openssl: Add a prf+ implementation based on OpenSSL's HKDF implementation

The HKDF-Expand() function defined in RFC 5869 is basically the same as
IKEv2's prf+(), so we can use the former to implement the latter.
However, we can only support HMAC-based PRFs this way, which should be
fine as others are rarely used.

src/libstrongswan/plugins/openssl/Makefile.am

@@ -33,6 +33,7 @@ libstrongswan_openssl_la_SOURCES = \
 	openssl_pkcs12.c openssl_pkcs12.h \
 	openssl_rng.c openssl_rng.h \
 	openssl_hmac.c openssl_hmac.h \
+	openssl_kdf.c openssl_kdf.h \
 	openssl_aead.c openssl_aead.h \
 	openssl_x_diffie_hellman.c openssl_x_diffie_hellman.h \
 	openssl_ed_private_key.c openssl_ed_private_key.h \

src/libstrongswan/plugins/openssl/openssl_kdf.c

@@ -0,0 +1,175 @@
+/*
+ * Copyright (C) 2022 Tobias Brunner, codelabs GmbH
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include <openssl/opensslv.h>
+#include <openssl/opensslconf.h>
+
+#if !defined(OPENSSL_NO_HMAC) && OPENSSL_VERSION_NUMBER >= 0x10101000L
+
+#include <openssl/evp.h>
+#include <openssl/kdf.h>
+
+#include "openssl_kdf.h"
+
+typedef struct private_kdf_t private_kdf_t;
+
+/**
+ * Private data.
+ */
+struct private_kdf_t {
+
+	/**
+	 * Public interface.
+	 */
+	kdf_t public;
+
+	/**
+	 * Hasher to use for underlying PRF.
+	 */
+	const EVP_MD *hasher;
+
+	/**
+	 * Key for KDF. Stored here because OpenSSL's HKDF API does not provide a
+	 * way to clear the "info" field in the context, new data is always
+	 * appended (up to 1024 bytes).
+	 */
+	chunk_t key;
+
+	/**
+	 * Salt for prf+ (see above).
+	 */
+	chunk_t salt;
+};
+
+METHOD(kdf_t, get_type, key_derivation_function_t,
+	private_kdf_t *this)
+{
+	return KDF_PRF_PLUS;
+}
+
+METHOD(kdf_t, get_bytes, bool,
+	private_kdf_t *this, size_t out_len, uint8_t *buffer)
+{
+	EVP_PKEY_CTX *ctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
+
+	if (!ctx ||
+		EVP_PKEY_derive_init(ctx) <= 0 ||
+		EVP_PKEY_CTX_set_hkdf_md(ctx, this->hasher) <= 0 ||
+		EVP_PKEY_CTX_hkdf_mode(ctx, EVP_PKEY_HKDEF_MODE_EXPAND_ONLY) <= 0 ||
+		EVP_PKEY_CTX_set1_hkdf_key(ctx, this->key.ptr, this->key.len) <= 0 ||
+		EVP_PKEY_CTX_add1_hkdf_info(ctx, this->salt.ptr, this->salt.len) <= 0 ||
+		EVP_PKEY_derive(ctx, buffer, &out_len) <= 0)
+	{
+		EVP_PKEY_CTX_free(ctx);
+		return FALSE;
+	}
+	EVP_PKEY_CTX_free(ctx);
+	return TRUE;
+}
+
+METHOD(kdf_t, allocate_bytes, bool,
+	private_kdf_t *this, size_t out_len, chunk_t *chunk)
+{
+	*chunk = chunk_alloc(out_len);
+
+	if (!get_bytes(this, out_len, chunk->ptr))
+	{
+		chunk_free(chunk);
+		return FALSE;
+	}
+	return TRUE;
+}
+
+METHOD(kdf_t, set_param, bool,
+	private_kdf_t *this, kdf_param_t param, ...)
+{
+	chunk_t chunk;
+
+	switch (param)
+	{
+		case KDF_PARAM_KEY:
+			VA_ARGS_GET(param, chunk);
+			chunk_clear(&this->key);
+			this->key = chunk_clone(chunk);
+			break;
+		case KDF_PARAM_SALT:
+			VA_ARGS_GET(param, chunk);
+			chunk_clear(&this->salt);
+			this->salt = chunk_clone(chunk);
+			break;
+	}
+	return TRUE;
+}
+
+METHOD(kdf_t, destroy, void,
+	private_kdf_t *this)
+{
+	chunk_clear(&this->salt);
+	chunk_clear(&this->key);
+	free(this);
+}
+
+/*
+ * Described in header
+ */
+kdf_t *openssl_kdf_create(key_derivation_function_t algo, va_list args)
+{
+	private_kdf_t *this;
+	pseudo_random_function_t prf_alg;
+	char *name, buf[8];
+
+	if (algo != KDF_PRF_PLUS)
+	{
+		return NULL;
+	}
+
+	VA_ARGS_VGET(args, prf_alg);
+	name = enum_to_name(hash_algorithm_short_names,
+						hasher_algorithm_from_prf(prf_alg));
+	if (!name)
+	{
+		return NULL;
+	}
+
+	INIT(this,
+		.public = {
+			.get_type = _get_type,
+			.get_bytes = _get_bytes,
+			.allocate_bytes = _allocate_bytes,
+			.set_param = _set_param,
+			.destroy = _destroy,
+		},
+		.hasher = EVP_get_digestbyname(name),
+		/* use a lengthy key to test the implementation below to make sure the
+		 * algorithms are usable, see openssl_hmac.c for details */
+		.key = chunk_clone(chunk_from_str("00000000000000000000000000000000")),
+	);
+
+	if (!this->hasher || !get_bytes(this, sizeof(buf), buf))
+	{
+		destroy(this);
+		return NULL;
+	}
+	return &this->public;
+}
+
+#endif /* OPENSSL_NO_HMAC && OPENSSL_VERSION_NUMBER */

src/libstrongswan/plugins/openssl/openssl_kdf.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2022 Tobias Brunner, codelabs GmbH
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+/**
+ * Implements key derivation functions (KDF) via OpenSSL, in particular prf+,
+ * which is implemented via OpenSSL's HKDF implementation.
+ *
+ * @defgroup openssl_kdf openssl_kdf
+ * @{ @ingroup openssl_p
+ */
+
+#ifndef OPENSSL_KDF_H_
+#define OPENSSL_KDF_H_
+
+#include <crypto/kdfs/kdf.h>
+
+/**
+ * Creates a new kdf_t object.
+ *
+ * @param algo		algorithm to instantiate
+ * @param args		algorithm-specific arguments
+ * @return			kdf_t object, NULL if not supported
+ */
+kdf_t *openssl_kdf_create(key_derivation_function_t algo, va_list args);
+
+#endif /** OPENSSL_KDF_H_ @}*/

src/libstrongswan/plugins/openssl/openssl_plugin.c

@@ -52,6 +52,7 @@
 #include "openssl_pkcs12.h"
 #include "openssl_rng.h"
 #include "openssl_hmac.h"
+#include "openssl_kdf.h"
 #include "openssl_aead.h"
 #include "openssl_x_diffie_hellman.h"
 #include "openssl_ed_public_key.h"
@@ -654,6 +655,11 @@ METHOD(plugin_t, get_features, int,
 			PLUGIN_PROVIDE(SIGNER, AUTH_HMAC_SHA2_512_256),
 			PLUGIN_PROVIDE(SIGNER, AUTH_HMAC_SHA2_512_512),
 #endif
+#if OPENSSL_VERSION_NUMBER >= 0x10101000L
+		/* HKDF is available since 1.1.0, expand-only mode only since 1.1.1 */
+		PLUGIN_REGISTER(KDF, openssl_kdf_create),
+			PLUGIN_PROVIDE(KDF, KDF_PRF_PLUS),
+#endif
 #endif /* OPENSSL_NO_HMAC */
 #if (OPENSSL_VERSION_NUMBER >= 0x1000100fL && !defined(OPENSSL_NO_AES)) || \
 	(OPENSSL_VERSION_NUMBER >= 0x1010000fL && !defined(OPENSSL_NO_CHACHA))