CODEWITHSUNDEEP
cencryptionaeslibgcrypt
Sean Mitchell
Sean Mitchell

Reputation: 457

Libgcrypt in C returning 42

I would like to preface this with, I am new to C and especially using a lib as large as libgcrypt, so if it is something incredibly simple please be understanding.

So, I have been trying to just encrypt some text just to kinda launchpad from and so that way I get some better confidence before going with just encrypting bytes which is one of the end goals of this little app if youd like to call it that. I have been following this, but still am struggling with getting it to work. The part that really confuses me is that I am passing it the out buffer which has been sized to match that of the in text. I do find it weird that this site tells me to make the out buffer the same size as the input even though usually when encrypting with AES the output is large in terms of bytes. Maybe I missed something that someone more experience can help me with, I have read over their code at least 3-4 times now and I feel like it must be something super simple I am missing here.

EDIT

client.c

#include "client.h"

void printGCRYPTError(gcry_error_t err) {
    if (err) {
         {
           fprintf (stderr, "Failure: %s/%s\n",
                    gcry_strsource (err),
                    gcry_strerror (err));
           fprintf (stdout, "Failure: %s/%s\n",
                    gcry_strsource (err),
                    gcry_strerror (err));
         }
    }
}

char* Encrypt(char* in, char* key, char* iv){
    size_t keyLength = gcry_cipher_get_algo_keylen(GCRY_CIPHER);
    size_t blkLength = gcry_cipher_get_algo_blklen(GCRY_CIPHER);

    if(strlen(key) != keyLength) {
        printf("Invalid Key Size. Expected %zu got %lu\n", keyLength, strlen(key));
        return NULL;
    }
    if(strlen(iv) != blkLength) {
        printf("Invalid BLK/IV Size. Expected %zu got %lu\n", blkLength, strlen(iv));
        return NULL;
    }

    gcry_cipher_hd_t handle;
    gcry_error_t err = 0;

    size_t inLen = strlen(in)+1;
    char * out = malloc(inLen);

    // Open
    err = gcry_cipher_open(&handle, GCRY_CIPHER, GCRY_C_MODE, 0);
    printGCRYPTError(err);

    // Set Key
    err = gcry_cipher_setkey(handle, key, keyLength);
    printGCRYPTError(err);

    // Set IV
    err = gcry_cipher_setiv(handle, iv, blkLength);
    printGCRYPTError(err);

    // Enc
    printf("Encrypting...\n");
    err = gcry_cipher_encrypt(handle, out, inLen, in, inLen);
    printGCRYPTError(err);
    printf("Encrypted.\n");
    gcry_cipher_final(handle);


    //Out
    printf("Plaintext Message: %s\n", in);
    printf("Encrypted Message: ");
    int index = 0;
    for (index = 0; index<inLen; index++) {
        printf("%02X", (unsigned char)out[index]);
    }
    printf("\n");

    // Cleanup!
    gcry_cipher_close(handle);

    // Return
    return out;
}

char* Decrypt(char* in, char* key, char* iv){
    size_t keyLength = gcry_cipher_get_algo_keylen(GCRY_CIPHER);
    size_t blkLength = gcry_cipher_get_algo_blklen(GCRY_CIPHER);

    if(strlen(key) != keyLength) {
        printf("Invalid Key Size. Expected %zu got %lu\n", keyLength, strlen(key));
        return NULL;
    }
    if(strlen(iv) != blkLength) {
        printf("Invalid BLK/IV Size. Expected %zu got %lu\n", blkLength, strlen(iv));
        return NULL;
    }

    gcry_cipher_hd_t handle;
    gcry_error_t err = 0;

    size_t inLen = strlen(in);
    char * out = malloc(inLen);

    // Open
    err = gcry_cipher_open(&handle, GCRY_CIPHER, GCRY_C_MODE, 0);
    printGCRYPTError(err);

    // Set Key
    err = gcry_cipher_setkey(handle, key, keyLength);
    printGCRYPTError(err);

    // Set IV
    err = gcry_cipher_setiv(handle, iv, blkLength);
    printGCRYPTError(err);

    // Dec
    printf("Decrypting...\n");
    err = gcry_cipher_decrypt(handle, out, inLen, in, inLen);
    printGCRYPTError(err);
    printf("Decrypted.\n");
    gcry_cipher_final(handle);

    //Out
    printf("Encrypted Message: ");
    int index = 0;
    for (index = 0; index<inLen; index++) {
        printf("%02X", (unsigned char)out[index]);
    }
    printf("\n");
    printf("Decrypted Message: %s\n", out);

    // Cleanup!
    gcry_cipher_close(handle);

    // Return
    return out;
}

int main() {
    //aesTest();
    char* encrypted = Encrypt("My test message", "sBK0hcXddz6mIKEsBK0hcXddz6mIKEAS", "sBK0uhSAUSUSHKES");
    char* decrypted = Decrypt(encrypted, "sBK0hcXddz6mIKEsBK0hcXddz6mIKEAS", "sBK0uhSAUSUSHKES");
}

Console Output

Encrypting...
Encrypted.
Plaintext Message: My test message
Encrypted Message: 153C01DD7821E5113C25D2038F3644F7
Decrypting...
Failure: gcrypt/Invalid length
Failure: gcrypt/Invalid length
Decrypted.
Encrypted Message: 0000000000001000000000000000101400
Decrypted Message:

Thanks for looking!

Upvotes: 2

Views: 331

Answers (3)

Sean Mitchell
Sean Mitchell

Reputation: 457

In addition to the key issues, I needed padding to be added to my AES plaintext before encryption.

Upvotes: 0

Trevor Keller
Trevor Keller

Reputation: 500

As selbie suggested, checking the error returned by gcry_cipher_encrypt is useful: it says "Failure: gcrypt/Invalid length."

The root cause appears to be inLen:

size_t inLen = strlen(in);
...
err = gcry_cipher_encrypt(handle, out, inLen, in, inLen);
printGCRYPTError(err);
gcry_cipher_final(handle);

returns the (expected?) output,

Message: My test message!
Encrypted Message = AEDBBB645CA54F16B5A279D77772A97B

Upvotes: 0

selbie
selbie

Reputation: 104464

As per the docs for libgcrypt:

Some encryption modes require that gcry_cipher_final is used before the final data chunk is passed to this function.

So I suspect you need to do two things:

  1. Check the error code result from gcry_cipher_encrypt consistent with how you were doing the prior function calls.

  2. Invoke gcry_cipher_final(handle) after you call encrypt - presuming you are doing a one-shot encryption of a buffer that is smaller than the block length of your cipher.

Upvotes: 0

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