EVP_SIGNINIT(3)

NAME
EVP_SignInit_ex, EVP_SignUpdate, EVP_SignFinal, EVP_SignInit - EVP signing functions

SYNOPSIS
libcrypto
#include <openssl/evp.h>
int
EVP_SignInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *engine);
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);

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

EVP_SignInit_ex() sets up the signing context ctx to use the digest type. Before calling this function, obtain ctx from EVP_MD_CTX_new(3) or call EVP_MD_CTX_reset(3) on it. The engine argument is always ignored and passing NULL is recommended.

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. sig must be at least EVP_PKEY_size(3) bytes in size. s is an OUT parameter, and not used as an IN parameter. 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(3) bytes will be written.

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

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.

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_free(3) or a memory leak will occur.

EVP_SignInit_ex(), EVP_SignUpdate(), and EVP_SignInit() are implemented as macros.

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

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

SEE ALSO
evp(3), EVP_DigestInit(3), EVP_PKEY_size(3), EVP_VerifyInit(3)

HISTORY
EVP_SignInit(), EVP_SignUpdate(), and EVP_SignFinal() first appeared in SSLeay 0.5.1 and have been available since OpenBSD 2.4.

EVP_SignInit_ex() first appeared in OpenSSL 0.9.7 and has been available since OpenBSD 3.2.

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_DigestSign* function.