#include <openssl/evp.h>
int EVP_DigestSignInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
int EVP_DigestSignUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
int EVP_DigestSignFinal(EVP_MD_CTX *ctx, unsigned char *sig, size_t *siglen);
EVP_DigestSignInit() sets up signing context ctx to use digest type from ENGINE impl and private key pkey. ctx must be initialized with EVP_MD_CTX_init() before calling this function. If pctx is not NULL the EVP_PKEY_CTX of the signing operation will be written to *pctx: this can be used to set alternative signing options.
EVP_DigestSignUpdate() hashes cnt bytes of data at d into the signature context ctx. This function can be called several times on the same ctx to include additional data. This function is currently implemented usig a macro.
EVP_DigestSignFinal() signs the data in ctx places the signature in sig. If sig is NULL then the maximum size of the output buffer is written to the siglen parameter. If sig is not NULL then before the call the siglen parameter should contain the length of the sig buffer, if the call is successful the signature is written to sig and the amount of data written to siglen.
The error codes can be obtained from ERR_get_error(3).
In previous versions of OpenSSL there was a link between message digest types and public key algorithms. This meant that ``clone'' digests such as EVP_dss1() needed to be used to sign using SHA1 and DSA. This is no longer necessary and the use of clone digest is now discouraged.
For some key types and parameters the random number generator must be seeded or the operation will fail.
The call to EVP_DigestSignFinal() internally finalizes a copy of the digest context. This means that calls to EVP_DigestSignUpdate() and EVP_DigestSignFinal() can be called later to digest and sign additional data.
Since only a copy of the digest context is ever finalized the context must be cleaned up after use by calling EVP_MD_CTX_cleanup() or a memory leak will occur.
The use of EVP_PKEY_size() with these functions is discouraged because some signature operations may have a signature length which depends on the parameters set. As a result EVP_PKEY_size() would have to return a value which indicates the maximum possible signature for any set of parameters.