EVP_DigestInit

EVP_DigestInit, EVP_DigestUpdate, EVP_DigestFinal ? EVP digest routines

 #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_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)

 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))

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

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

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

EVP_DigestFinal() 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() no additional calls to EVP_DigestUpdate() can be made, but EVP_DigestInit() can be called to initialiase a new digest operation.

EVP_MD_CTX_copy() 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.

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 initialised using, for example, OpenSSL_add_all_digests() for these functions to work.

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_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.

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.

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");
 }

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 EVP_sha1() must be used with RSA and EVP_dss1() must be used with DSS even though they are identical digests.

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

evp(3), hmac(3), md2(3), md5(3), mdc2(3), ripemd(3), sha(3), digest(1)

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