Proofs of poly_frommsg

cbmc/proofs/poly_frommsg/Makefile

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+
+include ../Makefile_params.common
+
+HARNESS_ENTRY = harness
+HARNESS_FILE = poly_frommsg_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 = poly_frommsg
+
+DEFINES +=
+INCLUDES +=
+
+REMOVE_FUNCTION_BODY +=
+UNWINDSET +=
+
+PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
+PROJECT_SOURCES += $(SRCDIR)/mlkem/poly.c
+
+CHECK_FUNCTION_CONTRACTS=$(MLKEM_NAMESPACE)poly_frommsg
+USE_FUNCTION_CONTRACTS=$(MLKEM_NAMESPACE)cmov_int16
+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)poly_frommsg
+
+# 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 = 8
+
+# 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/poly_frommsg/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/poly_frommsg/poly_frommsg_harness.c

@@ -0,0 +1,30 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: MIT-0
+
+/*
+ * Insert copyright notice
+ */
+
+/**
+ * @file poly_frommsg_harness.c
+ * @brief Implements the proof harness for poly_frommsg function.
+ */
+
+/*
+ * Insert project header files that
+ *   - include the declaration of the function
+ *   - include the types needed to declare function arguments
+ */
+#include <poly.h>
+
+/**
+ * @brief Starting point for formal analysis
+ *
+ */
+void harness(void) {
+  poly *a;
+  uint8_t *msg;
+
+  /* Contracts for this function are in poly.h */
+  poly_frommsg(a, msg);
+}

mlkem/poly.c

@@ -274,28 +274,25 @@ void poly_frombytes(poly *r, const uint8_t a[MLKEM_POLYBYTES]) {
 }
 #endif /* MLKEM_USE_NATIVE_POLY_FROMBYTES */
 
-/*************************************************
- * Name:        poly_frommsg
- *
- * Description: Convert 32-byte message to polynomial
- *
- * Arguments:   - poly *r: pointer to output polynomial
- *              - const uint8_t *msg: pointer to input message
- **************************************************/
 void poly_frommsg(poly *r, const uint8_t msg[MLKEM_INDCPA_MSGBYTES]) {
-  unsigned int i, j;
-
 #if (MLKEM_INDCPA_MSGBYTES != MLKEM_N / 8)
 #error "MLKEM_INDCPA_MSGBYTES must be equal to MLKEM_N/8 bytes!"
 #endif
 
-  for (i = 0; i < MLKEM_N / 8; i++) {
-    for (j = 0; j < 8; j++) {
-      r->coeffs[8 * i + j] = 0;
-      cmov_int16(r->coeffs + 8 * i + j, ((MLKEM_Q + 1) / 2), (msg[i] >> j) & 1);
+  for (int i = 0; i < MLKEM_N / 8; i++)  // clang-format off
+    ASSIGNS(i, OBJECT_WHOLE(r))
+    INVARIANT(i >= 0 && i <= MLKEM_N / 8)
+    INVARIANT(ARRAY_IN_BOUNDS(int, k, 0, (8 * i - 1), r->coeffs, 0, (MLKEM_Q - 1)))
+    {      // clang-format on
+      for (int j = 0; j < 8; j++)  // clang-format off
+        ASSIGNS(j, OBJECT_WHOLE(r))
+        INVARIANT(i >= 0 && i <  MLKEM_N / 8 && j >= 0 && j <= 8)
+        INVARIANT(ARRAY_IN_BOUNDS(int, k, 0, (8 * i + j - 1), r->coeffs, 0, (MLKEM_Q - 1)))
+        {  // clang-format on
+          r->coeffs[8 * i + j] = 0;
+          cmov_int16(&r->coeffs[8 * i + j], HALF_Q, (msg[i] >> j) & 1);
+        }
     }
-  }
-
   POLY_BOUND_MSG(r, MLKEM_Q, "poly_frommsg output");
 }
 

mlkem/poly.h

@@ -213,8 +213,23 @@ ASSIGNS(OBJECT_UPTO(r, sizeof(poly)))
 ENSURES(ARRAY_IN_BOUNDS(int, k, 0, (MLKEM_N - 1), r->coeffs, 0, 4095));
 // clang-format on
 
+
 #define poly_frommsg MLKEM_NAMESPACE(poly_frommsg)
-void poly_frommsg(poly *r, const uint8_t msg[MLKEM_INDCPA_MSGBYTES]);
+/*************************************************
+ * Name:        poly_frommsg
+ *
+ * Description: Convert 32-byte message to polynomial
+ *
+ * Arguments:   - poly *r: pointer to output polynomial
+ *              - const uint8_t *msg: pointer to input message
+ **************************************************/
+void poly_frommsg(poly *r, const uint8_t msg[MLKEM_INDCPA_MSGBYTES])
+    // clang-format off
+REQUIRES(IS_FRESH(msg, MLKEM_INDCPA_MSGBYTES))
+REQUIRES(IS_FRESH(r, sizeof(poly)))
+ASSIGNS(OBJECT_WHOLE(r))
+ENSURES(ARRAY_IN_BOUNDS(int, k, 0, (MLKEM_N - 1), r->coeffs, 0, (MLKEM_Q - 1)));
+// clang-format on
 
 
 #define poly_tomsg MLKEM_NAMESPACE(poly_tomsg)

mlkem/verify.h

@@ -29,7 +29,8 @@ void cmov_int16(int16_t *r, const int16_t v, const uint16_t b)
     // clang-format off
 REQUIRES(b == 0 || b == 1)
 REQUIRES(IS_FRESH(r, sizeof(int16_t)))
-ASSIGNS(OBJECT_WHOLE(r))
+ASSIGNS(OBJECT_UPTO(r, sizeof(int16_t)))
 ENSURES(*r == (b ? v : OLD(*r)));
 // clang-format on
+
 #endif