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) return ENOMEM;
  30
  31     int totalBytesRead = 0;
  32     if(skip_offset <= bufSize){
  33         bytesRead = fread(buffer, 1, skip_offset, file);
  34         totalBytesRead += bytesRead;
  35         // printf("totalBytesRead: %i\n", totalBytesRead);
  36         // printf("bytesRead: %i\n", bytesRead);
  37         SHA256_Update(&sha256, buffer, bytesRead);
  38     } else {
  39         int stillToRead = skip_offset-bytesRead;
  40         // printf("Initial stillToRead: %i\n", stillToRead);
  41         int readThisTime;
  42
  43         if(stillToRead>bufSize){
  44             readThisTime = bufSize;
  45         } else {
  46             readThisTime = stillToRead;
  47         }
  48         while((bytesRead = fread(buffer, 1, readThisTime, file)))
  49         {
  50             totalBytesRead += bytesRead;
  51             // printf("totalBytesRead: %i\n", totalBytesRead);
  52             // printf("readThisTime: %i\n", readThisTime);
  53             // printf("bytesRead: %i\n", bytesRead);
  54             SHA256_Update(&sha256, buffer, bytesRead);
  55             stillToRead = skip_offset-totalBytesRead;
  56             // printf("stillToRead: %i\n", stillToRead);
  57
  58             if(stillToRead>bufSize){
  59                 readThisTime = bufSize;
  60             } else {
  61                 readThisTime = stillToRead;
  62             }
  63         }
  64     }
  65
  66     fseek(file, skip_offset+skip_length, SEEK_SET);
  67
  68     /* Instead of the skipped area, calculate the sha256 of the same amount if 0x00s */
  69     int i = 0;
  70     for(i = 0; i < skip_length; i++) {
  71         SHA256_Update(&sha256, "\0", 1);
  72         totalBytesRead += 1;
  73     }
  74
  75     while((bytesRead = fread(buffer, 1, bufSize, file)))
  76     {
  77         totalBytesRead += bytesRead;
  78         // printf("totalBytesRead: %i\n", totalBytesRead);
  79         // printf("bytesRead: %i\n", bytesRead);
  80         SHA256_Update(&sha256, buffer, bytesRead);
  81     }
  82     SHA256_Final(hash, &sha256);
  83
  84     // fprintf(stderr, "totalBytesRead: %i\n", totalBytesRead);
  85
  86     for(i = 0; i < SHA256_DIGEST_LENGTH; i++)
  87     {
  88         sprintf(outputBuffer + (i * 2), "%02x", hash[i]);
  89     }
  90     outputBuffer[64] = 0;
  91
  92     return 0;
  93 }
  94
  95 int main(int argc,char **argv)
  96 {
  97     int res = 0;
  98
  99     if(argc < 2){
 100         fprintf(stderr, "Usage: %s file offset length\n", argv[0]);
 101         fprintf(stderr, "If no offset and length are provided, the ELF section '%s' is skipped\n\n", segment_name);
 102         fprintf(stderr, "Calculate a sha256 of a file except a skipped area from offset to offset+length bytes\n");
 103         fprintf(stderr, "which is replaced with 0x00 during checksum calculation.\n");
 104         fprintf(stderr, "This is useful when a signature is placed in the skipped area.\n");
 105         exit(1);
 106     }
 107
 108     unsigned long skip_offset = 0;
 109     unsigned long skip_length = 0;
 110     char *filename = argv[1];
 111
 112     struct stat st;
 113     if (stat(filename, &st) < 0) {
 114         fprintf(stderr, "not existing file: %s\n", filename);
 115         exit(1);
 116     }
 117
 118     if(argc < 4){
 119         get_elf_section_offset_and_lenghth(filename, ".sha256_sig", &skip_offset, &skip_length);
 120         if(skip_length > 0)
 121             fprintf(stderr, "Skipping ELF section %s with offset %lu, length %lu\n", segment_name, skip_offset, skip_length);
 122     } else if(argc == 4) {
 123         skip_offset = atoi(argv[2]);
 124         skip_length = atoi(argv[3]);
 125     } else {
 126         exit(1);
 127     }
 128
 129     int size = st.st_size;
 130     if(size < skip_offset+skip_length){
 131         fprintf(stderr, "offset+length cannot be less than the file size\n");
 132         exit(1);
 133     }
 134
 135     static char buffer[65];
 136     res = sha256_file(filename, buffer, skip_offset, skip_length);
 137     printf("%s\n", buffer);
 138     return res;
 139 }