MD5 筆記

MD5 Intro

MD5 訊息摘要演算法（英語：MD5 Message-Digest Algorithm），一種被廣泛使用的密碼雜湊函式，可以產生出一個128位元（16個字元(BYTES)）的雜湊值（hash value），用於確保資訊傳輸完整一致。(節自wiki)

簡易 MD5 工具 : md5sum

基本上 Unix, Linux 的作業系統都有預設 md5sum

• md5sum 用法

      # pipe
      $ echo "123" | md5sum
      ba1f2511fc30423bdbb183fe33f3dd0f  -
  
      # 讀檔
      $ md5sum .gitignore
      247bc32fd24d78844194917cb32d556a  .gitignore
  
      # check
      $ md5sum .gitignore > tmp.md5
      $ md5sum -c tmp.md5
      .gitignore: OK
  
      # pipe check
      $ echo "247bc32fd24d78844194917cb32d556a .gitignore" | md5sum -c
      .gitignore: OK
  

openssl/evp.h

The EVP digest routines are a high level interface to message digests (節自 Linux man page)。這裡註記一下 EVP 大概是代表 Envelope。雖然現在已經被改掉，但以前的 openssl/evp.h 的 #ifndef 是 HEADER_ENVELOPE_H。

• evp 大概的流程

      #include <stdio.h>
      #include <string.h>
      #include <openssl/evp.h>
  
      int main(int argc, char *argv[])
      {
          EVP_MD_CTX *mdctx;
          const EVP_MD *md;
          char mess1[] = "Test Message\n";
          char mess2[] = "Hello World\n";
          unsigned char md_value[EVP_MAX_MD_SIZE];
          unsigned int md_len, i;
  
          if (argv[1] == NULL) {
              printf("Usage: mdtest digestname\n");
              exit(1);
          }
  
          OpenSSL_add_all_algorithms(); // 有可能有些 algo 沒有 load, ex. RSA-SHA1
          md = EVP_get_digestbyname(argv[1]);
          if (md == NULL) {
              printf("Unknown message digest %s\n", argv[1]);
              exit(1);
          }
  
          mdctx = EVP_MD_CTX_new();
          EVP_DigestInit_ex(mdctx, md, NULL); // _ex() 較有效率, 以前的 code 可能是 EVP_DigestInit()
          EVP_DigestUpdate(mdctx, mess1, strlen(mess1));
          EVP_DigestUpdate(mdctx, mess2, strlen(mess2));
          EVP_DigestFinal_ex(mdctx, md_value, &md_len);
          EVP_MD_CTX_free(mdctx);
  
          printf("Digest is: ");
          for (i = 0; i < md_len; i++)
              printf("%02x", md_value[i]);
          printf("\n");
  
          exit(0);
      }
  

參考資料 :

1.MD5 wiki

2.MD5SUM wiki

3.linux man page

4.stackoverflow

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讀取 Linux file info (stat)

讀取 Linux 的檔案資訊

Linux 的檔案有很多資訊，即使只用到檔案大小也是很方便。

stat

• struct stat

  stat 的資料結構

      #include <sys/stat.h>
  
      struct stat {
          dev_t     st_dev;         /* ID of device containing file */
          ino_t     st_ino;         /* Inode number */
          mode_t    st_mode;        /* File type and mode */
          nlink_t   st_nlink;       /* Number of hard links */
          uid_t     st_uid;         /* User ID of owner */
          gid_t     st_gid;         /* Group ID of owner */
          dev_t     st_rdev;        /* Device ID (if special file) */
          off_t     st_size;        /* Total size, in bytes */
          blksize_t st_blksize;     /* Block size for filesystem I/O */
          blkcnt_t  st_blocks;      /* Number of 512B blocks allocated */
  
          /* Since Linux 2.6, the kernel supports nanosecond
              precision for the following timestamp fields.
              For the details before Linux 2.6, see NOTES. */
  
          struct timespec st_atim;  /* Time of last access */
          struct timespec st_mtim;  /* Time of last modification */
          struct timespec st_ctim;  /* Time of last status change */
  
          #define st_atime st_atim.tv_sec      /* Backward compatibility */
          #define st_mtime st_mtim.tv_sec
          #define st_ctime st_ctim.tv_sec
      };
  

• struct stat

  讀取檔案 (這裡讀 .gitignore)

      #include <stdio.h>    // for printf
      #include <time.h>     // for ctime
      #include <sys/stat.h>
  
      int main()
      {
          struct stat st;
          // 0 success, -1 failed
          // error code: errno
          if(stat(".gitignore", &st) == -1){
              return -1;
          }
          printf("I-node number:             %ld\n", (long) st.st_ino);
          printf("Mode:                      %lo (octal)\n", (unsigned long) st.st_mode);
          printf("Link count:                %ld\n", (long) st.st_nlink);
          printf("Ownership:                 UID=%ld   GID=%ld\n", (long) st.st_uid, (long) st.st_gid);
          printf("device containing file id: %ld\n", (long) st.st_dev);
          printf("device id:                 %ld\n", (long) st.st_rdev);
          printf("File size:                 %lld bytes\n", (long long) st.st_size);
          printf("Preferred I/O block size:  %ld bytes\n", (long) st.st_blksize);
          printf("Blocks allocated:          %lld\n", (long long) st.st_blocks);
          printf("Last status change:        %s", ctime(&st.st_ctime));
          printf("Last file access:          %s", ctime(&st.st_atime));
          printf("Last file modification:    %s", ctime(&st.st_mtime));
          return 0;
      }
  

      I-node number:             20057552
      Mode:                      100664 (octal)
      Link count:                1
      Ownership:                 UID=1000   GID=1000
      device containing file id: 66306
      device id:                 0
      File size:                 16 bytes
      Preferred I/O block size:  4096 bytes
      Blocks allocated:          8
      Last status change:        Tue Mar 29 14:03:01 2022
      Last file access:          Sun Oct  9 19:02:52 2022
      Last file modification:    Thu Jun 17 10:20:57 2021
  

參考資料 :

1.Linux manual page

2.程式人生

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