ECDSA_SIGN(3ossl)

📦
version 15.1 · 3 versions
, in freebsd 15.1

💜 ECDSA_SIGN(3ossl) — the deprecated low-level ECDSA squad

okay so bestie, this is a page from the OpenSSL manual (that's the 3ossl part, iykyk) and honestly? it's giving ✨vintage✨ because literally everything on it is deprecated. But you still gotta know what it does, so we're doing this together 🫶

🎀 NAME

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

✨ SYNOPSIS

#include <openssl/ecdsa.h>

So here's the tea: all of these have been deprecated since OpenSSL 3.0, and you can hide them entirely by defining OPENSSL_API_COMPAT with a suitable version value — peep openssl_user_macros(7) for that 🦋

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: if you want the whole lore on the ECDSA_SIG object, go read ECDSA_SIG_new(3) 💖 And for encoding/decoding ECDSA signatures to and from DER, that's i2d_ECDSA_SIG(3) and d2i_ECDSA_SIG(3) — iconic little converters, no cap 🦋

⚠️ okay but seriously: every single function described below is deprecated. Applications should be using the higher level EVP interface like EVP_DigestSignInit(3) or EVP_DigestVerifyInit(3) instead. That's the main character now, these are the nostalgic side characters 💀

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 (not the max), use EVP_PKEY_sign(3) with a NULL sig parameter — that's the hack ✨

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 is returned in siglen. Note note note: sig must point to ECDSA_size(eckey) bytes of memory, bestie, don't skimp on the buffer 😭 The parameter type is currently ignored (rude but okay). ECDSA_sign() is a wrapper function for ECDSA_sign_ex() with kinv and rp set to NULL.

ECDSA_do_sign() is similar to ECDSA_sign() except the signature is returned as a newly allocated ECDSA_SIG structure (or NULL on error). It's a wrapper function for ECDSA_do_sign_ex() with kinv and rp set to NULL. Same energy, different delivery method 🎁

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. Periodt.

ECDSA_do_verify() is similar to ECDSA_verify() except the signature is presented in the form of a pointer to an ECDSA_SIG structure. Vibe check but for signatures 🔥

🧚 the kinv and rp girlies (the "remaining functions")

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

  • 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(). It's giving meal-prep 🍓
  • 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 ignored. It's a whole thing 💀)
  • ECDSA_do_sign_ex() is 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).
🫶 friendly reminder that 0 and -1 are different things here — don't lump them together, bestie, that's how bugs are born 😭

🌈 EXAMPLES

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 👀

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 queen behavior, we love to see it 🎀

🦋 SEE ALSO

EC_KEY_new(3), EVP_DigestSignInit(3), EVP_DigestVerifyInit(3), EVP_PKEY_sign(3), i2d_ECDSA_SIG(3), d2i_ECDSA_SIG(3)

(the EVP ones are the glow-ups, fr fr — go befriend them ✨)

🌸 HISTORY

All functionality described here was deprecated in OpenSSL 3.0. It's giving "retired but remembered" 😭💜

Copyright 2004-2022 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 <https://www.openssl.org/source/license.html>.

okay that's the whole page bestie, you ate that 🫶✨ this was OpenSSL 3.5.6, dated 2026-04-07 — and yes, ECDSA_SIGN(3ossl) still says deprecated, some things never change 😌💅

go forth and use EVP, my love 🌈

📦 where this page comes from