www.pudn.com > miracl.zip > CRDECODE.CPP

/* 
 *   Cramer-Shoup public Key system - Hybrid Implementation  
 * 
 *   This program decrypts the contents of the file xxx.crs to the screen 
 *   usng the AES key recovered from xxx.key using the Cramer-Shoup 
 *   private key from private.crs 
 * 
 *   Requires: big.cpp 
 * 
 *   Copyright (c) 1999 Shamus Software Ltd. 
 */ 
 
#include  
#include  
#include  
#include  
 
using namespace std; 
 
Miracl precision(300,256); 
 
void strip(char *name) 
{ /* strip extension off filename */ 
    int i; 
    for (i=0;name[i]!='\0';i++) 
    { 
        if (name[i]!='.') continue; 
        name[i]='\0'; 
        break; 
    } 
} 
 
void hash(sha *s,Big x) 
{ /* hash in a big number */ 
    sha sh; 
    int ch; 
    while (x!=0) 
    { 
        ch=x%256; x/=256; 
        shs_process(s,ch); 
    } 
} 
 
int main() 
{ 
    int i,bits; 
    static char ifname[100]; 
    ifstream common("common.crs");    // construct file I/O streams  
    ifstream private_key("private.crs"); 
    ifstream key_file,ciphertext; 
    Big p,q,g1,g2,x1,x2,y1,y2,z,u1,u2,e,v,m,e1,e2,alpha; 
    char ch,H[20],iv[16],block[16];  
    miracl *mip=&precision; 
    sha sh; 
    aes a; 
 
// get common data  
    mip->IOBASE=16; 
    common >> bits; 
    common >> q >> p >> g1 >> g2; 
 
// get private key 
    private_key >> x1 >> x2 >> y1 >> y2 >> z; 
 
// figure out where input is coming from 
 
    cout << "file to be decoded = "; 
    cin >> ifname; 
 
    strip(ifname);          // open key file 
    strcat(ifname,".key"); 
    key_file.open(ifname,ios::in); 
    if (!key_file) 
    { 
        cout << "Key file " << ifname << " not found\n"; 
        return 0; 
    } 
 
    strip(ifname);          // open ciphertext 
    strcat(ifname,".crs"); 
    ciphertext.open(ifname,ios::binary|ios::in); 
    if (!ciphertext) 
    { 
        cout << "Unable to open file " << ifname << "\n"; 
        return 0; 
    } 
 
    mip->IOBASE=16; 
    key_file >> u1 >> u2 >> v; 
 
    shs_init(&sh); 
    hash(&sh,u1); 
    hash(&sh,u2); 
 
// first pass - scan in ciphertext, and calculate alpha=H(u1,u2,ciphertext) 
    cout << "Checking ciphertext validity\n" ; 
    forever 
    { 
        ciphertext.get(ch); 
        if (ciphertext.eof()) break; 
        shs_process(&sh,ch); 
    } 
 
    ciphertext.close(); 
    shs_hash(&sh,H); 
    mip->INPLEN=20;  
    mip->IOBASE=256; 
    alpha=(char *)H;      // alpha=H(u1,u2,ciphertext); 
 
    e1=(x1+y1*alpha)%q; 
    e2=(x2+y2*alpha)%q; 
 
// check validity of ciphertext 
 
    if (v!=pow(u1,e1,u2,e2,p)) 
    { 
        cout << "*** Cipher text is rejected ***" << endl; 
        return 0; 
    } 
 
    shs_init(&sh); 
    hash(&sh,pow(u1,z,p));                           
    shs_hash(&sh,H);      // H=hash(u1^z mod p) = AES Key 
 
    for (i=0;i<16;i++) iv[i]=i;  // Set CFB IV 
    aes_init(&a,MR_PCFB1,16,H,iv);        // Set 128 bit AES Key 
 
// decrypt file on second pass 
 
    ciphertext.open(ifname,ios::binary|ios::in); 
    cout << "decoding message\n"; 
    forever 
    { // decrypt input .. 
        ciphertext.get(ch); 
        if (ciphertext.eof()) break; 
        aes_decrypt(&a,&ch); 
        cout << ch; 
    } 
    aes_end(&a); 
    return 0; 
}