.. _eccpverifydsa:


ECCPVerifyDSA
=============


Verifies authenticity of the digital signature over a message digest
(ECDSA).


Syntax
------


IppStatus ippsECCPVerifyDSA(const IppsBigNumState\* pMsgDigest, const
IppsBigNumState\* pSignX, const IppsBigNumState\* pSignY, IppECResult\*
pResult, IppsECCPState\* pECC);


Include Files
-------------


``ippcp.h``


Parameters
----------


.. list-table:: 
   :header-rows: 0

   * - pMsgDigest   
     - Pointer to the message digest *msg*.
   * - pSignX   
     - Pointer to the integer *r* of the digital signature.
   * - pSignY   
     - Pointer to the integer *s* of the digital signature.
   * - pResult   
     - Pointer to the digital signature verification result.
   * - pECC   
     - Pointer to the context of the elliptic cryptosystem.




Description
-----------


The function verifies authenticity of the digital signature over a
message digest *msg*. The signature consists of two large integers: *r*
and *s*.


The scheme used to verify the signature is an elliptic curve analogue of
the DSA scheme and assumes that the following cryptosystem key be
hitherto set:


.. list-table:: 
   :header-rows: 0

   * - \ *regPubKey*\    
     - Message sender's regular public key.




The *regPubKey* is set by the function
`ECCPSetKeyPair <eccpsetkeypair.html>`__.


The result of the digital signature verification can take one of two
possible values:


.. list-table:: 
   :header-rows: 0

   * - ippECValid   
     - Digital signature is valid.
   * - ippECInvalidSignature   
     - Digital signature is not valid.




The call to the ECCPVerifyDSA function must be preceded by the call to
the
`ECCPSignDSA <eccpsigndsa.html>`__
function which computes the digital signature over the message digest
*msg* and represents the signature with two numbers: *r* and *s*.


The elliptic curve domain parameters must be hitherto defined by one of
the functions:
`ECCPSet <eccpset.html>`__
or
`ECCPSetStd <eccpsetstd.html>`__.


For more information on digital signatures, please refer to the
[`ANSI <bibliography.html>`__]
standard.


Return Values
-------------


.. list-table:: 
   :header-rows: 0

   * - ippStsNoErr    
     - Indicates no error. Any other value indicates an error or warning.
   * - ippStsNullPtrErr   
     - Indicates an error condition if any of the specified pointers is NULL.
   * - ippStsContextMatchErr   
     - Indicates an error condition if one of the contexts pointed by pMsgDigest, pSignX, pSignY, or ECC is not valid.
   * - ippStsMessageErr   
     - Indicates an error condition if the value of *msg* pointed by pMsgDigest falls outside the range of [1, *n*-1] where *n* is the order of the elliptic curve base point *G*.
   * - ippStsRangeErr   
     - Indicates an error condition if the value of pSignX or pSignY is less than 0.


.. rubric:: Related Information

:ref:`signing-verification-using-the-elliptic-curve-cryptography-functions-over-a-prime-finite-field`