sha1 example in C using openssl library - c

Question

I have a simplest possible example of sha256 written in C, using the openSSL library.
// compile with: gcc -o sha256 sha256.c -lcrypto
#include <openssl/sha.h>
#include <stdio.h>
int main(int argc, char **argv)
{
unsigned char buffer[BUFSIZ];
FILE *f;
SHA256_CTX ctx;
size_t len;
if (argc < 2) {
fprintf(stderr, "usage: %s <file>\n", argv[0]);
return 1;
}
f = fopen(argv[1], "r");
if (!f) {
fprintf(stderr, "couldn't open %s\n", argv[1]);
return 1;
}
SHA256_Init(&ctx);
do {
len = fread(buffer, 1, BUFSIZ, f);
SHA256_Update(&ctx, buffer, len);
} while (len == BUFSIZ);
SHA256_Final(buffer, &ctx);
fclose(f);
for (len = 0; len < SHA256_DIGEST_LENGTH; ++len)
printf("%02x", buffer[len]);
putchar('\n');
return 0;
}
I need the same for sha1, but I could not find similar simple example that actually works. The naive approach of replacing occurrences of SHA256 in the above code with SHA1 does not work (obviously).
How can I modify my program for SHA1 ?
UPDATE
as suggested by #dbush, I have used his EVP code, and integrated it into my program. My program now looks like this:
#include <stdio.h>
#include <openssl/sha.h>
#include <openssl/evp.h>
#include <openssl/err.h>
int main(int argc, char **argv)
{
FILE *f;
size_t len;
unsigned char buffer[BUFSIZ];
if (argc < 2) {
fprintf(stderr, "usage: %s <file>\n", argv[0]);
return 1;
}
f = fopen(argv[1], "r");
if (!f) {
fprintf(stderr, "couldn't open %s\n", argv[1]);
return 1;
}
EVP_MD_CTX hashctx;
//EVP_MD *hashptr = EVP_get_digestbyname("SHA256");
EVP_MD *hashptr = EVP_get_digestbyname("SHA1");
EVP_MD_CTX_init(&hashctx);
EVP_DigestInit_ex(&hashctx, hashptr, NULL));
do {
len = fread(buffer, 1, BUFSIZ, f);
EVP_DigestUpdate(&hashctx, buffer, len);
} while (len == BUFSIZ);
EVP_DigestFinal_ex(&hashctx, buffer, &len);
EVP_MD_CTX_cleanup(&hashctx);
fclose(f);
int i;
for (i = 0; i < len; ++i)
printf("%02x", buffer[i]);
return 0;
}
When I compile it using gcc -o evp evp.c -lcrypto, I get couple of errors, such as:
evp.c: In function ‘main’:
evp.c:29:19: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
evp.c:32:43: error: expected ‘;’ before ‘)’ token
evp.c:32:43: error: expected statement before ‘)’ token
evp.c:39:1: warning: passing argument 3 of ‘EVP_DigestFinal_ex’ from incompatible pointer type [enabled by default]
In file included from evp.c:4:0:
/usr/include/openssl/evp.h:574:5: note: expected ‘unsigned int *’ but argument is of type ‘size_t *’

Answer 1

Rather than using the SHA1 or SHA256 specific functions, use the EVP_Digest* family of functions which work with any hash.
...
// makes all algorithms available to the EVP* routines
OpenSSL_add_all_algorithms();
// load the error strings for ERR_error_string
ERR_load_crypto_strings();
EVP_MD_CTX hashctx;
//const EVP_MD *hashptr = EVP_get_digestbyname("SHA256");
const EVP_MD *hashptr = EVP_get_digestbyname("SHA1");
EVP_MD_CTX_init(&hashctx);
EVP_DigestInit_ex(&hashctx, hashptr, NULL);
do {
len = fread(buffer, 1, BUFSIZ, f);
EVP_DigestUpdate(&hashctx, buffer, len);
} while (len == BUFSIZ);
unsigned int outlen;
EVP_DigestFinal_ex(&hashctx, buffer, &outlen);
EVP_MD_CTX_cleanup(&hashctx);
fclose(f);
int i;
for (i = 0; i < outlen; ++i)
printf("%02x", buffer[i]);
I've omitted the error checking for brevity. To check for errors, do the following:
if (function_to_check() == 0) {
char errstr[1000];
ERR_error_string(ERR_get_error(), errstr);
printf("error: %s\n", errstr;
}
EDIT:
There were some error in the above code that have been corrected:
hashptr was declared EVP_MD *, is now const EVP_MD *.
The call to EVP_DigestInit_ex had an extra parenthesis at the end
The third parameter to EVP_DigestFinal_ex is specifically given an unsigned int * instead a size_t *, which may not necessarily be the same.
Added calls to two OpenSSL initialization functions at the top

Answer 2

You can simply do like this:-
#include <openssl/sha.h>
#include <stdio.h>
int main(int argc, char **argv)
{
unsigned char buffer[BUFSIZ];
FILE *f;
SHA_CTX ctx;
size_t len;
if (argc < 2) {
fprintf(stderr, "usage: %s <file>\n", argv[0]);
return 1;
}
f = fopen(argv[1], "r");
if (!f) {
fprintf(stderr, "couldn't open %s\n", argv[1]);
return 1;
}
SHA1_Init(&ctx);
do {
len = fread(buffer, 1, BUFSIZ, f);
SHA1_Update(&ctx, buffer, len);
} while (len == BUFSIZ);
SHA1_Final(buffer, &ctx);
fclose(f);
for (len = 0; len < SHA_DIGEST_LENGTH; ++len)
printf("%02x", buffer[len]);
putchar('\n');
return 0;
}