EVP_DigestInit(3)

     NAME
          EVP_MD_CTX_init, EVP_MD_CTX_create, EVP_DigestInit_ex,
          EVP_DigestUpdate, EVP_DigestFinal_ex, EVP_MD_CTX_cleanup,
          EVP_MD_CTX_destroy, EVP_MAX_MD_SIZE, EVP_MD_CTX_copy_ex,
          EVP_MD_CTX_copy, EVP_MD_type, EVP_MD_pkey_type, EVP_MD_size,
          EVP_MD_block_size, EVP_MD_CTX_md, EVP_MD_CTX_size,
          EVP_MD_CTX_block_size, EVP_MD_CTX_type, EVP_md_null,
          EVP_md2, EVP_md5, EVP_sha, EVP_sha1, EVP_dss, EVP_dss1,
          EVP_mdc2, EVP_ripemd160, EVP_get_digestbyname,
          EVP_get_digestbynid, EVP_get_digestbyobj - EVP digest
          routines

     SYNOPSIS
           #include <openssl/evp.h>

           void EVP_MD_CTX_init(EVP_MD_CTX *ctx);
           EVP_MD_CTX *EVP_MD_CTX_create(void);

           int EVP_DigestInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl);
           int EVP_DigestUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
           int EVP_DigestFinal_ex(EVP_MD_CTX *ctx, unsigned char *md,
                  unsigned int *s);

           int EVP_MD_CTX_cleanup(EVP_MD_CTX *ctx);
           void EVP_MD_CTX_destroy(EVP_MD_CTX *ctx);

           int EVP_MD_CTX_copy_ex(EVP_MD_CTX *out,const EVP_MD_CTX *in);

           int EVP_DigestInit(EVP_MD_CTX *ctx, const EVP_MD *type);
           int EVP_DigestFinal(EVP_MD_CTX *ctx, unsigned char *md,
                  unsigned int *s);

           int EVP_MD_CTX_copy(EVP_MD_CTX *out,EVP_MD_CTX *in);

           #define EVP_MAX_MD_SIZE (16+20) /* The SSLv3 md5+sha1 type */

           #define EVP_MD_type(e)                 ((e)->type)
           #define EVP_MD_pkey_type(e)            ((e)->pkey_type)
           #define EVP_MD_size(e)                 ((e)->md_size)
           #define EVP_MD_block_size(e)           ((e)->block_size)

           #define EVP_MD_CTX_md(e)               (e)->digest)
           #define EVP_MD_CTX_size(e)             EVP_MD_size((e)->digest)
           #define EVP_MD_CTX_block_size(e)       EVP_MD_block_size((e)->digest)
           #define EVP_MD_CTX_type(e)             EVP_MD_type((e)->digest)

           const EVP_MD *EVP_md_null(void);
           const EVP_MD *EVP_md2(void);
           const EVP_MD *EVP_md5(void);
           const EVP_MD *EVP_sha(void);
           const EVP_MD *EVP_sha1(void);
           const EVP_MD *EVP_dss(void);
           const EVP_MD *EVP_dss1(void);
           const EVP_MD *EVP_mdc2(void);
           const EVP_MD *EVP_ripemd160(void);

           const EVP_MD *EVP_get_digestbyname(const char *name);
           #define EVP_get_digestbynid(a) EVP_get_digestbyname(OBJ_nid2sn(a))
           #define EVP_get_digestbyobj(a) EVP_get_digestbynid(OBJ_obj2nid(a))


     DESCRIPTION
          The EVP digest routines are a high level interface to
          message digests.

          EVP_MD_CTX_init() initializes digest contet ctx.

          EVP_MD_CTX_create() allocates, initializes and returns a
          digest contet.

          EVP_DigestInit_ex() sets up digest context ctx to use a
          digest type from ENGINE impl. ctx must be initialized before
          calling this function. type will typically be supplied by a
          functionsuch as EVP_sha1().  If impl is NULL then the
          default implementation of digest type is used.

          EVP_DigestUpdate() hashes cnt bytes of data at d into the
          digest context ctx. This function can be called several
          times on the same ctx to hash additional data.

          EVP_DigestFinal_ex() retrieves the digest value from ctx and
          places it in md. If the s parameter is not NULL then the
          number of bytes of data written (i.e. the length of the
          digest) will be written to the integer at s, at most
          EVP_MAX_MD_SIZE bytes will be written.  After calling
          EVP_DigestFinal_ex() no additional calls to
          EVP_DigestUpdate() can be made, but EVP_DigestInit_ex() can
          be called to initialize a new digest operation.

          EVP_MD_CTX_cleanup() cleans up digest context ctx, it should
          be called after a digest context is no longer needed.

          EVP_MD_CTX_destroy() cleans up digest context ctx and frees
          up the space allocated to it, it should be called only on a
          context created using EVP_MD_CTX_create().

          EVP_MD_CTX_copy_ex() can be used to copy the message digest
          state from in to out. This is useful if large amounts of
          data are to be hashed which only differ in the last few
          bytes. out must be initialized before calling this function.

          EVP_DigestInit() behaves in the same way as
          EVP_DigestInit_ex() except the passed context ctx does not
          have to be initialized, and it always uses the default
          digest implementation.

          EVP_DigestFinal() is similar to EVP_DigestFinal_ex() except
          the digest contet ctx is automatically cleaned up.

          EVP_MD_CTX_copy() is similar to EVP_MD_CTX_copy_ex() except
          the destination out does not have to be initialized.

          EVP_MD_size() and EVP_MD_CTX_size() return the size of the
          message digest when passed an EVP_MD or an EVP_MD_CTX
          structure, i.e. the size of the hash.

          EVP_MD_block_size() and EVP_MD_CTX_block_size() return the
          block size of the message digest when passed an EVP_MD or an
          EVP_MD_CTX structure.

          EVP_MD_type() and EVP_MD_CTX_type() return the NID of the
          OBJECT IDENTIFIER representing the given message digest when
          passed an EVP_MD structure.  For example
          EVP_MD_type(EVP_sha1()) returns NID_sha1. This function is
          normally used when setting ASN1 OIDs.

          EVP_MD_CTX_md() returns the EVP_MD structure corresponding
          to the passed EVP_MD_CTX.

          EVP_MD_pkey_type() returns the NID of the public key signing
          algorithm associated with this digest. For example
          EVP_sha1() is associated with RSA so this will return
          NID_sha1WithRSAEncryption. This "link" between digests and
          signature algorithms may not be retained in future versions
          of OpenSSL.

          EVP_md2(), EVP_md5(), EVP_sha(), EVP_sha1(), EVP_mdc2() and
          EVP_ripemd160() return EVP_MD structures for the MD2, MD5,
          SHA, SHA1, MDC2 and RIPEMD160 digest algorithms
          respectively. The associated signature algorithm is RSA in
          each case.

          EVP_dss() and EVP_dss1() return EVP_MD structures for SHA
          and SHA1 digest algorithms but using DSS (DSA) for the
          signature algorithm.

          EVP_md_null() is a "null" message digest that does nothing:
          i.e. the hash it returns is of zero length.

          EVP_get_digestbyname(), EVP_get_digestbynid() and
          EVP_get_digestbyobj() return an EVP_MD structure when passed
          a digest name, a digest NID or an ASN1_OBJECT structure
          respectively. The digest table must be initialized using,
          for example, OpenSSL_add_all_digests() for these functions
          to work.

     RETURN VALUES
          EVP_DigestInit_ex(), EVP_DigestUpdate() and
          EVP_DigestFinal_ex() return 1 for success and 0 for failure.

          EVP_MD_CTX_copy_ex() returns 1 if successful or 0 for
          failure.

          EVP_MD_type(), EVP_MD_pkey_type() and EVP_MD_type() return
          the NID of the corresponding OBJECT IDENTIFIER or NID_undef
          if none exists.

          EVP_MD_size(), EVP_MD_block_size(), EVP_MD_CTX_size(e),
          EVP_MD_size(), EVP_MD_CTX_block_size()  and
          EVP_MD_block_size() return the digest or block size in
          bytes.

          EVP_md_null(), EVP_md2(), EVP_md5(), EVP_sha(), EVP_sha1(),
          EVP_dss(), EVP_dss1(), EVP_mdc2() and EVP_ripemd160() return
          pointers to the corresponding EVP_MD structures.

          EVP_get_digestbyname(), EVP_get_digestbynid() and
          EVP_get_digestbyobj() return either an EVP_MD structure or
          NULL if an error occurs.

     NOTES
          The EVP interface to message digests should almost always be
          used in preference to the low level interfaces. This is
          because the code then becomes transparent to the digest used
          and much more flexible.

          SHA1 is the digest of choice for new applications. The other
          digest algorithms are still in common use.

          For most applications the impl parameter to
          EVP_DigestInit_ex() will be set to NULL to use the default
          digest implementation.

          The functions EVP_DigestInit(), EVP_DigestFinal() and
          EVP_MD_CTX_copy() are obsolete but are retained to maintain
          compatibility with existing code. New applications should
          use EVP_DigestInit_ex(), EVP_DigestFinal_ex() and
          EVP_MD_CTX_copy_ex() because they can efficiently reuse a
          digest context instead of initializing and cleaning it up on
          each call and allow non default implementations of digests
          to be specified.

          In OpenSSL 0.9.7 and later if digest contexts are not
          cleaned up after use memory leaks will occur.

     EXAMPLE
          This example digests the data "Test Message\n" and "Hello
          World\n", using the digest name passed on the command line.

           #include <stdio.h>
           #include <openssl/evp.h>

           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];
           int md_len, i;

           OpenSSL_add_all_digests();

           if(!argv[1]) {
                  printf("Usage: mdtest digestname\n");
                  exit(1);
           }

           md = EVP_get_digestbyname(argv[1]);

           if(!md) {
                  printf("Unknown message digest %s\n", argv[1]);
                  exit(1);
           }

           EVP_MD_CTX_init(&mdctx);
           EVP_DigestInit_ex(&mdctx, md, NULL);
           EVP_DigestUpdate(&mdctx, mess1, strlen(mess1));
           EVP_DigestUpdate(&mdctx, mess2, strlen(mess2));
           EVP_DigestFinal_ex(&mdctx, md_value, &md_len);
           EVP_MD_CTX_cleanup(&mdctx);

           printf("Digest is: ");
           for(i = 0; i < md_len; i++) printf("%02x", md_value[i]);
           printf("\n");
           }


     BUGS
          The link between digests and signing algorithms results in a
          situation where EVP_sha1() must be used with RSA and
          EVP_dss1() must be used with DSS even though they are
          identical digests.

     SEE ALSO
          evp(3), hmac(3), md2(3), md5(3), mdc2(3), ripemd(3), sha(3),
          dgst(1)

     HISTORY
          EVP_DigestInit(), EVP_DigestUpdate() and EVP_DigestFinal()
          are available in all versions of SSLeay and OpenSSL.

          EVP_MD_CTX_init(), EVP_MD_CTX_create(),
          EVP_MD_CTX_copy_ex(), EVP_MD_CTX_cleanup(),
          EVP_MD_CTX_destroy(), EVP_DigestInit_ex() and
          EVP_DigestFinal_ex() were added in OpenSSL 0.9.7.

          EVP_md_null(), EVP_md2(), EVP_md5(), EVP_sha(), EVP_sha1(),
          EVP_dss(), EVP_dss1(), EVP_mdc2() and EVP_ripemd160() were
          changed to return truely const EVP_MD * in OpenSSL 0.9.7.