DABC 10 9 22] [CDAB 15 14 23] [BCDA 4 20 24] 
             
             
               [ABCD 9 5 25] [DABC 14 9 26] [CDAB 3 14 27] [BCDA 8 20 28] 
             
             
               [ABCD 13 5 29] [DABC 2 9 30] [CDAB 7 14 31] [BCDA 12 20 32] 
             
             
               // Round 3. 
             
             
               // Let [abcd k s i] denote the operation 
             
             
               // a = b + ((a + H(b,c,d) + W[k] + T[i]) &lt;&lt;&lt; s). 
             
             
               // Do the following 16 operations. 
             
             
               [ABCD 5 4 33] [DABC 8 11 34] [CDAB 11 16 35] [BCDA 14 23 36] 
             
             
               [ABCD 1 4 37] [DABC 4 11 38] [CDAB 7 16 39] [BCDA 10 23 40] 
             
             
               [ABCD 13 4 41] [DABC 0 11 42] [CDAB 3 16 43] [BCDA 6 23 44] 
             
             
               [ABCD 9 4 45] [DABC 12 11 46] [CDAB 15 16 47] [BCDA 2 23 48] 
             
             
               // Round 4. 
             
             
               // Let [abcd k s i] denote the operation 
             
             
               // a = b + ((a + I (b, c, d) + W [k] + T [i]) &lt;&lt;&lt; s). 
             
             
               // Do the following 16 operations. 
             
             
               [ABCD 0 6 49] [DABC 7 10 50] [CDAB 14 15 51] [BCDA 5 21 52] 
             
             
               [ABCD 12 6 53] [DABC 3 10 541 [CDAB 10 15 55] [BCDA 1 21 56] 
             
             
               [ABCD 8 6 57] [DABC 15 10 58] [CDAB 6 15 59] [BCDA 13 21 60] 
             
             
               [ABCD 4 6 61] [DABC 11 10 62] [CDAB 2 15 63] [BCDA 9 21 64] 
             
             
               H0 =H0+A, H1 =H1+B, H2 =H2+C, H3=H3+D 
             
             
                 
             
           
        
       
     
   
   The information below sets out the so-called atomic operations which are required to perform the different algorithm calculations. The following steps indicate the operation number, the operation performed, and the status of the read  140  and write  145  busses. Each operation listed below takes exactly one clock cycle. 
   SHA-1 Algorithm 
   Initialization 
                                       ##   operation   Readbus   Writebus                   01.   A := H0   H0   (copy)       02.   B := H1   H1   (copy)       03.   C := H2   H2   (copy)       04.   D := H3   H3   (copy)       05.   E := H4   H4   (copy)                    
0&lt;=t&lt;=15
 
                                       ##   operation   Readbus   Writebus                   01.   ACCU := B   B   (copy)       02.   TMP := ACCU and C   C   (and)       03.   ACCU := NOT B   B   (not)       04.   ACCU := ACCU and D   D   (and)       05.   ACCU := ACCU or TMP   TMP   (or)       06.   ACCU := ACCU + W[0]   W[0]   (+)       07.   ACCU := ACCU + E   E   (+)       08.   TMP := SL5(A)   A   (SL5)       09.   ACCU := ACCU + TMP   TMP   (+)       10.   TMP := ACCU + K[t]   K[t]   (+)       11.   E := D   D   (copy)       12.   D := C   C   (copy)       13.   C := SL30(B)   B   (SL30)       14.   B := A   A   (copy)       15.   A := TMP   TMP   (copy)       16.   ROTATE W[i]                    
16&lt;=t&lt;=19
 
                                       ##   operation   Readbus   Writebus                   01.   ACCU := B   B   (copy)       02.   TMP := ACCU and C   C   (and)       03.   ACCU := NOT B   B   (not)       04.   ACCU := ACCU and D   D   (and)       05.   TMP := ACCU or TMP   TMP   (or)       06.   ACCU := W[13]   W[13]   (copy)       07.   ACCU := ACCU xor W[8]   W[8]   (xor)       08.   ACCU := ACCU xor W[2]   W[2]   (xor)       09.   ACCU := ACCU xor W[0]   W[0]   (xor)       10.   W[0] := SL1   (ACCU)   (SL1)       11.   ACCU := W[0]   W[0]   (copy)       12.   ACCU := ACCU + TMP   TMP   (+)       13.   ACCU := ACCU + E   E   (+)       14.   TMP := SL5(A)   A   (SL5)       15.   ACCU := ACCU + TMP   TMP   (+)       16.   TMP := ACCU + K[t]   K[t]   (+)       17.   E := D   D   (copy)       18.   D := C   C   (copy)       19.   C := SL30(B)   B   (SL30)       20.   B := A   A   (copy)       21.   A := TMP   TMP   (copy)       22.   ROTATE W[i]                    
20&lt;=t&lt;=39 and 60&lt;=t&lt;=79
 
Final Round
 
                                       ##   operation   Readbus   Writebus                   01.   ACCU := A   A   (copy)       02.   H0 := ACCU + H0   H0   (+)       03.   ACCU := B   B   (copy)       04.   H1 := ACCU + H1   H1   (+)       05.   ACCU := C   C   (copy)       06.   H2 := ACCU + H2   H2   (+)       07.   ACCU := D   D   (copy)       08.   H3 := ACCU + H3   H3   (+)       09.   ACCU := E   E   (copy)       10.   H4 := ACCU + H4   H4   (+)                    
MD5 Algorithm.
 
Initialization
 
                                       ##   operation   Readbus   Writebus                   01.   A := H0   H0   (copy)       02.   B := H1   H1   (copy)       03.   C := H2   H2   (copy)       04.   D := H3   H3   (copy)                    
Round 1 (16 iterations): 0&lt;=i&lt;=15; k=0; s=7, 12, 17, 22, 7, 12, 17, 22 . . .
 
                                       ##   operation   Readbus   Writebus                   01.   ACCU := B   B   (copy)       02.   TMP := ACCU and C   C   (and)       03.   ACCU := NOT B   B   (not)       04.   ACCU := ACCU and D   D   (and)       05.   TMP := ACCU or TMP   TMP   (or)       06.   ACCU := W [k]   W [k]   (copy)       07.   ACCU := ACCU + A   A   (+)       08.   ACCU := ACCU + T[i]   T[i]   (+)       09.   TMP := ACCU + TMP   TMP   (+)       10.   ACCU := SL[s](TMP)   TMP   (SL[s])       11.   TMP := ACCU + B   B   (+)       12.   A := D   D   (copy)       13.   D := C   C   (copy)       14.   C := B   B   (copy)       15.   B := TMP   TMP   (copy)       16.   ROTATE W[k]                    
Preparation for Round 2
 
   01. ROTATE W[k] 
   Round 2 (16 iterations): 16&lt;=i&lt;=31; k=1; s=5, 9, 14, 20, 5, 9, 14, 20, 5 . . . 
                                       ##   operation   Readbus   Writebus                   01.   ACCU := B   B   (copy)       02.   TMP := ACCU and D   D   (and)       03.   ACCU := NOT D   D   (not)       04.   ACCU := ACCU and C   C   (and)       05.   TMP := ACCU or TMP   TMP   (or)       06.   ACCU := W[k]   W[k]   (copy)       07.   ACCU := ACCU + A   A   (+)       08.   ACCU := ACCU + T[i]   T[i]   (+)       09.   TMP := ACCU + TMP   TMP   (+)       10.   ACCU := SL[s](TMP)   TMP   (SL[s])       11.   TMP := ACCU + B   B   (+)       12.   A := D   D   (copy)       13.   D := C   C   (copy)       14.   C := B   B   (copy)       15.   B := TMP   TMP   (copy)       16.   ROTATE W[k]       17.   ROTATE W[k]       18.   ROTATE W[k]       19.   ROTATE W[k]       20.   ROTATE W[k]                    
Preparation for Round 3
 
                                               01.   ROTATE W[k]           02.   ROTATE W[k]           03.   ROTATE W[k]           04.   ROTATE W[k]                        
Round 3 (16 iterations): 32&lt;=i&lt;=47; k=5; s=4, 11, 16, 23, 4, 11, 16 . . .
 
   As an example of how the information above should be interpreted, step number 2 of the SHA-1 initialization section relates to the operation B:=H 1 , meaning that the register B is set to the value stored in H 1 . To achieve this, the tristate buffer  321  of register H 1  and the tristate buffer  301  of the copy logic are enabled together. At the same time, the clock to register B is enabled, resulting in the data in H 1  being written into B. The tristate buffer control and clock signals are generated by the control circuit  400 . 
   Similarly, step number 10 in the SHA-1 0&lt;=t&lt;=15 stage relates to the operation TMP:=ACCU+K[t]. The multiplexer and tristate buffer  332  is enabled for K[10]. The tristate buffer  304  is enabled for the ADD logic  215  and a gated clock signal is created and applied to the TMP register  120 . In this way, the rising clock signal causes the sum of the data in K[10] and ACCU to be written into the TMP register. 
   The last instruction in the 0&lt;=t&lt;=15 stage for SHA-1 (and the 0&lt;=i&lt;=15 stage for MD5) causes the entire Wi chain to be rotated, so that W 14  is loaded with the data previously in W 15 , W 13  receives the data previously in W 14 , and W 15  receives the data previously in W 0 . Advantageously, this instruction may be implemented in parallel with the instruction above it (Step 15) as the rotate instruction does not involve placing data onto the data bus. In this way, one clock cycle per iteration is saved, leading to a total saving of 80 cycles for SHA-1 and 64 cycles for MD5. 
   The embodiment presented has a bus width of 32 bits. However, it is possible to reduce the bus width to reduce the silicon area of the design at the expense of operational speed. If the bus width is reduced to 16 bits, each 32 bit XOR operation, for example, will take two cycles rather than one cycle if a 32 bit bus was used. 
   The present invention includes and novel feature or combination of features disclosed herein either explicitly or any generalization thereof irrespective of whether or not it relates to the claimed invention or mitigates any or all of the problems addressed. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims and the equivalents thereof.