NAME¶

SYNOPSIS¶

 #include <openssl/evp.h>

 EVP_MD_CTX *EVP_MD_CTX_new(void);
 int EVP_MD_CTX_reset(EVP_MD_CTX *ctx);
 void EVP_MD_CTX_free(EVP_MD_CTX *ctx);
 void EVP_MD_CTX_ctrl(EVP_MD_CTX *ctx, int cmd, int p1, void* p2);
 void EVP_MD_CTX_set_flags(EVP_MD_CTX *ctx, int flags);
 void EVP_MD_CTX_clear_flags(EVP_MD_CTX *ctx, int flags);
 int EVP_MD_CTX_test_flags(const EVP_MD_CTX *ctx, int flags);

 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_DigestFinalXOF(EVP_MD_CTX *ctx, unsigned char *md, size_t len);

 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);

 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);
 int EVP_MD_CTX_size(const EVP_MD *ctx);
 int EVP_MD_CTX_block_size(const EVP_MD *ctx);
 int EVP_MD_CTX_type(const EVP_MD *ctx);
 void *EVP_MD_CTX_md_data(const EVP_MD_CTX *ctx);

 const EVP_MD *EVP_md_null(void);

 const EVP_MD *EVP_get_digestbyname(const char *name);
 const EVP_MD *EVP_get_digestbynid(int type);
 const EVP_MD *EVP_get_digestbyobj(const ASN1_OBJECT *o);

 void EVP_MD_CTX_set_pkey_ctx(EVP_MD_CTX *ctx, EVP_PKEY_CTX *pctx);

DESCRIPTION¶

EVP_MD_CTX_new()

Allocates and returns a digest context.

EVP_MD_CTX_reset()

Resets the digest context ctx. This can be used to reuse an already existing context.

EVP_MD_CTX_free()

Cleans up digest context ctx and frees up the space allocated to it.

EVP_MD_CTX_ctrl()

Performs digest-specific control actions on context ctx.

EVP_MD_CTX_set_flags(), EVP_MD_CTX_clear_flags(), EVP_MD_CTX_test_flags()

Sets, clears and tests ctx flags. See "FLAGS" below for more information.

EVP_DigestInit_ex()

Sets up digest context ctx to use a digest type from ENGINE impl. 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 operation.

EVP_DigestFinalXOF()

Interfaces to extendable-output functions, XOFs, such as SHAKE128 and SHAKE256. It retrieves the digest value from ctx and places it in len-sized <B>md. After calling this function no additional calls to EVP_DigestUpdate() can be made, but EVP_DigestInit_ex() can be called to initialize a new operation.

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.

EVP_DigestInit()

Behaves in the same way as EVP_DigestInit_ex() except it always uses the default digest implementation.

EVP_DigestFinal()

Similar to EVP_DigestFinal_ex() except the digest context ctx is automatically cleaned up.

EVP_MD_CTX_copy()

Similar to EVP_MD_CTX_copy_ex() except the destination out does not have to be initialized.

EVP_MD_size(), 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(), 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(), 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_data()

Return the digest method private data for the passed EVP_MD_CTX. The space is allocated by OpenSSL and has the size originally set with EVP_MD_meth_set_app_datasize().

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. Since digests and signature algorithms are no longer linked this function is only retained for compatibility reasons.

EVP_md_null()

A "null" message digest that does nothing: i.e. the hash it returns is of zero length.

EVP_get_digestbyname(), EVP_get_digestbynid(), EVP_get_digestbyobj()

Returns an EVP_MD structure when passed a digest name, a digest NID or an ASN1_OBJECT structure respectively.

EVP_MD_CTX_set_pkey_ctx()

Assigns an EVP_PKEY_CTX to EVP_MD_CTX. This is usually used to provide a customzied EVP_PKEY_CTX to EVP_DigestSignInit(3) or EVP_DigestVerifyInit(3). The pctx passed to this function should be freed by the caller. A NULL pctx pointer is also allowed to clear the EVP_PKEY_CTX assigned to ctx. In such case, freeing the cleared EVP_PKEY_CTX or not depends on how the EVP_PKEY_CTX is created.

FLAGS¶

EVP_MD_CTX_FLAG_ONESHOT

This flag instructs the digest to optimize for one update only, if possible.

EVP_MD_CTX_FLAG_NO_INIT

This flag instructs EVP_DigestInit() and similar not to initialise the implementation specific data.

EVP_MD_CTX_FLAG_FINALISE

Some functions such as EVP_DigestSign only finalise copies of internal contexts so additional data can be included after the finalisation call. This is inefficient if this functionality is not required, and can be disabled with this flag.

RETURN VALUES¶

EVP_DigestInit_ex(), EVP_DigestUpdate(), EVP_DigestFinal_ex()

Returns 1 for success and 0 for failure.

EVP_MD_CTX_ctrl()

Returns 1 if successful or 0 for failure.

EVP_MD_CTX_copy_ex()

Returns 1 if successful or 0 for failure.

EVP_MD_type(), EVP_MD_pkey_type(), EVP_MD_type()

Returns the NID of the corresponding OBJECT IDENTIFIER or NID_undef if none exists.

EVP_MD_size(), EVP_MD_block_size(), EVP_MD_CTX_size(), EVP_MD_CTX_block_size()

Returns the digest or block size in bytes.

EVP_md_null()

Returns a pointer to the EVP_MD structure of the "null" message digest.

EVP_get_digestbyname(), EVP_get_digestbynid(), EVP_get_digestbyobj()

Returns either an EVP_MD structure or NULL if an error occurs.

EVP_MD_CTX_set_pkey_ctx()

This function has no return value.

NOTES¶

New applications should use the SHA-2 (such as EVP_sha256(3)) or the SHA-3 digest algorithms (such as EVP_sha3_512(3)). 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.

If digest contexts are not cleaned up after use, memory leaks will occur.

EVP_MD_CTX_size(), EVP_MD_CTX_block_size(), EVP_MD_CTX_type(), EVP_get_digestbynid() and EVP_get_digestbyobj() are defined as macros.

EVP_MD_CTX_ctrl() sends commands to message digests for additional configuration or control.

EXAMPLES¶

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

 int 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];
     unsigned int md_len, i;

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

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

     mdctx = EVP_MD_CTX_new();
     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_free(mdctx);

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

     exit(0);
 }

SEE ALSO¶

The full list of digest algorithms are provided below.

EVP_blake2b512(3), EVP_md2(3), EVP_md4(3), EVP_md5(3), EVP_mdc2(3), EVP_ripemd160(3), EVP_sha1(3), EVP_sha224(3), EVP_sha3_224(3), EVP_sm3(3), EVP_whirlpool(3)

HISTORY¶

The link between digests and signing algorithms was fixed in OpenSSL 1.0 and later, so now EVP_sha1() can be used with RSA and DSA.

The EVP_dss1() function was removed in OpenSSL 1.1.0.

The EVP_MD_CTX_set_pkey_ctx() function was added in 1.1.1.

COPYRIGHT¶

Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.