EVP_SignInit(3)

NAME
EVP_SignInit, EVP_SignUpdate, EVP_SignFinal - EVP signing functions

SYNOPSIS
 #include <openssl/evp.h>
 void EVP_SignInit(EVP_MD_CTX *ctx, const EVP_MD *type);
 void 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);
 int EVP_PKEY_size(EVP_PKEY *pkey);

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

EVP_SignInit() initializes a signing context ctx to using digest type: this will typically be supplied by a function such as EVP_sha1().

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. After calling EVP_SignFinal() no additional calls to EVP_SignUpdate() can be made, but EVP_SignInit() can be called to initialize a new signature operation.

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() and EVP_SignUpdate() do not return values.

EVP_SignFinal() returns 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.

BUGS
Several of the functions do not return values: maybe they should. Although the internal digest operations will never fail some future hardware based operations might.

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.