AES CCM Encryption and Decryption in Libgcrypt

Question

I have a problem encrypting/decrypting a simple 16 byte message in Libgcrypt while using CCM mode of operation and AES algorithm. In the documentation of Libgcrypt I cannot find which parameters to set for CCM (should I set IV or counter?).

I am stuck with the following code:

gcry_error_t     err;
gcry_cipher_hd_t hd;

char * key = "1234567890123456";
char * plainText = "VNiJkPzAWPFm1234";
size_t messageSize = strlen(plainText);
char * cipherText = malloc(messageSize);
char * recoveredText = malloc(messageSize);    

err = gcry_cipher_open(
    &hd,
    GCRY_CIPHER_AES128,
    GCRY_CIPHER_MODE_CCM,
    0);

err = gcry_cipher_setkey(hd, key, 16);

/* What to do here? */

err = gcry_cipher_encrypt(
    hd,
    cipherText,
    messageSize,
    plainText,
    messageSize);

err = gcry_cipher_decrypt(
    hd,
    recoveredText,
    messageSize,
    cipherText,
    messageSize);

How do I perform simple encryption/decryption using AES128 and CCM in Libgcrypt?

Answer 1

Example

The example is without additional non-encrypted data, therefore param[1] of gcry_cipher_ctl setting the GCRYCTL_SET_CCM_LENGTHS is 0. But it could be easily adjusted if required.

The functionality is divided into three parts:

• prepare (set the key, the nonce (IV), and the CCM lengths)
• encrypt function
• decrypt function

If you run it, it will output the following text to the console:

decrypted result: 'VNiJkPzAWPFm1234'

Code

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gcrypt.h>

void error(const char *what, gcry_error_t err) {
    fprintf(stderr, "%s failed: %s\n", what, gcry_strerror(err));
    exit(1);
}

typedef struct {
    const void *key;
    const void *nonce;
    size_t messageSize;
    int authTagLength;
} Config;

void prepare(Config config, gcry_cipher_hd_t *hd) {
    gcry_error_t err = gcry_cipher_open(hd, GCRY_CIPHER_AES128, GCRY_CIPHER_MODE_CCM, 0);
    if (err) { error("gcry_cipher_open", err); }

    err = gcry_cipher_setkey(*hd, config.key, strlen(config.key));
    if (err) { error("gcry_cipher_setkey", err); }

    err = gcry_cipher_setiv(*hd, config.nonce, strlen(config.nonce));
    if (err) { error("gcry_cipher_setiv", err); }

    uint64_t params[3];
    params[0] = config.messageSize;
    params[1] = 0;
    params[2] = config.authTagLength;
    err = gcry_cipher_ctl(*hd, GCRYCTL_SET_CCM_LENGTHS, params, sizeof(params));
    if (err) { error("gcry_cipher_ctl", err); }
}

void *encrypt(Config config, const void *plainText, void **tag) {
    gcry_cipher_hd_t hdEncrypt;
    prepare(config, &hdEncrypt);

    void *cipherText = malloc(config.messageSize);
    gcry_error_t err = gcry_cipher_encrypt(hdEncrypt, cipherText, config.messageSize, plainText, config.messageSize);
    if (err) { error("gcry_cipher_encrypt", err); }

    *tag = malloc(config.authTagLength);
    err = gcry_cipher_gettag(hdEncrypt, *tag, config.authTagLength);
    if (err) { error("gcry_cipher_encrypt", err); }

    gcry_cipher_close(hdEncrypt);

    return cipherText;
}

void *decrypt(Config config, void *cipherText, void *tag) {
    gcry_cipher_hd_t hdDecrypt;
    prepare(config, &hdDecrypt);

    void *recoveredText = malloc(config.messageSize);
    gcry_error_t err = gcry_cipher_decrypt(hdDecrypt, recoveredText, config.messageSize, cipherText, config.messageSize);
    if (err) { error("gcry_cipher_decrypt", err); }

    err = gcry_cipher_checktag(hdDecrypt, tag, config.authTagLength);
    if (gpg_err_code (err) == GPG_ERR_CHECKSUM) {
        error("Authentication", err);
    }

    gcry_cipher_close(hdDecrypt);
    return recoveredText;
}

int main() {
    const char *plainText = "VNiJkPzAWPFm1234";

    Config config;
    config.key = "1234567890123456";
    config.nonce = "0123456789012";
    config.messageSize = strlen(plainText) + 1;
    config.authTagLength = 10;

    void *tag;
    void *cipherText = encrypt(config, plainText, &tag);

    char *recoveredText = decrypt(config, cipherText, tag);
    printf("decrypted result: '%s'\n", recoveredText);

    free(tag);
    free(cipherText);
    free(recoveredText);

    return 0;
}