=pod

=head1 NAME

ECDSA_SIG_get0, ECDSA_SIG_get0_r, ECDSA_SIG_get0_s, ECDSA_SIG_set0,
ECDSA_SIG_new, ECDSA_SIG_free, 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

=head1 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 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);

=head1 DESCRIPTION

Note: these functions provide a low level interface to ECDSA. Most
applications should use the higher level B<EVP> interface such as
L<EVP_DigestSignInit(3)> or L<EVP_DigestVerifyInit(3)> instead.

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

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

ECDSA_SIG_free() frees the B<ECDSA_SIG> structure B<sig>.

ECDSA_SIG_get0() returns internal pointers the B<r> and B<s> values contained
in B<sig> and stores them in B<*pr> and B<*ps>, respectively.
The pointer B<pr> or B<ps> can be NULL, in which case the corresponding value
is not returned.

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

The B<r> and B<s> values can be set by calling ECDSA_SIG_set0() and passing the
new values for B<r> and B<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.

See L<i2d_ECDSA_SIG(3)> and L<d2i_ECDSA_SIG(3)> for information about encoding
and decoding ECDSA signatures to/from DER.

ECDSA_size() returns the maximum length of a DER encoded ECDSA signature
created with the private EC key B<eckey>.

ECDSA_sign() computes a digital signature of the B<dgstlen> bytes hash value
B<dgst> using the private EC key B<eckey>. The DER encoded signatures is
stored in B<sig> and its length is returned in B<sig_len>. Note: B<sig> must
point to ECDSA_size(eckey) bytes of memory. The parameter B<type> is currently
ignored. ECDSA_sign() is wrapper function for ECDSA_sign_ex() with B<kinv>
and B<rp> set to NULL.

ECDSA_do_sign() is similar to ECDSA_sign() except the signature is returned
as a newly allocated B<ECDSA_SIG> structure (or NULL on error). ECDSA_do_sign()
is a wrapper function for ECDSA_do_sign_ex() with B<kinv> and B<rp> set to
NULL.

ECDSA_verify() verifies that the signature in B<sig> of size B<siglen> is a
valid ECDSA signature of the hash value B<dgst> of size B<dgstlen> using the
public key B<eckey>.  The parameter B<type> is ignored.

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

The remaining functions utilise the internal B<kinv> and B<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 B<kinv> or B<r> is not B<NULL>.

ECDSA_sign_setup() may be used to precompute parts of the signing operation.
B<eckey> is the private EC key and B<ctx> is a pointer to B<BN_CTX> structure
(or NULL). The precomputed values or returned in B<kinv> and B<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 B<dgstlen> bytes hash value
B<dgst> using the private EC key B<eckey> and the optional pre-computed values
B<kinv> and B<rp>. The DER encoded signature is stored in B<sig> and its
length is returned in B<sig_len>. Note: B<sig> must point to ECDSA_size(eckey)
bytes of memory. The parameter B<type> is ignored.

ECDSA_do_sign_ex() is similar to ECDSA_sign_ex() except the signature is
returned as a newly allocated B<ECDSA_SIG> structure (or NULL on error).

=head1 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
B<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 L<ERR_get_error(3)>.

=head1 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 B<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 */

=head1 CONFORMING TO

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

=head1 SEE ALSO

L<EC_KEY_new(3)>,
L<EVP_DigestSignInit(3)>,
L<EVP_DigestVerifyInit(3)>,
L<i2d_ECDSA_SIG(3)>,
L<d2i_ECDSA_SIG(3)>

=head1 COPYRIGHT

Copyright 2004-2018 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
L<https://www.openssl.org/source/license.html>.

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