digest.c

   1 /*
   2         cc -o digest getsection.c digest.c -lssl -lcrypto
   3 */
   4
   5 #include <stdio.h>
   6 #include <string.h>
   7 #include <errno.h>
   8 #include <openssl/sha.h>
   9 #include <stdlib.h>
  10 #include <sys/stat.h>
  11 #include <linux/limits.h>
  12
  13 #include "getsection.h"
  14
  15 typedef unsigned char byte;
  16
  17 char segment_name[] = ".sha256_sig";
  18
  19 int sha256_file(char *path, char outputBuffer[65], int skip_offset, int skip_length)
  20 {
  21     FILE *file = fopen(path, "rb");
  22     if(!file) return(1);
  23     byte hash[SHA256_DIGEST_LENGTH];
  24     SHA256_CTX sha256;
  25     SHA256_Init(&sha256);
  26     const int bufSize = 1024*1024;
  27     byte *buffer = malloc(bufSize);
  28     int bytesRead = 0;
  29     if(!buffer) {
  30         fclose(file);
  31         return ENOMEM;
  32     }
  33
  34     int totalBytesRead = 0;
  35     if(skip_offset <= bufSize){
  36         bytesRead = fread(buffer, 1, skip_offset, file);
  37         totalBytesRead += bytesRead;
  38         // printf("totalBytesRead: %i\n", totalBytesRead);
  39         // printf("bytesRead: %i\n", bytesRead);
  40         SHA256_Update(&sha256, buffer, bytesRead);
  41     } else {
  42         int stillToRead = skip_offset-bytesRead;
  43         // printf("Initial stillToRead: %i\n", stillToRead);
  44         int readThisTime;
  45
  46         if(stillToRead>bufSize){
  47             readThisTime = bufSize;
  48         } else {
  49             readThisTime = stillToRead;
  50         }
  51         while((bytesRead = fread(buffer, 1, readThisTime, file)))
  52         {
  53             totalBytesRead += bytesRead;
  54             // printf("totalBytesRead: %i\n", totalBytesRead);
  55             // printf("readThisTime: %i\n", readThisTime);
  56             // printf("bytesRead: %i\n", bytesRead);
  57             SHA256_Update(&sha256, buffer, bytesRead);
  58             stillToRead = skip_offset-totalBytesRead;
  59             // printf("stillToRead: %i\n", stillToRead);
  60
  61             if(stillToRead>bufSize){
  62                 readThisTime = bufSize;
  63             } else {
  64                 readThisTime = stillToRead;
  65             }
  66         }
  67     }
  68
  69     fseek(file, skip_offset+skip_length, SEEK_SET);
  70
  71     /* Instead of the skipped area, calculate the sha256 of the same amount if 0x00s */
  72     int i = 0;
  73     for(i = 0; i < skip_length; i++) {
  74         SHA256_Update(&sha256, "\0", 1);
  75         totalBytesRead += 1;
  76     }
  77
  78     while((bytesRead = fread(buffer, 1, bufSize, file)))
  79     {
  80         totalBytesRead += bytesRead;
  81         // printf("totalBytesRead: %i\n", totalBytesRead);
  82         // printf("bytesRead: %i\n", bytesRead);
  83         SHA256_Update(&sha256, buffer, bytesRead);
  84     }
  85     SHA256_Final(hash, &sha256);
  86
  87     // fprintf(stderr, "totalBytesRead: %i\n", totalBytesRead);
  88
  89     for(i = 0; i < SHA256_DIGEST_LENGTH; i++)
  90     {
  91         sprintf(outputBuffer + (i * 2), "%02x", hash[i]);
  92     }
  93     outputBuffer[64] = 0;
  94
  95     fclose(file);
  96     free(buffer);
  97
  98     return 0;
  99 }
 100
 101 int main(int argc,char **argv)
 102 {
 103     int res = 0;
 104
 105     if(argc < 2){
 106         fprintf(stderr, "Usage: %s file offset length\n", argv[0]);
 107         fprintf(stderr, "If no offset and length are provided, the ELF section '%s' is skipped\n\n", segment_name);
 108         fprintf(stderr, "Calculate a sha256 of a file except a skipped area from offset to offset+length bytes\n");
 109         fprintf(stderr, "which is replaced with 0x00 during checksum calculation.\n");
 110         fprintf(stderr, "This is useful when a signature is placed in the skipped area.\n");
 111         exit(1);
 112     }
 113
 114     unsigned long skip_offset = 0;
 115     unsigned long skip_length = 0;
 116     char *filename = argv[1];
 117
 118     struct stat st;
 119     if (stat(filename, &st) < 0) {
 120         fprintf(stderr, "not existing file: %s\n", filename);
 121         exit(1);
 122     }
 123
 124     if(argc < 4){
 125         get_elf_section_offset_and_lenghth(filename, ".sha256_sig", &skip_offset, &skip_length);
 126         if(skip_length > 0)
 127             fprintf(stderr, "Skipping ELF section %s with offset %lu, length %lu\n", segment_name, skip_offset, skip_length);
 128     } else if(argc == 4) {
 129         skip_offset = atoi(argv[2]);
 130         skip_length = atoi(argv[3]);
 131     } else {
 132         exit(1);
 133     }
 134
 135     int size = st.st_size;
 136     if(size < skip_offset+skip_length){
 137         fprintf(stderr, "offset+length cannot be less than the file size\n");
 138         exit(1);
 139     }
 140
 141     static char buffer[65];
 142     res = sha256_file(filename, buffer, skip_offset, skip_length);
 143     printf("%s\n", buffer);
 144     return res;
 145 }