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ECDSA_SIG_NEW(3) Library Functions Manual ECDSA_SIG_NEW(3)

ECDSA_SIG_new, ECDSA_SIG_free, ECDSA_SIG_get0, ECDSA_SIG_set0, i2d_ECDSA_SIG, d2i_ECDSA_SIG, ECDSA_size, ECDSA_sign_setup, ECDSA_sign, ECDSA_sign_ex, ECDSA_verify, ECDSA_do_sign, ECDSA_do_sign_ex, ECDSA_do_verify, ECDSA_OpenSSL, ECDSA_get_default_method, ECDSA_set_default_method, ECDSA_set_method
Elliptic Curve Digital Signature Algorithm

#include <openssl/ecdsa.h>
ECDSA_SIG*
ECDSA_SIG_new(void);
void
ECDSA_SIG_free(ECDSA_SIG *sig);
void
ECDSA_SIG_get0(const ECDSA_SIG *sig, const BIGNUM **r, const BIGNUM **s);
int
ECDSA_SIG_set0(ECDSA_SIG *sig, BIGNUM *r, BIGNUM *s);
int
i2d_ECDSA_SIG(const ECDSA_SIG *sig, unsigned char **pp);
ECDSA_SIG*
d2i_ECDSA_SIG(ECDSA_SIG **sig, const unsigned char **pp, long len);
int
ECDSA_size(const EC_KEY *eckey);
int
ECDSA_sign_setup(EC_KEY *eckey, BN_CTX *ctx, BIGNUM **kinv, BIGNUM **rp);
int
ECDSA_sign(int type, const unsigned char *dgst, int dgstlen, unsigned char *sig, unsigned int *siglen, EC_KEY *eckey);
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);
int
ECDSA_verify(int type, const unsigned char *dgst, int dgstlen, const unsigned char *sig, int siglen, EC_KEY *eckey);
ECDSA_SIG*
ECDSA_do_sign(const unsigned char *dgst, int dgst_len, 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_do_verify(const unsigned char *dgst, int dgst_len, const ECDSA_SIG *sig, EC_KEY* eckey);
const ECDSA_METHOD*
ECDSA_OpenSSL(void);
const ECDSA_METHOD*
ECDSA_get_default_method(void);
void
ECDSA_set_default_method(const ECDSA_METHOD *meth);
int
ECDSA_set_method(EC_KEY *eckey, const ECDSA_METHOD *meth);

These functions provide a low level interface to ECDSA. Most applications should use the higher level EVP interface such as EVP_DigestSignInit(3) or EVP_DigestVerifyInit(3) instead. Creation of the required EC_KEY objects is described in EC_KEY_new(3).
The ECDSA_SIG structure consists of two BIGNUMs for the r and s value of an ECDSA signature (see X9.62 or FIPS 186-2).
struct { 
	BIGNUM *r; 
	BIGNUM *s; 
} ECDSA_SIG;
ECDSA_SIG_new() allocates a new ECDSA_SIG structure (note: this function also allocates the BIGNUMs) and initializes it.
ECDSA_SIG_free() frees the ECDSA_SIG structure sig.
ECDSA_SIG_get0() retrieves internal pointers the r and s values contained in sig.
ECDSA_SIG_set0() sets the r and s values in sig. Calling this function transfers the memory management of the values to sig. Therefore, the values that have been passed in should not be freed by the caller.
i2d_ECDSA_SIG() creates the DER encoding of the ECDSA signature sig and writes the encoded signature to *pp (note: if pp is NULL, i2d_ECDSA_SIG() returns the expected length in bytes of the DER-encoded signature). i2d_ECDSA_SIG() returns the length of the DER-encoded signature (or 0 on error).
d2i_ECDSA_SIG() decodes a DER-encoded ECDSA signature and returns the decoded signature in a newly allocated ECDSA_SIG structure. *sig points to the buffer containing the DER-encoded signature of size len.
ECDSA_size() returns the maximum length of a DER-encoded ECDSA signature created with the private EC key eckey.
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 a 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.
ECDSA_sign() is a wrapper function for ECDSA_sign_ex with kinv and rp set to NULL.
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() bytes of memory. The parameter type is ignored.
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.
ECDSA_do_sign() is a wrapper function for ECDSA_do_sign_ex() with kinv and rp set to NULL.
ECDSA_do_sign_ex() computes a digital signature of the dgst_len bytes hash value dgst using the private key eckey and the optional pre-computed values kinv and rp. The signature is returned in a newly allocated ECDSA_SIG structure (or NULL on error).
ECDSA_do_verify() verifies that the signature sig is a valid ECDSA signature of the hash value dgst of size dgst_len using the public key eckey.

ECDSA_size() returns the maximum length signature or 0 on error.
ECDSA_SIG_set0(), 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).

Creating an ECDSA signature of given SHA-1 hash value using the named curve secp192k1.
First step: create an EC_KEY object. This part is not ECDSA specific.
int ret; 
ECDSA_SIG *sig; 
EC_KEY *eckey; 
 
eckey = EC_KEY_new_by_curve_name(NID_secp192k1); 
if (eckey == NULL) { 
	/* error */ 
} 
if (!EC_KEY_generate_key(eckey)) { 
	/* error */ 
}
Second step: compute the ECDSA signature of a SHA-1 hash value using ECDSA_do_sign()
sig = ECDSA_do_sign(digest, 20, eckey); 
if (sig == NULL) { 
	/* error */ 
}
or using ECDSA_sign()
unsigned char *buffer, *pp; 
int buf_len; 
 
buf_len = ECDSA_size(eckey); 
buffer  = malloc(buf_len); 
pp = buffer; 
if (!ECDSA_sign(0, dgst, dgstlen, pp, &buf_len, eckey) { 
	/* error */ 
}
Third step: verify the created ECDSA signature using ECDSA_do_verify()
ret = ECDSA_do_verify(digest, 20, sig, eckey);
or using ECDSA_verify()
ret = ECDSA_verify(0, digest, 20, buffer, buf_len, eckey);
and finally evaluate the return value:
if (ret == -1) { 
	/* error */ 
} else if (ret == 0) { 
	/* incorrect signature */ 
} else { 
	/* ret == 1 */ 
	/* signature ok */ 
}

d2i_ECPKParameters(3), DSA_new(3), EC_GROUP_new(3), EC_KEY_new(3), ECDSA_set_ex_data(3), EVP_DigestSignInit(3), EVP_DigestVerifyInit(3), RSA_new(3)

ANSI X9.62, US Federal Information Processing Standard FIPS 186-2 (Digital Signature Standard, DSS)

ECDSA_SIG_new(), ECDSA_SIG_free(), i2d_ECDSA_SIG(), d2i_ECDSA_SIG(), ECDSA_size(), ECDSA_sign_setup(), ECDSA_sign(), ECDSA_sign_ex(), ECDSA_verify(), ECDSA_do_sign(), ECDSA_do_sign_ex(), ECDSA_do_verify(), ECDSA_OpenSSL(), ECDSA_get_default_method(), ECDSA_set_default_method(), and ECDSA_set_method() first appeared in OpenSSL 0.9.8 and have been available since OpenBSD 4.5.
ECDSA_SIG_get0() and ECDSA_SIG_set0() first appeared in OpenSSL 1.1.0 and have been available since OpenBSD 6.3.

Nils Larsch for the OpenSSL project.
March 23, 2018 OpenBSD-current