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EVP_PKEY_sign_init, EVP_PKEY_sign_init_ex, EVP_PKEY_sign - sign using a public key algorithm


 #include <openssl/evp.h>
 int EVP_PKEY_sign_init(EVP_PKEY_CTX *ctx);
 int EVP_PKEY_sign_init_ex(EVP_PKEY_CTX *ctx, const OSSL_PARAM params[]);
 int EVP_PKEY_sign(EVP_PKEY_CTX *ctx,
                   unsigned char *sig, size_t *siglen,
                   const unsigned char *tbs, size_t tbslen);


EVP_PKEY_sign_init() initializes a public key algorithm context ctx for signing using the algorithm given when the context was created using EVP_PKEY_CTX_new(3) or variants thereof. The algorithm is used to fetch a EVP_SIGNATURE method implicitly, see "Implicit fetch" in provider(7) for more information about implicit fetches.

EVP_PKEY_sign_init_ex() is the same as EVP_PKEY_sign_init() but additionally sets the passed parameters params on the context before returning.

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.


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.


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

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


EVP_PKEY_CTX_new(3), EVP_PKEY_CTX_ctrl(3), EVP_PKEY_encrypt(3), EVP_PKEY_decrypt(3), EVP_PKEY_verify(3), EVP_PKEY_verify_recover(3), EVP_PKEY_derive(3)


The EVP_PKEY_sign_init() and EVP_PKEY_sign() functions were added in OpenSSL 1.0.0.

The EVP_PKEY_sign_init_ex() function was added in OpenSSL 3.0.


Copyright 2006-2021 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <>.

2023-05-30 3.0.9