diff mbox series

[3/5] crypto: add RFC5869 HKDF

Message ID 12679948.uLZWGnKmhe@positron.chronox.de
State New
Headers show
Series Add KDF implementations to crypto API | expand

Commit Message

Stephan Mueller Jan. 4, 2021, 9:49 p.m. UTC
RFC5869 specifies an extract and expand two-step key derivation
function. The HKDF implementation is provided as a service function that
operates on a caller-provided HMAC cipher handle. The caller has to
allocate the HMAC cipher and then can invoke the HKDF service functions.
The HKDF implementation ensures that the entire state is kept with the
HMAC cipher handle which implies that no additional state is required to
be maintained by the HKDF implementation.

The extract function is invoked via the crypto_hkdf_setkey call. RFC5869
allows two optional parameters to be provided to the extract operation:
the salt and additional information. Both are to be provided with the
seed parameter where the salt is the first entry of the seed parameter
and all subsequent entries are handled as additional information. If
the caller intends to invoke the HKDF without salt, it has to provide a
NULL/0 entry as first entry in seed.

The expand function is invoked via the crypto_hkdf_generate and can be
invoked multiple times. This function allows the caller to provide a
context for the key derivation operation. As specified in RFC5869, it is
optional. In case such context is not provided, the caller must provide
NULL / 0 for the info / info_nvec parameters.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
---
 crypto/Kconfig        |   7 ++
 crypto/Makefile       |   1 +
 crypto/hkdf.c         | 226 ++++++++++++++++++++++++++++++++++++++++++
 include/crypto/hkdf.h |  48 +++++++++
 4 files changed, 282 insertions(+)
 create mode 100644 crypto/hkdf.c
 create mode 100644 include/crypto/hkdf.h

Comments

Eric Biggers Jan. 7, 2021, 7:30 a.m. UTC | #1
On Mon, Jan 04, 2021 at 10:49:13PM +0100, Stephan Müller wrote:
> RFC5869 specifies an extract and expand two-step key derivation

> function. The HKDF implementation is provided as a service function that

> operates on a caller-provided HMAC cipher handle.


HMAC isn't a "cipher".

> The extract function is invoked via the crypto_hkdf_setkey call.


Any reason not to call this crypto_hkdf_extract(), to match the specification?

> RFC5869

> allows two optional parameters to be provided to the extract operation:

> the salt and additional information. Both are to be provided with the

> seed parameter where the salt is the first entry of the seed parameter

> and all subsequent entries are handled as additional information. If

> the caller intends to invoke the HKDF without salt, it has to provide a

> NULL/0 entry as first entry in seed.


Where does "additional information" for extract come from?  RFC 5869 has:

	HKDF-Extract(salt, IKM) -> PRK

	Inputs:
	      salt     optional salt value (a non-secret random value);
		       if not provided, it is set to a string of HashLen zeros.
	      IKM      input keying material

There's no "additional information".

> 

> The expand function is invoked via the crypto_hkdf_generate and can be

> invoked multiple times. This function allows the caller to provide a

> context for the key derivation operation. As specified in RFC5869, it is

> optional. In case such context is not provided, the caller must provide

> NULL / 0 for the info / info_nvec parameters.


Any reason not to call this crypto_hkdf_expand() to match the specification?

- Eric
Stephan Mueller Jan. 7, 2021, 7:53 a.m. UTC | #2
Am Mittwoch, dem 06.01.2021 um 23:30 -0800 schrieb Eric Biggers:
> On Mon, Jan 04, 2021 at 10:49:13PM +0100, Stephan Müller wrote:

> > RFC5869 specifies an extract and expand two-step key derivation

> > function. The HKDF implementation is provided as a service function that

> > operates on a caller-provided HMAC cipher handle.

> 

> HMAC isn't a "cipher".

> 

> > The extract function is invoked via the crypto_hkdf_setkey call.

> 

> Any reason not to call this crypto_hkdf_extract(), to match the

> specification?


I named it to match the other KDF implementation. But you are right, I will
name it accordingly.

> 

> > RFC5869

> > allows two optional parameters to be provided to the extract operation:

> > the salt and additional information. Both are to be provided with the

> > seed parameter where the salt is the first entry of the seed parameter

> > and all subsequent entries are handled as additional information. If

> > the caller intends to invoke the HKDF without salt, it has to provide a

> > NULL/0 entry as first entry in seed.

> 

> Where does "additional information" for extract come from?  RFC 5869 has:

> 

>         HKDF-Extract(salt, IKM) -> PRK

> 

>         Inputs:

>               salt     optional salt value (a non-secret random value);

>                        if not provided, it is set to a string of HashLen

> zeros.

>               IKM      input keying material

> 

> There's no "additional information".


I used the terminology from SP800-108. I will update the description
accordingly. 
> 

> > 

> > The expand function is invoked via the crypto_hkdf_generate and can be

> > invoked multiple times. This function allows the caller to provide a

> > context for the key derivation operation. As specified in RFC5869, it is

> > optional. In case such context is not provided, the caller must provide

> > NULL / 0 for the info / info_nvec parameters.

> 

> Any reason not to call this crypto_hkdf_expand() to match the specification?


I will update the function name.

Thanks
Stephan
> 

> - Eric
Eric Biggers Jan. 7, 2021, 6:53 p.m. UTC | #3
On Thu, Jan 07, 2021 at 08:53:15AM +0100, Stephan Mueller wrote:
> > 

> > > RFC5869

> > > allows two optional parameters to be provided to the extract operation:

> > > the salt and additional information. Both are to be provided with the

> > > seed parameter where the salt is the first entry of the seed parameter

> > > and all subsequent entries are handled as additional information. If

> > > the caller intends to invoke the HKDF without salt, it has to provide a

> > > NULL/0 entry as first entry in seed.

> > 

> > Where does "additional information" for extract come from?  RFC 5869 has:

> > 

> >         HKDF-Extract(salt, IKM) -> PRK

> > 

> >         Inputs:

> >               salt     optional salt value (a non-secret random value);

> >                        if not provided, it is set to a string of HashLen

> > zeros.

> >               IKM      input keying material

> > 

> > There's no "additional information".

> 

> I used the terminology from SP800-108. I will update the description

> accordingly. 


For HKDF, it would be better to stick to the terminology used in RFC 5869
(https://tools.ietf.org/html/rfc5869), as generally that's what people are most
familiar with for HKDF.  It also matches the HKDF paper
(https://eprint.iacr.org/2010/264.pdf) more closely.

- Eric
diff mbox series

Patch

diff --git a/crypto/Kconfig b/crypto/Kconfig
index 9f375c2350f5..661287d7283b 100644
--- a/crypto/Kconfig
+++ b/crypto/Kconfig
@@ -1862,6 +1862,13 @@  config CRYPTO_JITTERENTROPY
 	  random numbers. This Jitterentropy RNG registers with
 	  the kernel crypto API and can be used by any caller.
 
+config CRYPTO_HKDF
+	tristate "Extract and Expand HKDF (RFC 5869)"
+	select CRYPTO_HASH
+	help
+	  Enable the extract and expand key derivation function compliant
+	  to RFC 5869.
+
 config CRYPTO_KDF800108_CTR
 	tristate "Counter KDF (SP800-108)"
 	select CRYPTO_HASH
diff --git a/crypto/Makefile b/crypto/Makefile
index 46845a70850d..55a4d8c31a45 100644
--- a/crypto/Makefile
+++ b/crypto/Makefile
@@ -201,4 +201,5 @@  obj-$(CONFIG_CRYPTO_SIMD) += crypto_simd.o
 #
 # Key derivation function
 #
+obj-$(CONFIG_CRYPTO_HKDF) += hkdf.o
 obj-$(CONFIG_CRYPTO_KDF800108_CTR) += kdf_sp800108.o
diff --git a/crypto/hkdf.c b/crypto/hkdf.c
new file mode 100644
index 000000000000..a3bf6d6b07ea
--- /dev/null
+++ b/crypto/hkdf.c
@@ -0,0 +1,226 @@ 
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * HMAC-based Extract-and-Expand Key Derivation Function (conformant to RFC5869)
+ *
+ * Copyright (C) 2020, Stephan Mueller <smueller@chronox.de>
+ */
+
+#include <linux/module.h>
+#include <crypto/hkdf.h>
+#include <crypto/internal/kdf_selftest.h>
+
+/*
+ * HKDF expand phase
+ */
+int crypto_hkdf_generate(struct crypto_shash *kmd,
+			 const struct kvec *info, unsigned int info_nvec,
+			 u8 *dst, unsigned int dlen)
+{
+	SHASH_DESC_ON_STACK(desc, kmd);
+	const unsigned int h = crypto_shash_digestsize(kmd);
+	unsigned int i;
+	int err = 0;
+	u8 *dst_orig = dst;
+	const u8 *prev = NULL;
+	u8 ctr = 0x01;
+
+	if (dlen > h * 255)
+		return -EINVAL;
+
+	desc->tfm = kmd;
+
+	/* T(1) and following */
+	while (dlen) {
+		err = crypto_shash_init(desc);
+		if (err)
+			goto out;
+
+		if (prev) {
+			err = crypto_shash_update(desc, prev, h);
+			if (err)
+				goto out;
+		}
+
+		for (i = 0; i < info_nvec; i++) {
+			err = crypto_shash_update(desc, info[i].iov_base,
+						  info[i].iov_len);
+			if (err)
+				goto out;
+		}
+
+		if (dlen < h) {
+			u8 tmpbuffer[HASH_MAX_DIGESTSIZE];
+
+			err = crypto_shash_finup(desc, &ctr, 1, tmpbuffer);
+			if (err)
+				goto out;
+			memcpy(dst, tmpbuffer, dlen);
+			memzero_explicit(tmpbuffer, h);
+			goto out;
+		}
+
+		err = crypto_shash_finup(desc, &ctr, 1, dst);
+		if (err)
+			goto out;
+
+		prev = dst;
+		dst += h;
+		dlen -= h;
+		ctr++;
+	}
+
+out:
+	if (err)
+		memzero_explicit(dst_orig, dlen);
+	shash_desc_zero(desc);
+	return err;
+}
+EXPORT_SYMBOL(crypto_hkdf_generate);
+
+/*
+ * HKDF extract phase.
+ */
+int crypto_hkdf_setkey(struct crypto_shash *kmd,
+		       const struct kvec *seed, unsigned int seed_nvec)
+{
+	SHASH_DESC_ON_STACK(desc, kmd);
+	unsigned int h = crypto_shash_digestsize(kmd);
+	int err;
+	static const u8 null_salt[HASH_MAX_DIGESTSIZE] = { 0 };
+	u8 prk[HASH_MAX_DIGESTSIZE];
+
+	/* Require at least the salt information */
+	if (seed_nvec < 1)
+		return -EINVAL;
+
+	desc->tfm = kmd;
+
+	if (seed[0].iov_len) {
+		/* Set the salt as HMAC key */
+		err = crypto_shash_setkey(kmd,
+					  seed[0].iov_base, seed[0].iov_len);
+	} else {
+		/* Set the default HMAC key as none was provided */
+		err = crypto_shash_setkey(kmd, null_salt, h);
+	}
+
+	if (err)
+		return err;
+
+	/* Extract the PRK */
+	if (seed_nvec > 1) {
+		unsigned int i;
+
+		err = crypto_shash_init(desc);
+		if (err)
+			goto err;
+
+		for (i = 1; i < seed_nvec; i++) {
+			err = crypto_shash_update(desc, seed[i].iov_base,
+						  seed[i].iov_len);
+			if (err)
+				goto err;
+		}
+
+		err = crypto_shash_final(desc, prk);
+	} else {
+		err = crypto_shash_digest(desc, NULL, 0, prk);
+	}
+	if (err)
+		goto err;
+
+	/* Set the PRK for the expand phase */
+	err = crypto_shash_setkey(kmd, prk, h);
+
+err:
+	shash_desc_zero(desc);
+	memzero_explicit(prk, h);
+	return err;
+}
+EXPORT_SYMBOL(crypto_hkdf_setkey);
+
+/* Test vectors from RFC 5869 appendix A */
+static const struct kdf_testvec hkdf_hmac_sha256_tv_template[] = {
+	{
+		.seed = { {
+			/* salt */
+			.iov_base = "\x00\x01\x02\x03\x04\x05\x06\x07"
+				    "\x08\x09\x0a\x0b\x0c",
+			.iov_len  = 13,
+		}, {
+			/* IKM */
+			.iov_base = "\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b"
+				    "\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b"
+				    "\x0b\x0b\x0b\x0b\x0b\x0b",
+			.iov_len  = 22,
+		} },
+		.seed_nvec	  = 2,
+		.info = {
+			.iov_base = "\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7"
+				    "\xf8\xf9",
+			.iov_len  = 10
+		},
+		.expected	  = "\x3c\xb2\x5f\x25\xfa\xac\xd5\x7a"
+				    "\x90\x43\x4f\x64\xd0\x36\x2f\x2a"
+				    "\x2d\x2d\x0a\x90\xcf\x1a\x5a\x4c"
+				    "\x5d\xb0\x2d\x56\xec\xc4\xc5\xbf"
+				    "\x34\x00\x72\x08\xd5\xb8\x87\x18"
+				    "\x58\x65",
+		.expectedlen	  = 42
+	}, {
+		.seed = { {
+			/* salt */
+			.iov_base = NULL,
+			.iov_len  = 0,
+		}, {
+			/* IKM */
+			.iov_base = "\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b"
+				    "\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b"
+				    "\x0b\x0b\x0b\x0b\x0b\x0b",
+			.iov_len  = 22,
+		} },
+		.seed_nvec	  = 2,
+		.info = {
+			.iov_base = NULL,
+			.iov_len  = 0
+		},
+		.expected	  = "\x8d\xa4\xe7\x75\xa5\x63\xc1\x8f"
+				    "\x71\x5f\x80\x2a\x06\x3c\x5a\x31"
+				    "\xb8\xa1\x1f\x5c\x5e\xe1\x87\x9e"
+				    "\xc3\x45\x4e\x5f\x3c\x73\x8d\x2d"
+				    "\x9d\x20\x13\x95\xfa\xa4\xb6\x1a"
+				    "\x96\xc8",
+		.expectedlen	  = 42
+	}
+};
+
+static int __init crypto_hkdf_init(void)
+{
+	unsigned int i;
+	int ret;
+
+	for (i = 0; i < ARRAY_SIZE(hkdf_hmac_sha256_tv_template); i++) {
+		ret = kdf_test(&hkdf_hmac_sha256_tv_template[i], "hmac(sha256)",
+			       crypto_hkdf_setkey, crypto_hkdf_generate);
+
+		if (ret) {
+			pr_warn("alg: self-tests for HKDF (hmac(sha256)) failed (rc=%d)\n",
+				ret);
+			return ret;
+		}
+	}
+
+	pr_info("alg: self-tests for HKDF (hmac(sha256)) passed\n");
+
+	return 0;
+}
+
+static void __exit crypto_hkdf_exit(void) { }
+
+module_init(crypto_hkdf_init);
+module_exit(crypto_hkdf_exit);
+
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Stephan Mueller <smueller@chronox.de>");
+MODULE_DESCRIPTION("HKDF HMAC-based Extract-and-Expand Key Derivation Function  (conformant to RFC5869)");
diff --git a/include/crypto/hkdf.h b/include/crypto/hkdf.h
new file mode 100644
index 000000000000..4e4b5444a7a6
--- /dev/null
+++ b/include/crypto/hkdf.h
@@ -0,0 +1,48 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+
+/*
+ * Copyright (C) 2020, Stephan Mueller <smueller@chronox.de>
+ */
+
+#ifndef _CRYPTO_HKDF_H
+#define _CRYPTO_HKDF_H
+
+#include <crypto/hash.h>
+#include <linux/uio.h>
+
+/**
+ * RFC 5869 HKDF expand operation
+ *
+ * @kmd Keyed message digest whose key was set with crypto_hkdf_setkey
+ * @info optional context and application specific information - this may be
+ *	 NULL
+ * @info_vec number of optional context/application specific information entries
+ * @dst destination buffer that the caller already allocated
+ * @dlen length of the destination buffer - the KDF derives that amount of
+ *	 bytes.
+ *
+ * @return 0 on success, < 0 on error
+ */
+int crypto_hkdf_generate(struct crypto_shash *kmd,
+			 const struct kvec *info, unsigned int info_nvec,
+			 u8 *dst, unsigned int dlen);
+
+/**
+ * RFC 5869 HKDF extract operation
+ *
+ * @kmd Keyed message digest allocated by the caller. The key should not have
+ *	been set.
+ * @seed Seed data to extract the HKDF state from - for HKDF, the seed is
+ *	 partitioned as follows:
+ *	 seed[0] is the salt - this kvec MUST be filled in. It is permissible
+ *	 to provide a NULL seed where .iov_base = NULL and .iov_len = 0.
+ *	 seed[>0] is the additional information data. It is permissible to
+ *	 provide zero or more kvec entries.
+ * @seed_nvec number of seed entries (must be at least 1)
+ *
+ * @return 0 on success, < 0 on error
+ */
+int crypto_hkdf_setkey(struct crypto_shash *kmd,
+		       const struct kvec *seed, unsigned int seed_nvec);
+
+#endif /* _CRYPTO_HKDF_H */