EVP_SignInit(3)

     NAME
          EVP_SignInit, EVP_SignUpdate, EVP_SignFinal - EVP signing
          functions

     SYNOPSIS
           #include <openssl/evp.h>

           int EVP_SignInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *impl);
           int EVP_SignUpdate(EVP_MD_CTX *ctx, const void *d, unsigned int cnt);
           int EVP_SignFinal(EVP_MD_CTX *ctx,unsigned char *sig,unsigned int *s, EVP_PKEY *pkey);

           void EVP_SignInit(EVP_MD_CTX *ctx, const EVP_MD *type);

           int EVP_PKEY_size(EVP_PKEY *pkey);


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

          EVP_SignInit_ex() sets up signing context ctx to use digest
          type from ENGINE impl. ctx must be initialized with
          EVP_MD_CTX_init() before calling this function.

          EVP_SignUpdate() 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.

          EVP_SignFinal() signs the data in ctx using the private key
          pkey and places the signature in sig. If the s parameter is
          not NULL then the number of bytes of data written (i.e. the
          length of the signature) will be written to the integer at
          s, at most EVP_PKEY_size(pkey) bytes will be written.

          EVP_SignInit() initializes a signing context ctx to use the
          default implementation of digest type.

          EVP_PKEY_size() returns the maximum size of a signature in
          bytes. The actual signature returned by EVP_SignFinal() may
          be smaller.

     RETURN VALUES
          EVP_SignInit_ex(), EVP_SignUpdate() and EVP_SignFinal()
          return 1 for success and 0 for failure.

          EVP_PKEY_size() returns the maximum size of a signature in
          bytes.

          The error codes can be obtained by 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.

          Due to the link between message digests and public key
          algorithms the correct digest algorithm must be used with
          the correct public key type. A list of algorithms and
          associated public key algorithms appears in
          EVP_DigestInit(3).

          When signing with DSA private keys the random number
          generator must be seeded or the operation will fail. The
          random number generator does not need to be seeded for RSA
          signatures.

          The call to EVP_SignFinal() internally finalizes a copy of
          the digest context.  This means that calls to
          EVP_SignUpdate() and EVP_SignFinal() 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.

     BUGS
          Older versions of this documentation wrongly stated that
          calls to EVP_SignUpdate() could not be made after calling
          EVP_SignFinal().

     SEE ALSO
          EVP_VerifyInit(3), EVP_DigestInit(3), err(3), evp(3),
          hmac(3), md2(3), md5(3), mdc2(3), ripemd(3), sha(3), dgst(1)

     HISTORY
          EVP_SignInit(), EVP_SignUpdate() and EVP_SignFinal() are
          available in all versions of SSLeay and OpenSSL.

          EVP_SignInit_ex() was added in OpenSSL 0.9.7.