printing contents from MD_CTX

Question

I am trying to understand how the hashing functions in md_rand.c works. Suppose I declare a variable MD_CTX m, and I am trying to see the contents of m after each MD_Update function.

MD_Init(&m);
printf("%X\n",*(&m));
MD_Update(&m,(unsigned char *)&(md_c[0]),sizeof(md_c));
printf("%X\n",*(&m));
MD_Update(&m,local_md,MD_DIGEST_LENGTH);
printf("%X\n",*(&m));
CRYPTO_w_lock(CRYPTO_LOCK_RAND);
MD_Update(&m,md,MD_DIGEST_LENGTH);
printf("%X\n",*(&m));       
MD_Final(md,&m);

However, I always get the same value printed out for all 4 printf statements after every MD_Update. What should I do to get the updated value in m?

jww · Accepted Answer · 2015-10-09T16:25:38.513

What should I do to get the updated value in m?

When you printf("%X\n",*(&m));, you are just printing the address of MD_CTX on the stack. It never changes, hence the reason for your observations.

A MD_CTX is an opaque structure, meaning you are not supposed to look into it. Its a typedef, and you can locate it with:

$ cd openssl-1.0.2d
$ grep -R EVP_MD * | grep struct | egrep "(\.h|\.c)"
crypto/ossl_typ.h:typedef struct env_md_st EVP_MD;
crypto/ossl_typ.h:typedef struct env_md_ctx_st EVP_MD_CTX;
engines/ccgost/gost_lcl.h:/* EVP_MD structure for GOST R 34.11 */
engines/ccgost/gost_lcl.h:/* EVP_MD structure for GOST 28147 in MAC mode */
include/openssl/ossl_typ.h:typedef struct env_md_st EVP_MD;
include/openssl/ossl_typ.h:typedef struct env_md_ctx_st EVP_MD_CTX;

Then, following struct env_md_ctx_st:

$ grep -R "struct env_md_ctx_st" * | grep "{"
crypto/evp/evp.h:struct env_md_ctx_st {
include/openssl/evp.h:struct env_md_ctx_st {

And finally, from evp.h around line 260:

struct env_md_ctx_st {
    const EVP_MD *digest;
    ENGINE *engine;             /* functional reference if 'digest' is
                                 * ENGINE-provided */
    unsigned long flags;
    void *md_data;
    /* Public key context for sign/verify */
    EVP_PKEY_CTX *pctx;
    /* Update function: usually copied from EVP_MD */
    int (*update) (EVP_MD_CTX *ctx, const void *data, size_t count);
} /* EVP_MD_CTX */ ;

And here's a related struct from around line 160:

struct env_md_st {
    int type;
    int pkey_type;
    int md_size;
    unsigned long flags;
    int (*init) (EVP_MD_CTX *ctx);
    int (*update) (EVP_MD_CTX *ctx, const void *data, size_t count);
    int (*final) (EVP_MD_CTX *ctx, unsigned char *md);
    int (*copy) (EVP_MD_CTX *to, const EVP_MD_CTX *from);
    int (*cleanup) (EVP_MD_CTX *ctx);
    /* FIXME: prototype these some day */
    int (*sign) (int type, const unsigned char *m, unsigned int m_length,
                 unsigned char *sigret, unsigned int *siglen, void *key);
    int (*verify) (int type, const unsigned char *m, unsigned int m_length,
                   const unsigned char *sigbuf, unsigned int siglen,
                   void *key);
    int required_pkey_type[5];  /* EVP_PKEY_xxx */
    int block_size;
    int ctx_size;               /* how big does the ctx->md_data need to be */
    /* control function */
    int (*md_ctrl) (EVP_MD_CTX *ctx, int cmd, int p1, void *p2);
} /* EVP_MD */ ;

printing contents from MD_CTX

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