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EVP digest

EVP

EVP_MD_CTX_init, EVP_MD_CTX_create, EVP_DigestInit_ex, EVP_DigestUp-
date, 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 con-
text 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 desti-
nation 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 IDEN-
TIFIER 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 sig-
nature 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 ini-
tialized using, for example, OpenSSL_add_all_digests() for these func-
tions 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 cor-
responding 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_DigestFi-
nal_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.

EOF


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