openssl/doc/crypto/ecdsa.pod

=pod

=head1 NAME

ecdsa - Elliptic Curve Digital Signature Algorithm

=head1 SYNOPSIS

 #include <openssl/ecdsa.h>

 ECDSA_SIG*	ECDSA_SIG_new(void);
 void		ECDSA_SIG_free(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);

 ECDSA_SIG*	ECDSA_do_sign(const unsigned char *dgst, int dgst_len,
			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_do_verify(const unsigned char *dgst, int dgst_len,
			const ECDSA_SIG *sig, EC_KEY* eckey);
 int		ECDSA_sign_setup(EC_KEY *eckey, BN_CTX *ctx,
			BIGNUM **kinv, BIGNUM **rp);
 int		ECDSA_sign(int type, const unsigned char *dgst,
			int dgstlen, unsigned char *sig,
			unsigned int *siglen, EC_KEY *eckey);
 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);
 int		ECDSA_verify(int type, const unsigned char *dgst,
			int dgstlen, const unsigned char *sig,
			int siglen, EC_KEY *eckey);
 int		ECDSA_size(const EC_KEY *eckey);

 const ECDSA_METHOD*	ECDSA_OpenSSL(void);
 void		ECDSA_set_default_method(const ECDSA_METHOD *meth);
 const ECDSA_METHOD*	ECDSA_get_default_method(void);
 int		ECDSA_set_method(EC_KEY *eckey,const ECDSA_METHOD *meth);

 int		ECDSA_get_ex_new_index(long argl, void *argp,
			CRYPTO_EX_new *new_func,
			CRYPTO_EX_dup *dup_func,
			CRYPTO_EX_free *free_func);
 int		ECDSA_set_ex_data(EC_KEY *d, int idx, void *arg);
 void*		ECDSA_get_ex_data(EC_KEY *d, int idx);

=head1 DESCRIPTION

The B<ECDSA_SIG> structure consists of two BIGNUMs for the
r and s value of a ECDSA signature (see X9.62 or FIPS 186-2).

 struct
	{
	BIGNUM *r;
	BIGNUM *s;
 } ECDSA_SIG;

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

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

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 B<i2d_ECDSA_SIG> returns the expected length in bytes of 
the DER encoded signature). B<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_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 B<ECDSA_sign_ex> or B<ECDSA_do_sign_ex>.

ECDSA_sign() is wrapper function for ECDSA_sign_ex with B<kinv>
and B<rp> set to NULL.

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 B<ECDSA_size> bytes of memory. The parameter B<type>
is ignored.

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

ECDSA_do_sign() is wrapper function for ECDSA_do_sign_ex with B<kinv>
and B<rp> set to NULL.

ECDSA_do_sign_ex() computes a digital signature of the B<dgst_len>
bytes hash value B<dgst> using the private key B<eckey> and the
optional pre-computed values B<kinv> and B<rp>. The signature is
returned in a newly allocated B<ECDSA_SIG> structure (or NULL on error).

ECDSA_do_verify() verifies that the signature B<sig> is a valid
ECDSA signature of the hash value B<dgst> of size B<dgst_len>
using the public key B<eckey>.

=head1 RETURN VALUES

ECDSA_size() returns the maximum length signature or 0 on error.

ECDSA_sign_setup() and ECDSA_sign() return 1 if successful or 0
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)|ERR_get_error(3)>.

=head1 EXAMPLES

Creating a ECDSA signature of given SHA-1 hash value using the
named curve secp192k1.

First step: create a EC_KEY object (note: this part is B<not> ECDSA
specific)

 int        ret;
 ECDSA_SIG *sig;
 EC_KEY    *eckey = EC_KEY_new();
 if (eckey == NULL)
	{
	/* error */
	}
 key->group = EC_GROUP_new_by_nid(NID_secp192k1);
 if (key->group == NULL)
	{
	/* error */
	}
 if (!EC_KEY_generate_key(eckey))
	{
	/* error */
	}

Second step: compute the ECDSA signature of a SHA-1 hash value 
using B<ECDSA_do_sign> 

 sig = ECDSA_do_sign(digest, 20, eckey);
 if (sig == NULL)
	{
	/* error */
	}

or using B<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);
	{
	/* error */
	}

Third step: verify the created ECDSA signature using B<ECDSA_do_verify>

 ret = ECDSA_do_verify(digest, 20, sig, eckey);

or using B<ECDSA_verify>

 ret = ECDSA_verify(0, digest, 20, buffer, buf_len, eckey);

and finally evaluate the return value:

 if (ret == -1)
	{
	/* error */
	}
 else if (ret == 0)
	{
	/* incorrect signature */
	}
 else	/* ret == 1 */
	{
	/* signature ok */
	}

=head1 CONFORMING TO

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

=head1 SEE ALSO

L<dsa(3)|dsa(3)>, L<rsa(3)|rsa(3)>

=head1 HISTORY

The ecdsa implementation was first introduced in OpenSSL 0.9.8

=head1 AUTHOR

Nils Larsch for the OpenSSL project (http://www.openssl.org).

=cut