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. 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().

       Since the private key is passed in the call to EVP_SignFinal() any error relating to the
       private key (for example an unsuitable key and digest combination) will not be indicated
       until after potentially large amounts of data have been passed through EVP_SignUpdate().

       It is not possible to change the signing parameters using these function.

       The previous two bugs are fixed in the newer EVP_SignDigest*() function.

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.