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           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_sha224, EVP_sha256, EVP_sha384, EVP_sha512,
           EVP_dss, EVP_dss1, EVP_mdc2, EVP_ripemd160, EVP_get_digestbyname,
           EVP_get_digestbynid, EVP_get_digestbyobj - EVP digest routines


            #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, size_t 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 64     /* SHA512 */
            int EVP_MD_type(const EVP_MD *md);
            int EVP_MD_pkey_type(const EVP_MD *md);
            int EVP_MD_size(const EVP_MD *md);
            int EVP_MD_block_size(const EVP_MD *md);
            const EVP_MD *EVP_MD_CTX_md(const EVP_MD_CTX *ctx);
            #define EVP_MD_CTX_size(e)             EVP_MD_size(EVP_MD_CTX_md(e))
            #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);
           EVP_MD_CTX_create() allocates, initializes and returns a digest
           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 function such 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
           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
           context 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
           SHA224, SHA256, SHA384, SHA512, MDC2 and RIPEMD160 digest algorithms
           EVP_dss() and EVP_dss1() return EVP_MD structures for SHA and SHA1
           digest algorithms but using DSS (DSA) for the signature algorithm.
           Note: there is no need to use these pseudo-digests in OpenSSL 1.0.0 and
           later, they are however retained for compatibility.
           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.


           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_sha224(), EVP_sha256(), EVP_sha384(), EVP_sha512(), 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.


           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.
           New applications should use the SHA2 digest algorithms such as SHA256.
           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(),
           structure changes (this will only happen with a major release of
           OpenSSL).  Applications wishing to avoid this should use
           EVP_MD_CTX_create() instead:
            EVP_MD_CTX *mctx;
            mctx = EVP_MD_CTX_create();


           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;
            if(!argv[1]) {
                   printf("Usage: mdtest digestname\n");
            md = EVP_get_digestbyname(argv[1]);
            if(!md) {
                   printf("Unknown message digest %s\n", argv[1]);
            mdctx = EVP_MD_CTX_create();
            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);
            printf("Digest is: ");
            for(i = 0; i < md_len; i++) printf("%02x", md_value[i]);


           evp(3), hmac(3), md2(3), md5(3), mdc2(3), ripemd(3), sha(3), dgst(1)
           no need to use EVP_dss1() any more.
           OpenSSL 1.0 and later does not include the MD2 digest algorithm in the
           default configuration due to its security weaknesses.

    1.0.1e 2016-01-07 EVP_DigestInit(3)


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