ECDSA_SIG_NEW(3)

NAME
ECDSA_SIG_get0, ECDSA_SIG_set0, ECDSA_SIG_new, ECDSA_SIG_free, i2d_ECDSA_SIG, d2i_ECDSA_SIG, ECDSA_size, ECDSA_sign, ECDSA_do_sign, ECDSA_verify, ECDSA_do_verify, ECDSA_sign_setup, ECDSA_sign_ex, ECDSA_do_sign_ex - low level elliptic curve digital signature algorithm (ECDSA) functions

SYNOPSIS
 #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 **pr, const BIGNUM **ps);
 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(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
Note: 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.

ECDSA_SIG is an opaque structure consisting of two BIGNUMs for the r and s value of an ECDSA signature (see X9.62 or FIPS 186-2).

ECDSA_SIG_new() allocates an empty ECDSA_SIG structure. Note: before OpenSSL 1.1.0 the: the r and s components were initialised.

ECDSA_SIG_free() frees the ECDSA_SIG structure sig.

ECDSA_SIG_get0() returns internal pointers the r and s values contained in sig.

The r and s values can be set by calling ECDSA_SIG_set0() and passing the new values for r and s as parameters to the function. Calling this function transfers the memory management of the values to the ECDSA_SIG object, and therefore the values that have been passed in should not be freed directly after this function has been called.

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() computes a digital signature of the dgstlen bytes hash value dgst using the private EC key eckey. The DER encoded signatures is stored in sig and its length is returned in sig_len. Note: sig must point to ECDSA_size(eckey) bytes of memory. The parameter type is currently ignored. ECDSA_sign() is 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). 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.

ECDSA_do_verify() is similar to ECDSA_verify() except the signature is presented in the form of a pointer to an ECDSA_SIG structure.

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.

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 or returned in kinv and rp and can be used in a later call to ECDSA_sign_ex() or ECDSA_do_sign_ex().

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 sig_len. Note: sig must point to ECDSA_size(eckey) bytes of memory. The parameter type is ignored.

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_SIG_set0() returns 1 on success or 0 on failure.

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).

EXAMPLES
Creating an ECDSA signature of a given SHA-256 hash value using the named curve prime256v1 (aka P-256).

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 FIPS 186-2 (Digital Signature Standard, DSS)

SEE ALSO
DSA_new(3), EVP_DigestSignInit(3), EVP_DigestVerifyInit(3)

COPYRIGHT
Copyright 2004-2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (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>.