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.