=pod

=head1 NAME

EVP_PKEY_sign_init, EVP_PKEY_sign - sign using a public key algorithm

=head1 SYNOPSIS

 #include <openssl/evp.h>

 int EVP_PKEY_sign_init(EVP_PKEY_CTX *ctx);
 int EVP_PKEY_sign(EVP_PKEY_CTX *ctx,
			unsigned char *sig, size_t *siglen,
			const unsigned char *tbs, size_t tbslen);

=head1 DESCRIPTION

The EVP_PKEY_sign_init() function initializes a public key algorithm
context using key B<pkey> for a signing operation.

The EVP_PKEY_sign() function performs a public key signing operation
using B<ctx>. The data to be signed is specified using the B<tbs> and
B<tbslen> parameters. If B<sig> is B<NULL> then the maximum size of the output
buffer is written to the B<siglen> parameter. If B<sig> is not B<NULL> then
before the call the B<siglen> parameter should contain the length of the
B<sig> buffer, if the call is successful the signature is written to
B<sig> and the amount of data written to B<siglen>.

=head1 NOTES

EVP_PKEY_sign() does not hash the data to be signed, and therefore is
normally used to sign digests. For signing arbitrary messages, see the
L<EVP_DigestSignInit(3)|EVP_DigestSignInit(3)> and
L<EVP_SignInit(3)|EVP_SignInit(3)> signing interfaces instead.

After the call to EVP_PKEY_sign_init() algorithm specific control
operations can be performed to set any appropriate parameters for the
operation (see L<EVP_PKEY_CTX_ctrl(3)|EVP_PKEY_CTX_ctrl(3)>).

The function EVP_PKEY_sign() can be called more than once on the same
context if several operations are performed using the same parameters.

=head1 RETURN VALUES

EVP_PKEY_sign_init() and 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.

=head1 EXAMPLE

Sign data using RSA with PKCS#1 padding and SHA256 digest:

 #include <openssl/evp.h>
 #include <openssl/rsa.h>

 EVP_PKEY_CTX *ctx;
 /* md is a SHA-256 digest in this example. */
 unsigned char *md, *sig;
 size_t mdlen = 32, siglen;
 EVP_PKEY *signing_key;

 /*
  * NB: assumes signing_key and md are set up before the next
  * step. signing_key must be an RSA private key and md must
  * point to the SHA-256 digest to be signed.
  */
 ctx = EVP_PKEY_CTX_new(signing_key, NULL /* no engine */);
 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 = OPENSSL_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 */


=head1 SEE ALSO

L<EVP_PKEY_CTX_new(3)|EVP_PKEY_CTX_new(3)>,
L<EVP_PKEY_CTX_ctrl(3)|EVP_PKEY_CTX_ctrl(3)>,
L<EVP_PKEY_encrypt(3)|EVP_PKEY_encrypt(3)>,
L<EVP_PKEY_decrypt(3)|EVP_PKEY_decrypt(3)>,
L<EVP_PKEY_verify(3)|EVP_PKEY_verify(3)>,
L<EVP_PKEY_verify_recover(3)|EVP_PKEY_verify_recover(3)>,
L<EVP_PKEY_derive(3)|EVP_PKEY_derive(3)> 

=head1 HISTORY

These functions were first added to OpenSSL 1.0.0.

=cut