Update and clarify ECDSA documentation.

Reviewed-by: NRichard Levitte <levitte@openssl.org>

doc/crypto/ecdsa.pod

@@ -2,113 +2,117 @@
 
 =head1 NAME
 
-ECDSA_SIG_new, ECDSA_SIG_free, i2d_ECDSA_SIG, d2i_ECDSA_SIG, ECDSA_size, ECDSA_sign_setup, ECDSA_sign, ECDSA_sign_ex, ECDSA_verify, ECDSA_do_sign, ECDSA_do_sign_ex, ECDSA_do_verify - Elliptic Curve Digital Signature Algorithm
+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);
- 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);
+ 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
 
-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).
+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.
 
- struct
-	{
-	BIGNUM *r;
-	BIGNUM *s;
- } ECDSA_SIG;
+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 initialize it.
+function also allocates the BIGNUMs) and initializes 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).
+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.
 
-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>.
+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).
 
-ECDSA_size() returns the maximum length of a DER encoded
-ECDSA signature created with the private EC key B<eckey>.
+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_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_size() returns the maximum length of a DER encoded ECDSA signature
+created with the private EC key B<eckey>.
 
-ECDSA_sign() is wrapper function for ECDSA_sign_ex with B<kinv>
+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_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_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_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_sign() is wrapper function for ECDSA_do_sign_ex with B<kinv>
-and B<rp> set to NULL.
+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_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_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_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>.
+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_setup() and ECDSA_sign() return 1 if successful 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.
@@ -116,8 +120,8 @@ The error codes can be obtained by L<ERR_get_error(3)>.
 
 =head1 EXAMPLES
 
-Creating a ECDSA signature of given SHA-1 hash value using the
-named curve secp192k1.
+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)
@@ -125,59 +129,50 @@ specific)
  int        ret;
  ECDSA_SIG *sig;
  EC_KEY    *eckey;
- eckey = EC_KEY_new_by_curve_name(NID_secp192k1);
- if (eckey == 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>
+ 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);
-	{
-	/* error */
-	}
+ if (ECDSA_sign(0, dgst, dgstlen, pp, &buf_len, eckey) == 0) {
+    /* error */
+ }
 
-Third step: verify the created ECDSA signature using B<ECDSA_do_verify>
+Third step: verify the created ECDSA signature using ECDSA_do_verify():
 
- ret = ECDSA_do_verify(digest, 20, sig, eckey);
+ ret = ECDSA_do_verify(digest, 32, sig, eckey);
 
-or using B<ECDSA_verify>
+or using ECDSA_verify():
 
- ret = ECDSA_verify(0, digest, 20, buffer, buf_len, eckey);
+ ret = ECDSA_verify(0, digest, 32, 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 */
-	}
+ if (ret == 1) {
+    /* signature ok */
+ } else if (ret == 0) {
+    /* incorrect signature */
+ } else {
+    /* error */
+ }
 
 =head1 CONFORMING TO
 
@@ -186,6 +181,9 @@ ANSI X9.62, US Federal Information Processing Standard FIPS 186-2
 
 =head1 SEE ALSO
 
-L<dsa(3)>, L<rsa(3)>
+L<dsa(3)>,
+L<rsa(3)>,
+L<EVP_DigestSignInit(3)>,
+L<EVP_DigestVerifyInit(3)>
 
 =cut