openssl/doc/crypto/EVP_DigestInit.pod

=pod

=head1 NAME

EVP_DigestInit, EVP_DigestUpdate, EVP_DigestFinal - EVP digest routines

=head1 SYNOPSIS

 #include <openssl/evp.h>

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

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

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

 #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_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)		((e)->digest)

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

=head1 DESCRIPTION

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

B<EVP_DigestInit()> initialises a digest context B<ctx> to use a digest
B<type>: this will typically be supplied by a function such as
B<EVP_sha1()>.

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

B<EVP_DigestFinal()> retrieves the digest value from B<ctx> and places
it in B<md>. If the B<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 B<s>, at most B<EVP_MAX_MD_SIZE> bytes will be written.
After calling B<EVP_DigestFinal> no additional calls to B<EVP_DigestUpdate>
can be made, but B<EVP_DigestInit> can be called to initialiase a new
digest operation.

B<EVP_MD_CTX_copy> can be used to copy the message digest state from
B<in> to B<out>. This is useful if large amounts of data are to be
hashed which only differ in the last few bytes.

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

B<EVP_MD_block_size> and B<EVP_MD_CTX_block_size> return the block size of the
message digest when passed an B<EVP_MD> or an B<EVP_MD_CTX> structure.

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

B<EVP_MD_CTX_type> return the B<EVP_MD> structure corresponding to the passed
B<EVP_MD_CTX>.

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

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

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

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

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

=head1 RETURN VALUES

EVP_DigestInit(), EVP_DigestUpdate() and EVP_DigestFinal() do not return values.

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

B<EVP_get_digestbyname>, B<EVP_get_digestbynid> and B<EVP_get_digestbyobj>
return either an B<EVP_MD> structure or NULL if an error occurs.

=head1 NOTES

The B<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.

=head1 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_DigestInit(&mdctx, md);
 EVP_DigestUpdate(&mdctx, mess1, strlen(mess1));
 EVP_DigestUpdate(&mdctx, mess2, strlen(mess2));
 EVP_DigestFinal(&mdctx, md_value, &md_len);

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

=head1 BUGS

B<EVP_MD_CTX_type> is not a good name because its name wrongly implies it does
the same as B<EVP_MD_type> but takes an B<EVP_MD_CTX> parameter instead.

Several of the functions do not return values: maybe they should. Although the
internal digest operations will never fail some future hardware based operations
might.

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

The size of an B<EVP_MD_CTX> structure is determined at compile time: this results
in code that must be recompiled if the size of B<EVP_MD_CTX> increases.

=head1 SEE ALSO

=head1 HISTORY

=cut