ecdsa

DESCRIPTION

The 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 ECDSA_SIG structure (note: this function also allocates the BIGNUMs) and initialize it.

ECDSA_SIG_free() frees the ECDSA_SIG structure sig.

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_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() is wrapper function for ECDSA_sign_ex with kinv and rp set to NULL.

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 signatures is stored in sig and it's length is returned in sig_len. Note: sig must point to ECDSA_size bytes of memory. The parameter type is ignored.

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

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

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

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

RETURN VALUES

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

ECDSA_sign_setup() and ECDSA_sign() return 1 if successful or -1 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 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 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 ECDSA_do_sign

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

Third step: verify the created ECDSA signature using ECDSA_do_verify

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

or using 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 */
       }

CONFORMING TO

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

SEE ALSO

dsa(3), rsa(3)

HISTORY

The ecdsa implementation was first introduced in OpenSSL 0.9.8

AUTHOR

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