NAME

       EVP_DigestSignInit, EVP_DigestSignUpdate, EVP_DigestSignFinal - EVP signing functions

SYNOPSIS

        #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);

DESCRIPTION

       The EVP signature routines are a high level interface to digital signatures.

       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.

RETURN VALUES

       EVP_DigestSignInit() EVP_DigestSignUpdate() and EVP_DigestSignaFinal() return 1 for
       success and 0 or a negative value for failure. In particular a return value of -2
       indicates the operation is not supported by the public key algorithm.

       The error codes can be obtained from ERR_get_error(3).

NOTES

       The EVP interface to digital signatures should almost always be used in preference to the
       low level interfaces. This is because the code then becomes transparent to the algorithm
       used and much more flexible.

       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.

SEE ALSO

       EVP_DigestVerifyInit(3), EVP_DigestInit(3), err(3), evp(3), hmac(3), md2(3), md5(3),
       mdc2(3), ripemd(3), sha(3), dgst(1)

HISTORY

       EVP_DigestSignInit(), EVP_DigestSignUpdate() and EVP_DigestSignFinal() were first added to
       OpenSSL 1.0.0.