Trouble encrypting/decrypting with ecc in libgrcypt

[Will Dignazio]

Hello gnupg-users,

I’m stuck trying to decrypt a simple string in a test program. I seem to 
correctly go through all of steps to generate a key pair, use the public key of 
the pair to encrypt, and the secret key to decrypt. However, the value returned 
after decryption seems to be mangled.

Would anyone be willing to lend a moment to explain what I’m doing wrong, or 
any misunderstanding I may have?

My program is as follows (please forgive the lack of resource deallocation, 
this is just a test program):

#include <gcrypt.h>

int main(void) {
    gcry_error_t err = 0;
    gcry_ctx_t ctx = NULL;
    gcry_sexp_t keyparams = NULL;
    gcry_sexp_t keypair = NULL;
    gcry_sexp_t pubkey = NULL;
    gcry_sexp_t seckey = NULL;
    gcry_sexp_t encrypted_data = NULL;
    gcry_sexp_t decrypted_data = NULL;
    gcry_sexp_t enc_data = NULL;
    gcry_mpi_t datampi = NULL;
    const char *sexp = "(genkey (ecc (curve \"NIST P-256\") (flags param 
eddsa)))";
    size_t erroff = 0;

    /* Tell Libgcrypt that initialization has completed. */
    gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0);
    
    err = gcry_sexp_build(&keyparams, &erroff, sexp);
    if (err) {
        fprintf(stderr, "Failed to build keypair sexp: %s\n", 
gcry_strerror(err));
        return 1;
    }

    err = gcry_pk_genkey(&keypair, keyparams);
    if (err) {
        fprintf(stderr, "Error initializing keypair: %s\n", gcry_strerror(err));
        return 1;
    }

    err = gcry_pk_testkey(keypair);
    if (err) {
        fprintf(stderr, "testkey failed\n");
        return 1;
    }

    err = gcry_mpi_ec_new(&ctx, keypair, "NIST P-256");
    if (err) {
        fprintf(stderr, "Failed to allocate mpi context: %s\n", 
gcry_strerror(err));
        return 1;
    }

    err = gcry_pubkey_get_sexp(&pubkey, GCRY_PK_GET_PUBKEY, ctx);
    if (err) {
        fprintf(stderr, "Failed to parse public key from keypair sexp: %s\n", 
gcry_strerror(err));
        return 1;
    }

    err = gcry_pubkey_get_sexp(&seckey, GCRY_PK_GET_SECKEY, ctx);
    if (err) {
        fprintf(stderr, "Failed to parse secret key from keypair sexp: %s\n", 
gcry_strerror(err));
        return 1;
    }

    const char *data = "This is the data";
    size_t len = strlen(data);
    
    err = gcry_mpi_scan(&datampi, GCRYMPI_FMT_STD, (const char*)data, len, 
NULL);
    if (err) {
        fprintf(stderr, "Failed to scan data for ecnryption: %s\n", 
gcry_strerror(err));
        return 1;
    }
    
    err = gcry_sexp_build(&enc_data, &erroff, "(data (flags raw) (value %m))", 
datampi);
    if (err || erroff) {
        fprintf(stderr, "Failed to build encryption sexp: %s\n", 
gcry_strerror(err));
        return 1;
    }

    err = gcry_pk_encrypt(&encrypted_data, enc_data, pubkey);
    if (err) {
        fprintf(stderr, "Failed to encrypt data sexp: %s\n", 
gcry_strerror(err));
        return 1;
    }

    gcry_sexp_dump(encrypted_data);
    printf("\n");

    err = gcry_pk_decrypt(&decrypted_data, encrypted_data, seckey);
    if (err) {
        fprintf(stderr, "Failed to decrypt data%s\n", gcry_strerror(err));
        return 1;
    }

    gcry_sexp_dump(decrypted_data);
    printf("\n");

    datampi = gcry_sexp_nth_mpi(decrypted_data, 1, GCRYMPI_FMT_USG);
    if (datampi == NULL) {
        fprintf(stderr, "Failed to extract value: %s\n", gcry_strerror(err));
        return 1;
    }

    size_t written;
    unsigned char *buffer;
    gcry_mpi_aprint(GCRYMPI_FMT_USG, &buffer, &written, datampi);

    printf("%s\n", buffer);
}


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