how to use CryptoAPI in the linux kernel 2.6

Question

I have been looking for some time but have not found anywhere near sufficient documentation / examples on how to use the CryptoAPI that comes with linux in the creation of syscalls / in kernel land.

If anyone knows of a good source please let me know, I would like to know how to do SHA1 / MD5 and Blowfish / AES within the kernel space only.

Answer 1

There are a couple of places in the kernel which use the crypto module: the eCryptfs file system (linux/fs/ecryptfs/) and the 802.11 wireless stack (linux/drivers/staging/rtl8187se/ieee80211/). Both of these use AES, but you may be able to extrapolate what you find there to MD5.

Answer 2

Another good example is from the 2.6.18 kernel source in security/seclvl.c

Note: You can change CRYPTO_TFM_REQ_MAY_SLEEP if needed

static int
plaintext_to_sha1(unsigned char *hash, const char *plaintext, unsigned int len)
{
  struct crypto_tfm *tfm;
  struct scatterlist sg;
  if (len > PAGE_SIZE) {
    seclvl_printk(0, KERN_ERR, "Plaintext password too large (%d "
            "characters).  Largest possible is %lu "
            "bytes.\n", len, PAGE_SIZE);
    return -EINVAL;
  }
  tfm = crypto_alloc_tfm("sha1", CRYPTO_TFM_REQ_MAY_SLEEP);
  if (tfm == NULL) {
    seclvl_printk(0, KERN_ERR,
            "Failed to load transform for SHA1\n");
    return -EINVAL;
  }
  sg_init_one(&sg, (u8 *)plaintext, len);
  crypto_digest_init(tfm);
  crypto_digest_update(tfm, &sg, 1);
  crypto_digest_final(tfm, hash);
  crypto_free_tfm(tfm);
  return 0;
}

Answer 3

The best place to start is Documentation/crytpo in the kernel sources. dm-crypt is one of the many components that probably uses the kernel crypto API and you can refer to it to get an idea about usage.

Answer 4

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/crypto.h>
#include <linux/err.h>
#include <linux/scatterlist.h>

#define SHA1_LENGTH     20

static int __init sha1_init(void)
{
    struct scatterlist sg;
    struct crypto_hash *tfm;
    struct hash_desc desc;
    unsigned char output[SHA1_LENGTH];
    unsigned char buf[10];
    int i;

    printk(KERN_INFO "sha1: %s\n", __FUNCTION__);

    memset(buf, 'A', 10);
    memset(output, 0x00, SHA1_LENGTH);

    tfm = crypto_alloc_hash("sha1", 0, CRYPTO_ALG_ASYNC);

    desc.tfm = tfm;
    desc.flags = 0;

    sg_init_one(&sg, buf, 10);
    crypto_hash_init(&desc);

    crypto_hash_update(&desc, &sg, 10);
    crypto_hash_final(&desc, output);

    for (i = 0; i < 20; i++) {
        printk(KERN_ERR "%d-%d\n", output[i], i);
    }

    crypto_free_hash(tfm);

    return 0;
}

static void __exit sha1_exit(void)
{
    printk(KERN_INFO "sha1: %s\n", __FUNCTION__);
}

module_init(sha1_init);
module_exit(sha1_exit);

MODULE_LICENSE("Dual MIT/GPL");
MODULE_AUTHOR("Me");

Answer 5

One critical note:

Never compare the return value of crypto_alloc_hash function to NULL for detecting the failure.

Steps:

Always use IS_ERR function for this purpose. Comparing to NULL does not capture the error, hence you get segmentation faults later on.

If IS_ERR returns fail, you possibly have a missing crypto algorithm compiled into your kernel image (or as a module). Make sure you have selected the appropriate crypto algo. form make menuconfig.

Answer 6

Cryptodev-linux

https://github.com/cryptodev-linux/cryptodev-linux

It is a kernel module that exposes the kernel crypto API to userspace through /dev/crypto .

SHA calculation example: https://github.com/cryptodev-linux/cryptodev-linux/blob/da730106c2558c8e0c8e1b1b1812d32ef9574ab7/examples/sha.c

As others have mentioned, the kernel does not seem to expose the crypto API to userspace itself, which is a shame since the kernel can already use native hardware accelerated crypto functions internally.

Crypto operations cryptodev supports: https://github.com/nmav/cryptodev-linux/blob/383922cabeea7dca354415e8c590f8e932f4d7a8/crypto/cryptodev.h

Crypto operations Linux x86 supports: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/x86/crypto?id=refs/tags/v4.0