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Digital Signatures

 

 

 

 

 

 

 

(C) DSA Signature Create and Verify

Shows how to create a DSA (DSS) signature for the contents of a file. The first step is to create an SHA-1 hash of the file contents. The hash is signed using the Digital Signature Algorithm and the signature bytes are retrieved as a hex-encoded string.

The 2nd part of the example loads the signature and verifies it against the hash.

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#include <C_CkCrypt2.h>
#include <C_CkDsa.h>

void ChilkatSample(void)
    {
    BOOL success;
    HCkCrypt2 crypt;
    const char *hashStr;
    HCkDsa dsa;
    const char *pemPrivateKey;
    const char *hexSig;
    HCkDsa dsa2;
    const char *pemPublicKey;



    //  Use Chilkat Crypt to hash the contents of a file.
    //  This unlocks both the crypt and DSA objects..
    crypt = CkCrypt2_Create();
    success = CkCrypt2_UnlockComponent(crypt,"Anything for 30-day trial.");
    if (success != TRUE) {
        printf("%s\n",CkCrypt2_lastErrorText(crypt));
        CkCrypt2_Dispose(crypt);
        return;
    }

    CkCrypt2_putEncodingMode(crypt,"hex");
    CkCrypt2_putHashAlgorithm(crypt,"sha-1");

    //  Return the SHA-1 hash of a file.  The file may be any size.
    //  The Chilkat Crypt component will stream the file when
    //  computing the hash, keeping the memory usage constant
    //  and reasonable.
    //  The 20-byte SHA-1 hash is returned as a hex-encoded string.
    hashStr = CkCrypt2_hashFileENC(crypt,"hamlet.xml");

    dsa = CkDsa_Create();

    //  Load a DSA private key from a PEM file.  Chilkat DSA
    //  provides the ability to load and save DSA public and private
    //  keys from encrypted or non-encrypted PEM or DER.
    //  The LoadText method is for convenience only.  You may
    //  use any means to load the contents of a PEM file into
    //  a string.

    pemPrivateKey = CkDsa_loadText(dsa,"dsa_priv.pem");
    success = CkDsa_FromPem(dsa,pemPrivateKey);
    if (success != TRUE) {
        printf("%s\n",CkDsa_lastErrorText(dsa));
        CkCrypt2_Dispose(crypt);
        CkDsa_Dispose(dsa);
        return;
    }

    //  You may optionally verify the key to ensure that it is a valid
    //  DSA key.
    success = CkDsa_VerifyKey(dsa);
    if (success != TRUE) {
        printf("%s\n",CkDsa_lastErrorText(dsa));
        CkCrypt2_Dispose(crypt);
        CkDsa_Dispose(dsa);
        return;
    }

    //  Load the hash to be signed into the DSA object:
    success = CkDsa_SetEncodedHash(dsa,"hex",hashStr);
    if (success != TRUE) {
        printf("%s\n",CkDsa_lastErrorText(dsa));
        CkCrypt2_Dispose(crypt);
        CkDsa_Dispose(dsa);
        return;
    }

    //  Now that the DSA object contains both the private key and hash,
    //  it is ready to create the signature:
    success = CkDsa_SignHash(dsa);
    if (success != TRUE) {
        printf("%s\n",CkDsa_lastErrorText(dsa));
        CkCrypt2_Dispose(crypt);
        CkDsa_Dispose(dsa);
        return;
    }

    //  If SignHash is successful, the DSA object contains the
    //  signature.  It may be accessed as a hex or base64 encoded
    //  string.  (It is also possible to access directly in byte array form via
    //  the "Signature" property.)
    hexSig = CkDsa_getEncodedSignature(dsa,"hex");
    printf("Signature:\n");
    printf("%s\n",hexSig);

    //  -----------------------------------------------------------
    //  Step 2: Verify the DSA Signature
    //  -----------------------------------------------------------

    dsa2 = CkDsa_Create();

    //  Load the DSA public key to be used for verification:

    pemPublicKey = CkDsa_loadText(dsa2,"dsa_pub.pem");
    success = CkDsa_FromPublicPem(dsa2,pemPublicKey);
    if (success != TRUE) {
        printf("%s\n",CkDsa_lastErrorText(dsa2));
        CkCrypt2_Dispose(crypt);
        CkDsa_Dispose(dsa);
        CkDsa_Dispose(dsa2);
        return;
    }

    //  Load the hash to be verified against the signature.
    success = CkDsa_SetEncodedHash(dsa2,"hex",hashStr);
    if (success != TRUE) {
        printf("%s\n",CkDsa_lastErrorText(dsa2));
        CkCrypt2_Dispose(crypt);
        CkDsa_Dispose(dsa);
        CkDsa_Dispose(dsa2);
        return;
    }

    //  Load the signature:
    success = CkDsa_SetEncodedSignature(dsa2,"hex",hexSig);
    if (success != TRUE) {
        printf("%s\n",CkDsa_lastErrorText(dsa2));
        CkCrypt2_Dispose(crypt);
        CkDsa_Dispose(dsa);
        CkDsa_Dispose(dsa2);
        return;
    }

    //  Verify:
    success = CkDsa_Verify(dsa2);
    if (success != TRUE) {
        printf("%s\n",CkDsa_lastErrorText(dsa2));
    }
    else {
        printf("DSA Signature Verified!\n");
    }



    CkCrypt2_Dispose(crypt);
    CkDsa_Dispose(dsa);
    CkDsa_Dispose(dsa2);

    }

 

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