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DSS Keys

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Generating and Retrieving DSS Keys

DSS Keys can be generated by a call to the CryptGenKey function. The call to CryptGenKey requires that either AT_SIGNATURE or CALG_DSS_SIGN be passed in the Algid argument. This call will generate the P (prime modulus), Q (prime), G (generator), X (secret exponent), and Y (public key) values from scratch and persist them in a key BLOB to local storage.

To generate a DSS signature key pair

  1. Call the CryptAcquireContext function to get a handle to the Microsoft DSS Cryptographic Provider.
  2. Call CryptGenKey to generate the keys. Either AT_SIGNATURE or CALG_DSS_SIGN must be passed in for the Algid argument and the upper 16 bits of the dwFlags argument must be set to the desired key size. If the upper 16 bits are zero, then the default key size of 1,024 bits will be used. A HCRYPTKEY handle is returned in the hKey argument.

To retrieve a pointer to previously generated signature keys

  1. Call CryptAcquireContext to get a handle to the Microsoft DSS Cryptographic Provider.
  2. Call the CryptGetUserKey function with the dwKeySpec argument set to either AT_SIGNATURE or CALG_DSS_SIGN.

To retrieve the P, Q, and G values

  1. Call CryptAcquireContext to get a handle to the Microsoft DSS Cryptographic Provider.
  2. Call CryptGetUserKey with the dwKeySpec argument set to either AT_SIGNATURE or CALG_DSS_SIGN.
  3. Call CryptGetKeyParam with the hKey argument set to the pointer retrieved in the previous step. The dwParam argument must be set to the desired flag; KP_P, KP_Q, or KP_G. The value is returned in the pbData argument, and the length of the data is returned in the pdwDataLen argument. The value is returned with no header information and in little-endian format.

Generating DSS Signatures

Data to be signed must first be hashed by using the SHA algorithm. After that data is hashed, a DSS signature is generated by calling the CryptSignHash function.

To generate a DSS signature

  1. Call CryptAcquireContext to get a handle to the Microsoft DSS Cryptographic Provider.
  2. Call CryptCreateHash with the Algid argument set to CALG_SHA to get a handle to a SHA hash object.
  3. Call CryptHashData with the hHash argument set to the handle retrieved in the previous step. This creates a hash of the data and returns a handle to the hash in the phHash argument of the CryptCreateHash function call.
  4. Call CryptSignHash with the hHash argument set to the handle retrieved in the previous step. Either AT_SIGNATURE or CALG_DSS_SIGN may be passed in the dwKeySpec parameter. The signature is returned to the address provided in the pbSignature argument, and the length of the signature is returned to the address provided in the pdwSigLen argument. A NULL pointer may be passed in the pbSignature argument, and, in this case, the signature is not generated, but the length of the signature is returned to the address provided in the pdwSigLen parameter.

Verifying a DSS Signature

To verify a DSS signature, the DSS public key of the signer must be imported, the signed data must be hashed, and then the signature can be verified.

To verify a DSS signature

  1. Call CryptAcquireContext to get a handle to the Microsoft DSS Cryptographic Provider.

  2. Call CryptImportKey to import the signer's DSS public key.

  3. Call CryptCreateHash with the Algid argument set to CALG_SHA to get a handle to a SHA hash object.

  4. Call CryptHashData with the hHash argument set to the handle retrieved in the previous step and with pbData pointing to the signed data. This creates a hash of the data and returns a handle to the hash in the phHash argument of the CryptCreateHash function call.

  5. Call CryptVerifySignature with the following settings:

    hHash is set to the handle to the hash performed in the previous step.

    pbSignature points to the signature to be verified.

    dwSigLen is set to the length of the signature.

    hPubKey is set to the handle of the public key imported in step 2.

    dwFlags is set to zero.

Exporting DSS Keys

When you send signed data to someone where the signature will need to be verified by the recipient, the signer's public key must be provided to the recipient and is usually sent along with the signed data. Therefore, it is necessary to be able to export the DSS keys in a key BLOB format.

To export the DSS public key

  1. Call CryptAcquireContext to get a handle to the Microsoft DSS Cryptographic Provider.
  2. Call CryptGetUserKey with the dwKeySpec argument set to either AT_SIGNATURE or CALG_DSS_SIGN.
  3. Call CryptExportKey with hKey set to the handle retrieved in the previous step, dwBlobType set to PUBLICKEYBLOB, and dwFlags set to zero. The DSS public key BLOB is returned in pbData, and the length of the key BLOB is returned in pdwDataLen. A NULL pointer may be passed in pbData, and, in this case, just the length of the DSS key BLOB will be returned. The BLOB returned when making the call to CryptExportKey is in the format described in DSS Provider Key BLOBs.

To export the DSS private key

  • Follow the same procedure as for exporting a DSS public key, except that when making the call to CryptExportKey, dwBlobType is set to PRIVATEKEYBLOB. The BLOB returned when making the call to CryptExportKey is in the format described in DSS Provider Key BLOBs.