The EVP digest routines are a high level interface to message digests.
EVP_MD_CTX_init() initializes digest context ctx.
EVP_MD_CTX_create() allocates, initializes and returns a digest context.
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 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_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 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. 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.