NAME

       ECDSA_SIG_get0, ECDSA_SIG_get0_r, ECDSA_SIG_get0_s, 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);
        const BIGNUM *ECDSA_SIG_get0_r(const ECDSA_SIG *sig);
        const BIGNUM *ECDSA_SIG_get0_s(const ECDSA_SIG *sig);
        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 and stores
       them in *pr and *ps, respectively.  The pointer pr or ps can be NULL, in which case the
       corresponding value is not returned.

       The values r, s can also be retrieved separately by the corresponding function
       ECDSA_SIG_get0_r() and ECDSA_SIG_get0_s(), respectively.

       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_new() returns NULL if the allocation fails.

       ECDSA_SIG_set0() returns 1 on success or 0 on failure.

       ECDSA_SIG_get0_r() and ECDSA_SIG_get0_s() return the corresponding value, or NULL if it is
       unset.

       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 2004-2018 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>.