---
name: "libressl-EVP_PKEY_sign"
section: "3"
platform: "linux"
package: "libressl"
version: "4.4.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/libressl-EVP_PKEY_sign"
---

# libressl-EVP\_PKEY\_sign(3)

sign using a public key algorithm

## NAME

**EVP\_PKEY\_sign\_init**, **EVP\_PKEY\_sign** – sign using a public key algorithm

## SYNOPSIS

Crypto Library (libcrypto, -lcrypto)

**#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*);

## DESCRIPTION

The **EVP\_PKEY\_sign\_init**() function initializes a public key algorithm context using the key *ctx-\>pkey* for a signing operation.

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

**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 EVP\_DigestSignInit(3) and 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 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.

## 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.

## EXAMPLES

Sign data using RSA with PKCS#1 padding and SHA-256 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 = 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 */
```

## SEE ALSO

EVP\_PKEY\_CTX\_ctrl(3), EVP\_PKEY\_CTX\_new(3), EVP\_PKEY\_decrypt(3), EVP\_PKEY\_derive(3), EVP\_PKEY\_encrypt(3), EVP\_PKEY\_verify(3), EVP\_PKEY\_verify\_recover(3)

## HISTORY

**EVP\_PKEY\_sign\_init**() and **EVP\_PKEY\_sign**() first appeared in OpenSSL 1.0.0 and have been available since OpenBSD 4.9.