EVP_PKEY_SIGN(3) | Library Functions Manual | EVP_PKEY_SIGN(3)

# NAME

`EVP_PKEY_sign_init`, `EVP_PKEY_sign` — sign using a public key algorithm

# SYNOPSIS

library “libcrypto”
`#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.

June 8, 2025 | Debian