[英]C getting equal hash values for different strings openssl
i am pretty new to c. 我对c很新。 I am trying to compare two files with md5.
我想用md5比较两个文件。 I wrote a function which should return the hash values.
我写了一个应该返回哈希值的函数。 But when comparing the values of different files or buffers, it says that they have the same hash.
但是在比较不同文件或缓冲区的值时,它表示它们具有相同的哈希值。
unsigned char* getMD5(void *buffer, size_t bsize) {
EVP_MD_CTX *mdctx;
const EVP_MD *md;
unsigned char hashwert[EVP_MAX_MD_SIZE];
int hashwert_laenge;
OpenSSL_add_all_digests();
md = EVP_get_digestbyname("MD5");
mdctx = EVP_MD_CTX_create();
EVP_DigestInit_ex(mdctx, md, NULL);
EVP_DigestUpdate(mdctx, buffer, bsize);
EVP_DigestFinal_ex(mdctx, hashwert, &hashwert_laenge);
EVP_MD_CTX_destroy(mdctx);
return hashwert;
}
//in main...
char mess[] = "abc";
cahr mess2[] = "bcd";
if(strcmp(getMD5(mess, strlen(mess)),getMD5(mess2, strlen(mess2))==0) {
printf("euqal\n");
}else {
printf("not equal \n");
}
I always get that the buffers are equal, even if they are not. 我总是得到缓冲区是相等的,即使它们不是。 Regards
问候
You should compile with all warnings enabled and debugging info, eg with gcc -Wall -g
on Linux. 您应该在启用所有警告和调试信息的情况下进行编译,例如在Linux上使用
gcc -Wall -g
。
It will have warned you: function returns address of local variable . 它会警告你: 函数返回局部变量的地址 。
Newbies and expert C programmers usually should improve their code till no warnings are given. 新手和专家C程序员通常应该改进他们的代码,直到没有给出警告。 If your code triggers a warning that you really cannot avoid you should at least comment very carefully why.
如果您的代码触发了一个您无法避免的警告,那么至少应该仔细评论原因。
You cannot meaningfully return the address of some local array. 你无法有意义地返回一些本地数组的地址。
You could return strdup(hashwert);
你可以
return strdup(hashwert);
and have the convention that the calling function (the caller) should free
the result. 并且具有调用函数(调用者)应该
free
结果的约定。
Or you could have a different API, for example having hashwert
be a parameter of your function. 或者您可以使用不同的API,例如将
hashwert
作为函数的参数。
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