ECDSA_SIGN(3ossl)

✨ ECDSA_SIGN ✨ (that's `3ossl` bestie, OpenSSL's very own)

okay so picture this: it's the low-level, deprecated, kinda vintage corner of OpenSSL where all the raw elliptic curve digital signature algorithm (ECDSA) stuff lives. The icons in question are ECDSA_size, ECDSA_sign, ECDSA_do_sign, ECDSA_verify, ECDSA_do_verify, ECDSA_sign_setup, ECDSA_sign_ex, and ECDSA_do_sign_ex. They all still exist, but they're giving "retired but still shows up to the party" energy ngl 💅

🎀 SYNOPSIS 🎀

#include <openssl/ecdsa.h>

So here's the tea: every single one of these functions has been deprecated since OpenSSL 3.0, and you can make them vanish entirely by defining OPENSSL_API_COMPAT with a suitable version value — check openssl_user_macros(7) for the deets.

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 🦋

First up, the supporting cast: peek at ECDSA_SIG_new(3) if you wanna know what the ECDSA_SIG object actually is. And for the whole encode/decode-to-and-from-DER situation, i2d_ECDSA_SIG(3) and d2i_ECDSA_SIG(3) have you covered 🫶

⚠️ Real talk: every function on this page is deprecated. Like, all of them. Applications should use the higher level EVP interface instead, such as EVP_DigestSignInit(3) or EVP_DigestVerifyInit(3). That's the main character now, these are the nostalgia cameos.

ECDSA_size() — returns the maximum length of a DER encoded ECDSA signature created with the private EC key eckey. Want the actual signature size instead of the max? Use EVP_PKEY_sign(3) with a NULL sig parameter ✨
ECDSA_sign() — computes a digital signature of the dgstlen bytes hash value dgst using the private EC key eckey. The DER encoded signature gets stored in sig and its length comes back in siglen. Note: sig must point to ECDSA_size(eckey) bytes of memory — no shortcuts, she needs room 🎀 The parameter type is currently ignored. ECDSA_sign() is literally a wrapper function for ECDSA_sign_ex() with kinv and rp set to NULL.
ECDSA_do_sign() — similar to ECDSA_sign() except the signature comes back as a newly allocated ECDSA_SIG structure (or NULL on error). Also 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 (she's just there, vibing).
ECDSA_do_verify() — similar to ECDSA_verify() except the signature is presented in the form of a pointer to an ECDSA_SIG structure. Same energy, different packaging 💅

Okay now the remaining functions utilise the internal kinv and r values used during signature computation. Most applications will never need to call these, and some external ECDSA ENGINE implementations may not support them at all if either kinv or r is not NULL. So like… proceed with awareness, bestie 🙈

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(). Prep work = self care ✨
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.
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. Those error codes can be obtained by ERR_get_error(3) 🫶

🔥 EXAMPLES 🔥

Okay so we're creating an ECDSA signature of a given SHA-256 hash value using the named curve prime256v1 (aka P-256). Heads up: this example uses deprecated functionality. See "DESCRIPTION" above, iykyk 👑

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). Standards-compliant and cute, we love to see it 🌈

👑 HISTORY 👑

All functionality described here was deprecated in OpenSSL 3.0. That's the whole arc 💀

🍓 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>.

aaaand that's the page!! 🧚 mwah, go use EVP_DigestSignInit(3) instead of these, love u 💖 — signed, sealed, delivered by OpenSSL 3.5.0, dated 2026-09-25, on the 3ossl manual page, periodt 🌟

📦 where this page comes from

opensuse-leap 16.0: openssl-3-doc 3.5.0-160000.11.1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms