ecdsa - Elliptic Curve Digital Signature Algorithm

 #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);

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.

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).

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 */
        }

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

dsa(3), rsa(3)

The ecdsa implementation was first introduced in OpenSSL 0.9.8

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