.Dd $Mdocdate: November 3 2016 $
.Dt EVP_PKEY_SIGN 3
.Os
.Sh NAME
.Nm EVP_PKEY_sign_init ,
.Nm EVP_PKEY_sign
.Nd sign using a public key algorithm
.Sh SYNOPSIS
.In openssl/evp.h
.Ft int
.Fo EVP_PKEY_sign_init
.Fa "EVP_PKEY_CTX *ctx"
.Fc
.Ft int
.Fo EVP_PKEY_sign
.Fa "EVP_PKEY_CTX *ctx"
.Fa "unsigned char *sig"
.Fa "size_t *siglen"
.Fa "const unsigned char *tbs"
.Fa "size_t tbslen"
.Fc
.Sh DESCRIPTION
The
.Fn EVP_PKEY_sign_init
function initializes a public key algorithm context using the key
.Fa ctx->pkey
for a signing operation.
.Pp
The
.Fn EVP_PKEY_sign
function performs a public key signing operation using
.Fa ctx .
The data to be signed is specified using the
.Fa tbs
and
.Fa tbslen
parameters.
If
.Fa sig
is
.Dv NULL ,
then the maximum size of the output buffer is written to the
.Fa siglen
parameter.
If
.Fa sig
is not
.Dv NULL ,
then before the call the
.Fa siglen
parameter should contain the length of the
.Fa sig
buffer.
If the call is successful the signature is written to
.Fa sig
and the amount of data written to
.Fa siglen .
.Pp
After the call to
.Fn EVP_PKEY_sign_init ,
algorithm specific control operations can be performed to set any
appropriate parameters for the operation.
.Pp
The function
.Fn EVP_PKEY_sign
can be called more than once on the same context if several operations
are performed using the same parameters.
.Sh RETURN VALUES
.Fn EVP_PKEY_sign_init
and
.Fn EVP_PKEY_sign
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.
.Sh EXAMPLES
Sign data using RSA with PKCS#1 padding and SHA256 digest:
.Bd -literal
#include <openssl/evp.h>
#include <openssl/rsa.h>

EVP_PKEY_CTX *ctx;
unsigned char *md, *sig;
size_t mdlen, siglen;
EVP_PKEY *signing_key;
/* NB: assumes signing_key, md and mdlen are already set up
 * and that signing_key is an RSA private key
 */
ctx = EVP_PKEY_CTX_new(signing_key);
if (!ctx)
	/* Error occurred */
if (EVP_PKEY_sign_init(ctx) <= 0)
	/* Error */
if (EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PADDING) <= 0)
	/* Error */
if (EVP_PKEY_CTX_set_signature_md(ctx, EVP_sha256()) <= 0)
	/* Error */

/* Determine buffer length */
if (EVP_PKEY_sign(ctx, NULL, &siglen, md, mdlen) <= 0)
	/* Error */

sig = malloc(siglen);

if (!sig)
	/* malloc failure */

if (EVP_PKEY_sign(ctx, sig, &siglen, md, mdlen) <= 0)
	/* Error */

/* Signature is siglen bytes written to buffer sig */
.Ed
.Sh SEE ALSO
.Xr EVP_PKEY_CTX_new 3 ,
.Xr EVP_PKEY_decrypt 3 ,
.Xr EVP_PKEY_derive 3 ,
.Xr EVP_PKEY_encrypt 3 ,
.Xr EVP_PKEY_verify 3 ,
.Xr EVP_PKEY_verify_recover 3
.Sh HISTORY
These functions were first added to OpenSSL 1.0.0.