void AES_ecb_decrypt()

in src/aes.c [445:519]

• 63 lines of code
• 4 McCabe index (conditional complexity)

void AES_ecb_decrypt(const amcl_aes *a,uchar *buff)
{
    int i;
    int j;
    int k;
    unsign32 p[4];
    unsign32 q[4];
    unsign32 *x;
    unsign32 *y;
    unsign32 *t;

    for (i=j=0; i<NB; i++,j+=4)
    {
        p[i]=pack(&buff[j]);
        p[i]^=a->rkey[i];
    }

    k=NB;
    x=p;
    y=q;

    /* State alternates between x and y */
    for (i=1; i<a->Nr; i++)
    {
        /* Nr is number of rounds. May be odd. */

        y[0]=a->rkey[k]^rtable[MR_TOBYTE(x[0])]^
             ROTL8(rtable[MR_TOBYTE(x[3]>>8)])^
             ROTL16(rtable[MR_TOBYTE(x[2]>>16)])^
             ROTL24(rtable[x[1]>>24]);
        y[1]=a->rkey[k+1]^rtable[MR_TOBYTE(x[1])]^
             ROTL8(rtable[MR_TOBYTE(x[0]>>8)])^
             ROTL16(rtable[MR_TOBYTE(x[3]>>16)])^
             ROTL24(rtable[x[2]>>24]);
        y[2]=a->rkey[k+2]^rtable[MR_TOBYTE(x[2])]^
             ROTL8(rtable[MR_TOBYTE(x[1]>>8)])^
             ROTL16(rtable[MR_TOBYTE(x[0]>>16)])^
             ROTL24(rtable[x[3]>>24]);
        y[3]=a->rkey[k+3]^rtable[MR_TOBYTE(x[3])]^
             ROTL8(rtable[MR_TOBYTE(x[2]>>8)])^
             ROTL16(rtable[MR_TOBYTE(x[1]>>16)])^
             ROTL24(rtable[x[0]>>24]);

        k+=4;
        t=x;
        x=y;
        y=t;      /* swap pointers */
    }


    /* Last Round */
    y[0]=a->rkey[k]^(unsign32)rbsub[MR_TOBYTE(x[0])]^
         ROTL8((unsign32)rbsub[MR_TOBYTE(x[3]>>8)])^
         ROTL16((unsign32)rbsub[MR_TOBYTE(x[2]>>16)])^
         ROTL24((unsign32)rbsub[x[1]>>24]);
    y[1]=a->rkey[k+1]^(unsign32)rbsub[MR_TOBYTE(x[1])]^
         ROTL8((unsign32)rbsub[MR_TOBYTE(x[0]>>8)])^
         ROTL16((unsign32)rbsub[MR_TOBYTE(x[3]>>16)])^
         ROTL24((unsign32)rbsub[x[2]>>24]);
    y[2]=a->rkey[k+2]^(unsign32)rbsub[MR_TOBYTE(x[2])]^
         ROTL8((unsign32)rbsub[MR_TOBYTE(x[1]>>8)])^
         ROTL16((unsign32)rbsub[MR_TOBYTE(x[0]>>16)])^
         ROTL24((unsign32)rbsub[x[3]>>24]);
    y[3]=a->rkey[k+3]^(unsign32)rbsub[MR_TOBYTE(x[3])]^
         ROTL8((unsign32)rbsub[MR_TOBYTE(x[2]>>8)])^
         ROTL16((unsign32)rbsub[MR_TOBYTE(x[1]>>16)])^
         ROTL24((unsign32)rbsub[x[0]>>24]);

    for (i=j=0; i<NB; i++,j+=4)
    {
        unpack(y[i],&buff[j]);
        x[i]=y[i]=0;   /* clean up stack */
    }

}