Add ECDSA documentation.

Submitted by: Nils Larsch

diff --git a/doc/crypto/ecdsa.pod b/doc/crypto/ecdsa.pod
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+=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_DATA*   ECDSA_DATA_new(void);
+ ECDSA_DATA*   ECDSA_DATA_new_method(ENGINE *eng);
+ void          ECDSA_DATA_free(ECDSA_DATA *data);
+ ECDSA_DATA*   ecdsa_check(EC_KEY *eckey);
+
+ ECDSA_SIG*    ECDSA_do_sign(const unsigned char *dgst, int dgst_len,
+                       EC_KEY *eckey);
+ int           ECDSA_do_verify(const unsigned char *dgst, int dgst_len,
+                       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_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>.
+
+The B<ECDSA_DATA> structure extends the B<EC_KEY_METH_DATA>
+structure with ECDSA specific data.
+
+ struct
+       {
+       /* EC_KEY_METH_DATA part */
+       int  (*init)(EC_KEY *);
+       void (*finish)(EC_KEY *);
+       /* method (ECDSA) specific part */
+       BIGNUM  *kinv;  /* signing pre-calc */
+       BIGNUM  *r;     /* signing pre-calc */
+       ...
+       } 
+ ECDSA_DATA;
+
+B<kinv> and B<r> are used to store precomputed values (see
+B<ECDSA_sign_setup>). 
+
+ECDSA_DATA_new() returns a newly allocated and initialized
+B<ECDSA_DATA> structure (or NULL on error). 
+
+ECDSA_DATA_free() frees the B<ECDSA_DATA> structure B<data>.
+
+ecdsa_check() returns the pointer to the B<ECDSA_DATA>
+structure in B<EC_KEY-E<gt>meth_data> (if B<EC_KEY-E<gt>meth_data>
+is not a pointer to a B<ECDSA_DATA> structure then the old
+data is freed and a new B<ECDSA_DATA> structure is allocated
+using B<ECDSA_DATA_new>).
+
+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> or B<ECDSA_do_sign> when placed in
+B<ECDSA_DATA-E<gt>kinv> and B<ECDSA_DATA-E<gt>r>.
+
+ECDSA_sign() computes a digital signature of the B<dgstlen> bytes
+hash value B<dgst> using the private EC key B<eckey> and places
+the DER encoding of the created signature in B<sig>. The length
+of the created signature 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() computes a digital signature of the B<dgst_len>
+bytes hash value B<dgst> using the private key B<eckey> and 
+returns the signature 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 -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 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