ECDSA_SIGN(3SSL)

📦

version 4.0.3 · 14 versions

💖 ECDSA_SIGN — the deprecated ECDSA girlies 💖

okay so bestie, we're in ECDSA_SIGN(3SSL) territory, and I need you to sit down for this one because literally every function on this page is deprecated and I'm not exaggerating. It's the low-level elliptic curve digital signature algorithm (ECDSA) crew: ECDSA_size, ECDSA_sign, ECDSA_do_sign, ECDSA_verify, ECDSA_do_verify, ECDSA_sign_setup, ECDSA_sign_ex, ECDSA_do_sign_ex. They still work, they're just… vintage. Like a flip phone. Cute, iconic, but babe, we have EVP now. 🫶

✨ NAME ✨

ECDSA_size, ECDSA_sign, ECDSA_do_sign, ECDSA_verify, ECDSA_do_verify, ECDSA_sign_setup, ECDSA_sign_ex, ECDSA_do_sign_ex — deprecated low-level elliptic curve digital signature algorithm (ECDSA) functions 💅

🌸 SYNOPSIS 🌸

#include <openssl/ecdsa.h>

The following functions have been deprecated since OpenSSL 3.0, and can be hidden entirely by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

int ECDSA_size(const EC_KEY *eckey);

int ECDSA_sign(int type, const unsigned char *dgst, int dgstlen,
               unsigned char *sig, unsigned int *siglen, EC_KEY *eckey);
ECDSA_SIG *ECDSA_do_sign(const unsigned char *dgst, int dgst_len,
                         EC_KEY *eckey);

int ECDSA_verify(int type, const unsigned char *dgst, int dgstlen,
                 const unsigned char *sig, int siglen, EC_KEY *eckey);
int ECDSA_do_verify(const unsigned char *dgst, int dgst_len,
                    const ECDSA_SIG *sig, EC_KEY* eckey);

ECDSA_SIG *ECDSA_do_sign_ex(const unsigned char *dgst, int dgstlen,
                            const BIGNUM *kinv, const BIGNUM *rp,
                            EC_KEY *eckey);
int ECDSA_sign_setup(EC_KEY *eckey, BN_CTX *ctx, BIGNUM **kinv, BIGNUM **rp);
int ECDSA_sign_ex(int type, const unsigned char *dgst, int dgstlen,
                  unsigned char *sig, unsigned int *siglen,
                  const BIGNUM *kinv, const BIGNUM *rp, EC_KEY *eckey);

🦋 DESCRIPTION 🦋

Wanna know what an ECDSA_SIG object even is? Go read ECDSA_SIG_new(3), she explains herself.

Encoding and decoding ECDSA signatures to/from DER? That's i2d_ECDSA_SIG(3) and d2i_ECDSA_SIG(3), respectively. Slay.

⚠️ Real talk first: ALL of the functions described below are deprecated. Applications should use the higher level EVP interface such as EVP_DigestSignInit(3) or EVP_DigestVerifyInit(3) instead. Periodt. 💀

ECDSA_size() — returns the maximum length of a DER encoded ECDSA signature created with the private EC key eckey. And if you want the actual signature size instead of the juicy maximum? Use EVP_PKEY_sign(3) with a NULL sig parameter. iykyk. ✨
ECDSA_sign() — computes a digital signature of the dgstlen bytes hash value dgst using the private EC key eckey. The DER encoded signature is stored in sig and its length is returned in siglen. Note: sig must point to ECDSA_size(eckey) bytes of memory — like, do not skimp on the buffer, bestie. The parameter type is currently ignored (she's just there for vibes). ECDSA_sign() is a wrapper function for ECDSA_sign_ex() with kinv and rp set to NULL.
ECDSA_do_sign() — same energy as ECDSA_sign() except the signature is returned as a newly allocated ECDSA_SIG structure (or NULL on error). ECDSA_do_sign() is a wrapper function for ECDSA_do_sign_ex() with kinv and rp set to NULL.
ECDSA_verify() — verifies that the signature in sig of size siglen is a valid ECDSA signature of the hash value dgst of size dgstlen using the public key eckey. The parameter type is ignored (again, decorative 💅).
ECDSA_do_verify() — similar to ECDSA_verify() except the signature is presented in the form of a pointer to an ECDSA_SIG structure.
The remaining functions — these utilise the internal kinv and r values used during signature computation. Most applications will never need to call these. And honestly? If you're most applications, you don't. 🧚
ECDSA_sign_setup() — may be used to precompute parts of the signing operation. eckey is the private EC key and ctx is a pointer to BN_CTX structure (or NULL). The precomputed values are returned in kinv and rp and can be used in a later call to ECDSA_sign_ex() or ECDSA_do_sign_ex(). Pre-gaming the math, icon behaviour. 🍓
ECDSA_sign_ex() — computes a digital signature of the dgstlen bytes hash value dgst using the private EC key eckey and the optional pre-computed values kinv and rp. The DER encoded signature is stored in sig and its length is returned in siglen. Note: sig must point to ECDSA_size(eckey) bytes of memory. The parameter type is ignored. (Yes, still. She's committed to the bit.)
ECDSA_do_sign_ex() — similar to ECDSA_sign_ex() except the signature is returned as a newly allocated ECDSA_SIG structure (or NULL on error).

👑 RETURN VALUES 👑

ECDSA_size() — returns the maximum length signature, or 0 on error.
ECDSA_sign(), ECDSA_sign_ex() and ECDSA_sign_setup() — return 1 if successful or 0 on error.
ECDSA_do_sign() and ECDSA_do_sign_ex() — return a pointer to an allocated ECDSA_SIG structure or NULL on error.
ECDSA_verify() and ECDSA_do_verify() — return 1 for a valid signature, 0 for an invalid signature and -1 on error. The error codes can be obtained by ERR_get_error(3). So like, 0 and -1 are NOT the same thing, do not conflate them, this is not a drill 😭

🌈 EXAMPLES 🌈

Creating an ECDSA signature of a given SHA-256 hash value using the named curve prime256v1 (aka P-256). This example uses deprecated functionality. See "DESCRIPTION". (I did warn you. 💖)

First step: create an EC_KEY object (note: this part is not ECDSA specific)

int ret;
ECDSA_SIG *sig;
EC_KEY *eckey;

eckey = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
if (eckey == NULL)
    /* error */
if (EC_KEY_generate_key(eckey) == 0)
    /* error */

Second step: compute the ECDSA signature of a SHA-256 hash value using ECDSA_do_sign():

sig = ECDSA_do_sign(digest, 32, eckey);
if (sig == NULL)
    /* error */

or using ECDSA_sign():

unsigned char *buffer, *pp;
int buf_len;

buf_len = ECDSA_size(eckey);
buffer = OPENSSL_malloc(buf_len);
pp = buffer;
if (ECDSA_sign(0, dgst, dgstlen, pp, &buf_len, eckey) == 0)
    /* error */

Third step: verify the created ECDSA signature using ECDSA_do_verify():

ret = ECDSA_do_verify(digest, 32, sig, eckey);

or using ECDSA_verify():

ret = ECDSA_verify(0, digest, 32, buffer, buf_len, eckey);

and finally evaluate the return value:

if (ret == 1)
    /* signature ok */
else if (ret == 0)
    /* incorrect signature */
else
    /* error */

⭐ CONFORMING TO ⭐

ANSI X9.62, US Federal Information Processing Standard FIPS186-2 (Digital Signature Standard, DSS)

🎀 HISTORY 🎀

All functionality described here was deprecated in OpenSSL 3.0. (not me being sad about it 🥲)

💌 COPYRIGHT 💌

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 <https://www.openssl.org/source/license.html>.

and that's the whole page, bestie!! 💖✨🦋 This was OpenSSL 4.0.3, dated 2026-09-29, from the ECDSA_SIGN(3SSL) manual. Go forth, use EVP, and drink some water. mwah 🫶🌸

📦 where this page comes from

ubuntu stonking: libssl-doc 4.0.3-1ubuntu1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms