openssl/doc/crypto/ecdsa.pod

=pod

=head1 NAME

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.

=head1 SYNOPSIS

 #include <openssl/ecdsa.h>

 ECDSA_SIG *ECDSA_SIG_new(void);
 void ECDSA_SIG_free(ECDSA_SIG *sig);
 void ECDSA_SIG_get0(BIGNUM **pr, BIGNUM **ps, ECDSA_SIG *sig);
 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);

=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 a new B<ECDSA_SIG> structure (note: this
function also allocates the BIGNUMs) and initializes it.

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>. The values can then be examined or initialised.

i2d_ECDSA_SIG() creates the DER encoding of the ECDSA signature B<sig> and
writes the encoded signature to B<*pp> (note: if B<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 B<ECDSA_SIG> structure. B<*sig> points to the
buffer containing the DER encoded signature of size B<len>.

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 it's 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 signatures is stored in B<sig> and it's
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_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 a 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<dsa(3)>,
L<rsa(3)>,
L<EVP_DigestSignInit(3)>,
L<EVP_DigestVerifyInit(3)>

=cut