C

Verify an RSA Signature Against the Signed String

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Demonstrates how to validate an RSA Signature against the string that was signed. C

#include <C_CkStringBuilder.h>
#include <C_CkPublicKey.h>
#include <C_CkRsa.h>

void ChilkatSample(void)
    {
    BOOL success;
    HCkStringBuilder sb;
    BOOL crlfLineEnding;
    int i;
    const char *base64_rsa_sig;
    HCkPublicKey pubKey;
    HCkRsa rsa;
    const char *signedString;

    success = FALSE;

    //  Create the same string we previously signed in this example: 
    //  RSA Sign utf-8 Bytes of String to get Base64 RSA Signature

    sb = CkStringBuilder_Create();
    crlfLineEnding = TRUE;

    for (i = 0; i <= 10; i++) {
        CkStringBuilder_AppendLine(sb,"This is a test.",crlfLineEnding);
    }

    base64_rsa_sig = "gWgpEXQqvXN6wh0MuFXPiw2xCb8cnmdizBT1TD1Tpm2GlJ8gnD59DGSj35GGXk1tM+mCOvMa2uW/9gmz8p6A90JLbn918i/2wKGvcde4wXnzPO7JADSNn2UwYzf3rp4/q/JrHv1GZETRJhABSFqTxOgdmdbEJQE/zijOSctCtQAM2CTb6t2BO8uHKSSUAwPH2cbeWxqsaVRLT4ruk5nZOxhoLgLMSjupiSzbE8zSTkkFOEHD+Mbb2xwg85AI0E4DsUnp+hLwwtS0blesgSyvtBY8oJODJeFcJsR7JDOTPdzHQHgJRl/2A4ij5xT91TpXy7ok43jhT6O9j7Q3qrafIQ==";

    //  Get the public key to be used for signature verification.
    pubKey = CkPublicKey_Create();
    success = CkPublicKey_LoadFromFile(pubKey,"rsaKeys/chilkat-rsa-2048.pem");
    if (success == FALSE) {
        printf("%s\n",CkPublicKey_lastErrorText(pubKey));
        CkStringBuilder_Dispose(sb);
        CkPublicKey_Dispose(pubKey);
        return;
    }

    rsa = CkRsa_Create();
    success = CkRsa_UsePublicKey(rsa,pubKey);
    if (success == FALSE) {
        printf("%s\n",CkRsa_lastErrorText(rsa));
        CkStringBuilder_Dispose(sb);
        CkPublicKey_Dispose(pubKey);
        CkRsa_Dispose(rsa);
        return;
    }

    //  Verify the string against the signature.
    //  We signed the utf-8 byte representation of the string, so we must indicate
    //  to also validate against the utf-8 byte representation.
    CkRsa_putCharset(rsa,"utf-8");
    CkRsa_putEncodingMode(rsa,"base64");
    signedString = CkStringBuilder_getAsString(sb);
    success = CkRsa_VerifyStringENC(rsa,signedString,"sha256",base64_rsa_sig);

    if (success == FALSE) {
        printf("%s\n",CkRsa_lastErrorText(rsa));
        printf("Signature invalid.\n");
    }
    else {
        printf("Signature valid.\n");
    }



    CkStringBuilder_Dispose(sb);
    CkPublicKey_Dispose(pubKey);
    CkRsa_Dispose(rsa);

    }