OpenSSL ECDSA signature validity - c

Question

From OpenSSL documentation
Creating an ECDSA signature of a given SHA-256 hash value using the named curve prime256v1 (aka P-256).
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 */
}
EOF
This brings me to an understanding that I need to verify every signature I create with ECDSA_do_sign or ECDSA_sign, do I? Can it happen that a created signature is not valid?

Answer 1

From NIST PUB 186-4 - Digital Signature Standard section 4.7:
Signature verification may be performed by any party (i.e., the signatory, the intended recipient or any other party) using the signatory’s public key. A signatory may wish to verify that the computed signature is correct, perhaps before sending the signed message to the intended recipient. The intended recipient (or any other party) verifies the signature to determine its
authenticity.
(where the signatory is the signature creator)