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![Eric Biggers](/assets/img/avatar_default.png)
Rewrite the skcipher API example, changing it to encrypt a buffer with AES-256-XTS. This addresses various problems with the previous example: - It requests a specific driver "cbc-aes-aesni", which is unusual. Normally users ask for "cbc(aes)", not a specific driver. - It encrypts only a single AES block. For the reader, that doesn't clearly distinguish the "skcipher" API from the "cipher" API. - Showing how to encrypt something with bare CBC is arguably a poor choice of example, as it doesn't follow modern crypto trends. Now, usually authenticated encryption is recommended, in which case the user would use the AEAD API, not skcipher. Disk encryption is still a legitimate use for skcipher, but for that usually XTS is recommended. - Many other bugs and poor coding practices, such as not setting CRYPTO_TFM_REQ_MAY_SLEEP, unnecessarily allocating a heap buffer for the IV, unnecessary NULL checks, using a pointless wrapper struct, and forgetting to set an error code in one case. Signed-off-by: Eric Biggers <ebiggers@google.com> Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
187 lines
5.7 KiB
ReStructuredText
187 lines
5.7 KiB
ReStructuredText
Code Examples
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=============
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Code Example For Symmetric Key Cipher Operation
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-----------------------------------------------
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This code encrypts some data with AES-256-XTS. For sake of example,
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all inputs are random bytes, the encryption is done in-place, and it's
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assumed the code is running in a context where it can sleep.
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::
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static int test_skcipher(void)
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{
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struct crypto_skcipher *tfm = NULL;
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struct skcipher_request *req = NULL;
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u8 *data = NULL;
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const size_t datasize = 512; /* data size in bytes */
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struct scatterlist sg;
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DECLARE_CRYPTO_WAIT(wait);
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u8 iv[16]; /* AES-256-XTS takes a 16-byte IV */
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u8 key[64]; /* AES-256-XTS takes a 64-byte key */
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int err;
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/*
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* Allocate a tfm (a transformation object) and set the key.
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*
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* In real-world use, a tfm and key are typically used for many
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* encryption/decryption operations. But in this example, we'll just do a
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* single encryption operation with it (which is not very efficient).
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*/
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tfm = crypto_alloc_skcipher("xts(aes)", 0, 0);
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if (IS_ERR(tfm)) {
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pr_err("Error allocating xts(aes) handle: %ld\n", PTR_ERR(tfm));
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return PTR_ERR(tfm);
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}
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get_random_bytes(key, sizeof(key));
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err = crypto_skcipher_setkey(tfm, key, sizeof(key));
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if (err) {
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pr_err("Error setting key: %d\n", err);
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goto out;
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}
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/* Allocate a request object */
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req = skcipher_request_alloc(tfm, GFP_KERNEL);
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if (!req) {
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err = -ENOMEM;
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goto out;
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}
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/* Prepare the input data */
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data = kmalloc(datasize, GFP_KERNEL);
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if (!data) {
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err = -ENOMEM;
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goto out;
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}
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get_random_bytes(data, datasize);
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/* Initialize the IV */
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get_random_bytes(iv, sizeof(iv));
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/*
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* Encrypt the data in-place.
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*
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* For simplicity, in this example we wait for the request to complete
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* before proceeding, even if the underlying implementation is asynchronous.
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*
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* To decrypt instead of encrypt, just change crypto_skcipher_encrypt() to
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* crypto_skcipher_decrypt().
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*/
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sg_init_one(&sg, data, datasize);
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skcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG |
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CRYPTO_TFM_REQ_MAY_SLEEP,
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crypto_req_done, &wait);
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skcipher_request_set_crypt(req, &sg, &sg, datasize, iv);
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err = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
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if (err) {
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pr_err("Error encrypting data: %d\n", err);
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goto out;
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}
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pr_debug("Encryption was successful\n");
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out:
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crypto_free_skcipher(tfm);
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skcipher_request_free(req);
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kfree(data);
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return err;
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}
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Code Example For Use of Operational State Memory With SHASH
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-----------------------------------------------------------
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::
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struct sdesc {
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struct shash_desc shash;
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char ctx[];
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};
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static struct sdesc *init_sdesc(struct crypto_shash *alg)
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{
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struct sdesc *sdesc;
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int size;
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size = sizeof(struct shash_desc) + crypto_shash_descsize(alg);
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sdesc = kmalloc(size, GFP_KERNEL);
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if (!sdesc)
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return ERR_PTR(-ENOMEM);
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sdesc->shash.tfm = alg;
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return sdesc;
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}
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static int calc_hash(struct crypto_shash *alg,
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const unsigned char *data, unsigned int datalen,
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unsigned char *digest)
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{
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struct sdesc *sdesc;
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int ret;
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sdesc = init_sdesc(alg);
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if (IS_ERR(sdesc)) {
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pr_info("can't alloc sdesc\n");
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return PTR_ERR(sdesc);
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}
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ret = crypto_shash_digest(&sdesc->shash, data, datalen, digest);
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kfree(sdesc);
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return ret;
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}
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static int test_hash(const unsigned char *data, unsigned int datalen,
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unsigned char *digest)
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{
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struct crypto_shash *alg;
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char *hash_alg_name = "sha1-padlock-nano";
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int ret;
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alg = crypto_alloc_shash(hash_alg_name, 0, 0);
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if (IS_ERR(alg)) {
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pr_info("can't alloc alg %s\n", hash_alg_name);
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return PTR_ERR(alg);
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}
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ret = calc_hash(alg, data, datalen, digest);
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crypto_free_shash(alg);
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return ret;
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}
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Code Example For Random Number Generator Usage
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----------------------------------------------
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::
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static int get_random_numbers(u8 *buf, unsigned int len)
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{
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struct crypto_rng *rng = NULL;
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char *drbg = "drbg_nopr_sha256"; /* Hash DRBG with SHA-256, no PR */
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int ret;
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if (!buf || !len) {
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pr_debug("No output buffer provided\n");
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return -EINVAL;
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}
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rng = crypto_alloc_rng(drbg, 0, 0);
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if (IS_ERR(rng)) {
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pr_debug("could not allocate RNG handle for %s\n", drbg);
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return PTR_ERR(rng);
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}
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ret = crypto_rng_get_bytes(rng, buf, len);
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if (ret < 0)
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pr_debug("generation of random numbers failed\n");
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else if (ret == 0)
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pr_debug("RNG returned no data");
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else
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pr_debug("RNG returned %d bytes of data\n", ret);
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out:
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crypto_free_rng(rng);
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return ret;
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}
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