aescfb.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Minimal library implementation of AES in CFB mode
 *
 * Copyright 2023 Google LLC
 */

#include <crypto/aes.h>
#include <crypto/algapi.h>
#include <linux/export.h>
#include <linux/module.h>
#include <asm/irqflags.h>

static void aescfb_encrypt_block(const struct crypto_aes_ctx *ctx, void *dst,
				 const void *src)
{
	unsigned long flags;

	/*
	 * In AES-CFB, the AES encryption operates on known 'plaintext' (the IV
	 * and ciphertext), making it susceptible to timing attacks on the
	 * encryption key. The AES library already mitigates this risk to some
	 * extent by pulling the entire S-box into the caches before doing any
	 * substitutions, but this strategy is more effective when running with
	 * interrupts disabled.
	 */
	local_irq_save(flags);
	aes_encrypt(ctx, dst, src);
	local_irq_restore(flags);
}

/**
 * aescfb_encrypt - Perform AES-CFB encryption on a block of data
 *
 * @ctx:	The AES-CFB key schedule
 * @dst:	Pointer to the ciphertext output buffer
 * @src:	Pointer the plaintext (may equal @dst for encryption in place)
 * @len:	The size in bytes of the plaintext and ciphertext.
 * @iv:		The initialization vector (IV) to use for this block of data
 */
void aescfb_encrypt(const struct crypto_aes_ctx *ctx, u8 *dst, const u8 *src,
		    int len, const u8 iv[AES_BLOCK_SIZE])
{
	u8 ks[AES_BLOCK_SIZE];
	const u8 *v = iv;

	while (len > 0) {
		aescfb_encrypt_block(ctx, ks, v);
		crypto_xor_cpy(dst, src, ks, min(len, AES_BLOCK_SIZE));
		v = dst;

		dst += AES_BLOCK_SIZE;
		src += AES_BLOCK_SIZE;
		len -= AES_BLOCK_SIZE;
	}

	memzero_explicit(ks, sizeof(ks));
}
EXPORT_SYMBOL(aescfb_encrypt);

/**
 * aescfb_decrypt - Perform AES-CFB decryption on a block of data
 *
 * @ctx:	The AES-CFB key schedule
 * @dst:	Pointer to the plaintext output buffer
 * @src:	Pointer the ciphertext (may equal @dst for decryption in place)
 * @len:	The size in bytes of the plaintext and ciphertext.
 * @iv:		The initialization vector (IV) to use for this block of data
 */
void aescfb_decrypt(const struct crypto_aes_ctx *ctx, u8 *dst, const u8 *src,
		    int len, const u8 iv[AES_BLOCK_SIZE])
{
	u8 ks[2][AES_BLOCK_SIZE];

	aescfb_encrypt_block(ctx, ks[0], iv);

	for (int i = 0; len > 0; i ^= 1) {
		if (len > AES_BLOCK_SIZE)
			/*
			 * Generate the keystream for the next block before
			 * performing the XOR, as that may update in place and
			 * overwrite the ciphertext.
			 */
			aescfb_encrypt_block(ctx, ks[!i], src);

		crypto_xor_cpy(dst, src, ks[i], min(len, AES_BLOCK_SIZE));

		dst += AES_BLOCK_SIZE;
		src += AES_BLOCK_SIZE;
		len -= AES_BLOCK_SIZE;
	}

	memzero_explicit(ks, sizeof(ks));
}
EXPORT_SYMBOL(aescfb_decrypt);

MODULE_DESCRIPTION("Generic AES-CFB library");
MODULE_AUTHOR("Ard Biesheuvel <[email protected]>");
MODULE_LICENSE("GPL");

#ifdef CONFIG_CRYPTO_SELFTESTS

/*
 * Test code below. Vectors taken from crypto/testmgr.h
 */

static struct {
	u8	ptext[64] __nonstring;
	u8	ctext[64] __nonstring;

	u8	key[AES_MAX_KEY_SIZE] __nonstring;
	u8	iv[AES_BLOCK_SIZE] __nonstring;

	int	klen;
	int	len;
} const aescfb_tv[] __initconst = {
	{ /* From NIST SP800-38A */
		.key    = "\x2b\x7e\x15\x16\x28\xae\xd2\xa6"
			  "\xab\xf7\x15\x88\x09\xcf\x4f\x3c",
		.klen	= 16,
		.iv	= "\x00\x01\x02\x03\x04\x05\x06\x07"
			  "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
		.ptext	= "\x6b\xc1\xbe\xe2\x2e\x40\x9f\x96"
			  "\xe9\x3d\x7e\x11\x73\x93\x17\x2a"
			  "\xae\x2d\x8a\x57\x1e\x03\xac\x9c"
			  "\x9e\xb7\x6f\xac\x45\xaf\x8e\x51"
			  "\x30\xc8\x1c\x46\xa3\x5c\xe4\x11"
			  "\xe5\xfb\xc1\x19\x1a\x0a\x52\xef"
			  "\xf6\x9f\x24\x45\xdf\x4f\x9b\x17"
			  "\xad\x2b\x41\x7b\xe6\x6c\x37\x10",
		.ctext	= "\x3b\x3f\xd9\x2e\xb7\x2d\xad\x20"
			  "\x33\x34\x49\xf8\xe8\x3c\xfb\x4a"
			  "\xc8\xa6\x45\x37\xa0\xb3\xa9\x3f"
			  "\xcd\xe3\xcd\xad\x9f\x1c\xe5\x8b"
			  "\x26\x75\x1f\x67\xa3\xcb\xb1\x40"
			  "\xb1\x80\x8c\xf1\x87\xa4\xf4\xdf"
			  "\xc0\x4b\x05\x35\x7c\x5d\x1c\x0e"
			  "\xea\xc4\xc6\x6f\x9f\xf7\xf2\xe6",
		.len	= 64,
	}, {
		.key	= "\x8e\x73\xb0\xf7\xda\x0e\x64\x52"
			  "\xc8\x10\xf3\x2b\x80\x90\x79\xe5"
			  "\x62\xf8\xea\xd2\x52\x2c\x6b\x7b",
		.klen	= 24,
		.iv	= "\x00\x01\x02\x03\x04\x05\x06\x07"
			  "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
		.ptext	= "\x6b\xc1\xbe\xe2\x2e\x40\x9f\x96"
			  "\xe9\x3d\x7e\x11\x73\x93\x17\x2a"
			  "\xae\x2d\x8a\x57\x1e\x03\xac\x9c"
			  "\x9e\xb7\x6f\xac\x45\xaf\x8e\x51"
			  "\x30\xc8\x1c\x46\xa3\x5c\xe4\x11"
			  "\xe5\xfb\xc1\x19\x1a\x0a\x52\xef"
			  "\xf6\x9f\x24\x45\xdf\x4f\x9b\x17"
			  "\xad\x2b\x41\x7b\xe6\x6c\x37\x10",
		.ctext	= "\xcd\xc8\x0d\x6f\xdd\xf1\x8c\xab"
			  "\x34\xc2\x59\x09\xc9\x9a\x41\x74"
			  "\x67\xce\x7f\x7f\x81\x17\x36\x21"
			  "\x96\x1a\x2b\x70\x17\x1d\x3d\x7a"
			  "\x2e\x1e\x8a\x1d\xd5\x9b\x88\xb1"
			  "\xc8\xe6\x0f\xed\x1e\xfa\xc4\xc9"
			  "\xc0\x5f\x9f\x9c\xa9\x83\x4f\xa0"
			  "\x42\xae\x8f\xba\x58\x4b\x09\xff",
		.len	= 64,
	}, {
		.key	= "\x60\x3d\xeb\x10\x15\xca\x71\xbe"
			  "\x2b\x73\xae\xf0\x85\x7d\x77\x81"
			  "\x1f\x35\x2c\x07\x3b\x61\x08\xd7"
			  "\x2d\x98\x10\xa3\x09\x14\xdf\xf4",
		.klen	= 32,
		.iv	= "\x00\x01\x02\x03\x04\x05\x06\x07"
			  "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
		.ptext	= "\x6b\xc1\xbe\xe2\x2e\x40\x9f\x96"
			  "\xe9\x3d\x7e\x11\x73\x93\x17\x2a"
			  "\xae\x2d\x8a\x57\x1e\x03\xac\x9c"
			  "\x9e\xb7\x6f\xac\x45\xaf\x8e\x51"
			  "\x30\xc8\x1c\x46\xa3\x5c\xe4\x11"
			  "\xe5\xfb\xc1\x19\x1a\x0a\x52\xef"
			  "\xf6\x9f\x24\x45\xdf\x4f\x9b\x17"
			  "\xad\x2b\x41\x7b\xe6\x6c\x37\x10",
		.ctext	= "\xdc\x7e\x84\xbf\xda\x79\x16\x4b"
			  "\x7e\xcd\x84\x86\x98\x5d\x38\x60"
			  "\x39\xff\xed\x14\x3b\x28\xb1\xc8"
			  "\x32\x11\x3c\x63\x31\xe5\x40\x7b"
			  "\xdf\x10\x13\x24\x15\xe5\x4b\x92"
			  "\xa1\x3e\xd0\xa8\x26\x7a\xe2\xf9"
			  "\x75\xa3\x85\x74\x1a\xb9\xce\xf8"
			  "\x20\x31\x62\x3d\x55\xb1\xe4\x71",
		.len	= 64,
	}, { /* > 16 bytes, not a multiple of 16 bytes */
		.key	= "\x2b\x7e\x15\x16\x28\xae\xd2\xa6"
			  "\xab\xf7\x15\x88\x09\xcf\x4f\x3c",
		.klen	= 16,
		.iv	= "\x00\x01\x02\x03\x04\x05\x06\x07"
			  "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
		.ptext	= "\x6b\xc1\xbe\xe2\x2e\x40\x9f\x96"
			  "\xe9\x3d\x7e\x11\x73\x93\x17\x2a"
			  "\xae",
		.ctext	= "\x3b\x3f\xd9\x2e\xb7\x2d\xad\x20"
			  "\x33\x34\x49\xf8\xe8\x3c\xfb\x4a"
			  "\xc8",
		.len	= 17,
	}, { /* < 16 bytes */
		.key	= "\x2b\x7e\x15\x16\x28\xae\xd2\xa6"
			  "\xab\xf7\x15\x88\x09\xcf\x4f\x3c",
		.klen	= 16,
		.iv	= "\x00\x01\x02\x03\x04\x05\x06\x07"
			  "\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f",
		.ptext	= "\x6b\xc1\xbe\xe2\x2e\x40\x9f",
		.ctext	= "\x3b\x3f\xd9\x2e\xb7\x2d\xad",
		.len	= 7,
	},
};

static int __init libaescfb_init(void)
{
	for (int i = 0; i < ARRAY_SIZE(aescfb_tv); i++) {
		struct crypto_aes_ctx ctx;
		u8 buf[64];

		if (aes_expandkey(&ctx, aescfb_tv[i].key, aescfb_tv[i].klen)) {
			pr_err("aes_expandkey() failed on vector %d\n", i);
			return -ENODEV;
		}

		aescfb_encrypt(&ctx, buf, aescfb_tv[i].ptext, aescfb_tv[i].len,
			       aescfb_tv[i].iv);
		if (memcmp(buf, aescfb_tv[i].ctext, aescfb_tv[i].len)) {
			pr_err("aescfb_encrypt() #1 failed on vector %d\n", i);
			return -ENODEV;
		}

		/* decrypt in place */
		aescfb_decrypt(&ctx, buf, buf, aescfb_tv[i].len, aescfb_tv[i].iv);
		if (memcmp(buf, aescfb_tv[i].ptext, aescfb_tv[i].len)) {
			pr_err("aescfb_decrypt() failed on vector %d\n", i);
			return -ENODEV;
		}

		/* encrypt in place */
		aescfb_encrypt(&ctx, buf, buf, aescfb_tv[i].len, aescfb_tv[i].iv);
		if (memcmp(buf, aescfb_tv[i].ctext, aescfb_tv[i].len)) {
			pr_err("aescfb_encrypt() #2 failed on vector %d\n", i);

			return -ENODEV;
		}

	}
	return 0;
}
module_init(libaescfb_init);

static void __exit libaescfb_exit(void)
{
}
module_exit(libaescfb_exit);
#endif