CBMC: Add proof for crypto_kem_keypair

Straightforward extension of crypto_kem_keypair_derand(),
axiomatizing `randombytes()`.

Signed-off-by: Hanno Becker <[email protected]>

cbmc/proofs/crypto_kem_enc_derand/Makefile

@@ -19,7 +19,7 @@ PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
 PROJECT_SOURCES += $(SRCDIR)/mlkem/kem.c
 
 CHECK_FUNCTION_CONTRACTS=$(MLKEM_NAMESPACE)enc_derand
-USE_FUNCTION_CONTRACTS=$(FIPS202_NAMESPACE)sha3_256 $(FIPS202_NAMESPACE)sha3_512 $(MLKEM_NAMESPACE)indcpa_enc $(MLKEM_NAMESPACE)polyvec_frombytes $(MLKEM_NAMESPACE)polyvec_reduce $(MLKEM_NAMESPACE)polyvec_tobytes pk_cmp_polyvec
+USE_FUNCTION_CONTRACTS=$(FIPS202_NAMESPACE)sha3_256 $(FIPS202_NAMESPACE)sha3_512 $(MLKEM_NAMESPACE)indcpa_enc $(MLKEM_NAMESPACE)polyvec_frombytes $(MLKEM_NAMESPACE)polyvec_reduce $(MLKEM_NAMESPACE)polyvec_tobytes memcmp
 APPLY_LOOP_CONTRACTS=on
 USE_DYNAMIC_FRAMES=1
 

cbmc/proofs/crypto_kem_keypair/Makefile

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+
+include ../Makefile_params.common
+
+HARNESS_ENTRY = harness
+HARNESS_FILE = crypto_kem_keypair_harness
+
+# This should be a unique identifier for this proof, and will appear on the
+# Litani dashboard. It can be human-readable and contain spaces if you wish.
+PROOF_UID = crypto_kem_keypair
+
+DEFINES +=
+INCLUDES +=
+
+REMOVE_FUNCTION_BODY +=
+UNWINDSET +=
+
+PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
+PROJECT_SOURCES += $(SRCDIR)/mlkem/kem.c
+
+CHECK_FUNCTION_CONTRACTS=$(MLKEM_NAMESPACE)keypair
+USE_FUNCTION_CONTRACTS=$(MLKEM_NAMESPACE)keypair_derand randombytes
+APPLY_LOOP_CONTRACTS=on
+USE_DYNAMIC_FRAMES=1
+
+# Disable any setting of EXTERNAL_SAT_SOLVER, and choose SMT backend instead
+EXTERNAL_SAT_SOLVER=
+CBMCFLAGS=--smt2
+
+FUNCTION_NAME = $(MLKEM_NAMESPACE)keypair
+
+# If this proof is found to consume huge amounts of RAM, you can set the
+# EXPENSIVE variable. With new enough versions of the proof tools, this will
+# restrict the number of EXPENSIVE CBMC jobs running at once. See the
+# documentation in Makefile.common under the "Job Pools" heading for details.
+# EXPENSIVE = true
+
+# This function is large enough to need...
+CBMC_OBJECT_BITS = 10
+
+# If you require access to a file-local ("static") function or object to conduct
+# your proof, set the following (and do not include the original source file
+# ("mlkem/poly.c") in PROJECT_SOURCES).
+# REWRITTEN_SOURCES = $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i
+# include ../Makefile.common
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_SOURCE = $(SRCDIR)/mlkem/poly.c
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_FUNCTIONS = foo bar
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_OBJECTS = baz
+# Care is required with variables on the left-hand side: REWRITTEN_SOURCES must
+# be set before including Makefile.common, but any use of variables on the
+# left-hand side requires those variables to be defined. Hence, _SOURCE,
+# _FUNCTIONS, _OBJECTS is set after including Makefile.common.
+
+include ../Makefile.common

cbmc/proofs/crypto_kem_keypair/cbmc-proof.txt

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: Apache-2.0
+
+# This file marks this directory as containing a CBMC proof.

cbmc/proofs/crypto_kem_keypair/crypto_kem_keypair_harness.c

@@ -0,0 +1,27 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: MIT-0
+
+/*
+ * Insert copyright notice
+ */
+
+/**
+ * @file crypto_kem_keypair_harness.c
+ * @brief Implements the proof harness for crypto_kem_keypair function.
+ */
+
+/*
+ * Insert project header files that
+ *   - include the declaration of the function
+ *   - include the types needed to declare function arguments
+ */
+#include <kem.h>
+
+/**
+ * @brief Starting point for formal analysis
+ *
+ */
+void harness(void) {
+  uint8_t *a, *b;
+  crypto_kem_keypair(a, b);
+}

mlkem/kem.c

@@ -9,17 +9,13 @@
 #include "symmetric.h"
 #include "verify.h"
 
-// This helper function is only to prevent CBMC from inlining
-// `memcmp`, and is expected to be inlined by the compiler.
-static int pk_cmp_polyvec(
-    const uint8_t pk[MLKEM_PUBLICKEYBYTES],
-    const uint8_t pv[MLKEM_POLYVECBYTES])  // clang-format off
-  REQUIRES(IS_FRESH(pk, MLKEM_PUBLICKEYBYTES))
-  REQUIRES(IS_FRESH(pv, MLKEM_POLYVECBYTES))
+#if defined(CBMC)
+// Redeclaration with contract needed for CBMC only
+int memcmp(const void *str1, const void *str2, size_t n)  // clang-format off
+  REQUIRES(IS_FRESH(str1, n))
+  REQUIRES(IS_FRESH(str2, n));
 // clang-format on
-{
-  return memcmp(pk, pv, MLKEM_POLYVECBYTES);
-}
+#endif
 
 /*************************************************
  * Name:        check_pk
@@ -40,7 +36,7 @@ static int check_pk(const uint8_t pk[MLKEM_PUBLICKEYBYTES]) {
   polyvec_reduce(&p);
   polyvec_tobytes(p_reencoded, &p);
   // Data is public, so a variable-time memcmp() is OK
-  if (pk_cmp_polyvec(pk, p_reencoded)) {
+  if (memcmp(pk, p_reencoded, MLKEM_POLYVECBYTES)) {
     return -1;
   }
   return 0;
@@ -99,19 +95,6 @@ int crypto_kem_keypair_derand(uint8_t *pk, uint8_t *sk, const uint8_t *coins) {
   return 0;
 }
 
-/*************************************************
- * Name:        crypto_kem_keypair
- *
- * Description: Generates public and private key
- *              for CCA-secure ML-KEM key encapsulation mechanism
- *
- * Arguments:   - uint8_t *pk: pointer to output public key
- *                (an already allocated array of MLKEM_PUBLICKEYBYTES bytes)
- *              - uint8_t *sk: pointer to output private key
- *                (an already allocated array of MLKEM_SECRETKEYBYTES bytes)
- *
- * Returns 0 (success)
- **************************************************/
 int crypto_kem_keypair(uint8_t *pk, uint8_t *sk) {
   uint8_t coins[2 * MLKEM_SYMBYTES] ALIGN;
   randombytes(coins, 2 * MLKEM_SYMBYTES);

mlkem/kem.h

@@ -30,7 +30,25 @@ int crypto_kem_keypair_derand(uint8_t *pk, uint8_t *sk,
 // clang-format on
 
 #define crypto_kem_keypair MLKEM_NAMESPACE(keypair)
-int crypto_kem_keypair(uint8_t *pk, uint8_t *sk);
+/*************************************************
+ * Name:        crypto_kem_keypair
+ *
+ * Description: Generates public and private key
+ *              for CCA-secure ML-KEM key encapsulation mechanism
+ *
+ * Arguments:   - uint8_t *pk: pointer to output public key
+ *                (an already allocated array of MLKEM_PUBLICKEYBYTES bytes)
+ *              - uint8_t *sk: pointer to output private key
+ *                (an already allocated array of MLKEM_SECRETKEYBYTES bytes)
+ *
+ * Returns 0 (success)
+ **************************************************/
+int crypto_kem_keypair(uint8_t *pk, uint8_t *sk)  // clang-format off
+  REQUIRES(IS_FRESH(pk, MLKEM_PUBLICKEYBYTES))
+  REQUIRES(IS_FRESH(sk, MLKEM_SECRETKEYBYTES))
+  ASSIGNS(OBJECT_WHOLE(pk))
+  ASSIGNS(OBJECT_WHOLE(sk));
+// clang-format on
 
 #define crypto_kem_enc_derand MLKEM_NAMESPACE(enc_derand)
 /*************************************************