compute md5 2

[flubbard]

Does anyone know of a way to programmatically compute the md5 on a file from within c. I would like to use md5 as a hash to determine if the locally stored file matches a remote file but I am having trouble finding an example where the md5 is returned to the program.

Any help would be appreciated.

 

[TrojanWarBlade]

It might help if you specify your OS since you may already have the functionality available to you.

(Windoze by chance?!!)


Trojan.

 

[flubbard]

OS is linux. I can call md5sum from the command line but i do not know how to get this value back into the program.

 

[TrojanWarBlade]

Try "man MD5"

md5(3) OpenSSL md5(3)

NAME
MD2, MD4, MD5, MD2_Init, MD2_Update, MD2_Final, MD4_Init, MD4_Update,
MD4_Final, MD5_Init, MD5_Update, MD5_Final - MD2, MD4, and MD5 hash
functions

SYNOPSIS
#include <openssl/md2.h>

unsigned char *MD2(const unsigned char *d, unsigned long n,
unsigned char *md);

void MD2_Init(MD2_CTX *c);
void MD2_Update(MD2_CTX *c, const unsigned char *data,
unsigned long len);
void MD2_Final(unsigned char *md, MD2_CTX *c);

#include <openssl/md4.h>

unsigned char *MD4(const unsigned char *d, unsigned long n,
unsigned char *md);

void MD4_Init(MD4_CTX *c);
void MD4_Update(MD4_CTX *c, const void *data,
unsigned long len);
void MD4_Final(unsigned char *md, MD4_CTX *c);

#include <openssl/md5.h>

unsigned char *MD5(const unsigned char *d, unsigned long n,
unsigned char *md);

void MD5_Init(MD5_CTX *c);
void MD5_Update(MD5_CTX *c, const void *data,
unsigned long len);
void MD5_Final(unsigned char *md, MD5_CTX *c);

DESCRIPTION
MD2, MD4, and MD5 are cryptographic hash functions with a 128 bit out-
put.

MD2(), MD4(), and MD5() compute the MD2, MD4, and MD5 message digest of
the n bytes at d and place it in md (which must have space for
MD2_DIGEST_LENGTH == MD4_DIGEST_LENGTH == MD5_DIGEST_LENGTH == 16 bytes
of output). If md is NULL, the digest is placed in a static array.

Trojan.

 

[flubbard]

when attempting to use the MD5 function, I get an error: Undefined reference to 'MD5'. I have #include <openssl/md5.h> Do I need to include other compiler options?

 

[TrojanWarBlade]

You might need to link in the openssl the library with -l


Trojan.

 

[flubbard]

It does not seem to work. I am trying g++ md5_test.c -lopenssl and I receive and error that the compiler can't find openssl. I do have a md5.h in my /usr/include/openssl directory.

Any other thoughts?

 

[TrojanWarBlade]

Try "g++ md5_test.c -lssl".


Trojan.

 

[jstreich]

Trojan, I give a star... VERY useful answer, I think I'll find myself using it in the near future.

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http://game-master.us/phpx-3.4.0/

[/plug]

 

[flubbard]

Seems to compile now. I'm still getting a different answer everytime it is run so I will have to take a look at that, but at least it is compiling.

Thanks for the help.

 

[ArkM]

See also:

http://www.cr0.net:8040/code/crypto/md5/

GPL, no dependencies (except <string.h>.

 

[flubbard]

thanks to you both. I was able to use a combination of the two to get the result I was looking for. It seems to function now.