fix fuzz 5 reloaded: modexp - endianness issue for exponent MSB (#266)

* fix fuzz 5 reloaded: endianness issue for exponent MSB

* refactoring typo in test vs gmp

constantine/math/elliptic/ec_scalar_mul_vartime.nim

@@ -361,7 +361,7 @@ func scalarMul_vartime*[scalBits; EC](
 
   const L = scalBits.ceilDiv_vartime(M) + 1
 
-  let usedBits = scalar.limbs.getBits_vartime()
+  let usedBits = scalar.limbs.getBits_LE_vartime()
 
   when scalBits == EC.F.C.getCurveOrderBitwidth() and
        EC.F.C.hasEndomorphismAcceleration():

constantine/math_arbitrary_precision/arithmetic/bigints_views.nim

@@ -55,8 +55,8 @@ func powOddMod_vartime*(
   debug:
     doAssert bool(M.isOdd())
 
-  let aBits  = a.getBits_vartime()
-  let mBits  = M.getBits_vartime()
+  let aBits  = a.getBits_LE_vartime()
+  let mBits  = M.getBits_LE_vartime()
   let L      = wordsRequired(mBits)
   let m0ninv = M[0].negInvModWord()
   var rMont  = allocStackArray(SecretWord, L)
@@ -105,17 +105,17 @@ func powMod_vartime*(
 
   # Special cases: early returns
   # -------------------------------------------------------------------
-  let mBits = M.getBits_vartime()
+  let mBits = M.getBits_LE_vartime()
   if mBits < 2: # Check if modulus = 0 or 1
     r.setZero()
     return
 
-  let eBits = exponent.getBits_vartime()
+  let eBits = exponent.getBits_BE_vartime()
   if eBits == 0: # Check if exponent == 0
     r.setOne()   # a⁰ = 1 and 0⁰ = 1
     return
 
-  let aBits = a.getBits_vartime()
+  let aBits = a.getBits_LE_vartime()
   if aBits < 2:  # Check if a == 0 or a == 1
     r[0] = a[0]
     for i in 1 ..< r.len:

constantine/math_arbitrary_precision/arithmetic/limbs_mod2k.nim

@@ -109,7 +109,7 @@ func powMod2k_vartime*(
   for i in 0 ..< r.len:
     r[i] = Zero
 
-  let msb = getMSB_vartime(exponent)
+  let msb = getMSB_BE_vartime(exponent)
 
   if msb == -1: # exponent is 0
     r[0] = One  # x⁰ = 1, even for 0⁰

constantine/math_arbitrary_precision/arithmetic/limbs_views.nim

@@ -105,18 +105,40 @@ func ccopyWords*(
 # Bit operations
 # ------------------------------------------------------------
 
-func getMSB_vartime*[T](a: openArray[T]): int =
+func getMSB_BE_vartime*(a: openArray[byte]): int =
   ## Returns the position of the most significant bit
   ## of `a`.
   ## Returns -1 if a == 0
+  ##
+  ## Input MUST be ordered from most to least significant byte
   result = -1
-  for i in countdown(a.len-1, 0):
-    if bool(a[i] != T(0)):
-      return int(log2_vartime(uint64 a[i])) + 8*sizeof(T)*i
+  for i in 0 ..< a.len:
+    if bool a[i] != 0:
+      return int(log2_vartime(uint32 a[i])) + 8*sizeof(byte)*(a.len-1-i)
 
-func getBits_vartime*[T](a: openArray[T]): int {.inline.} =
+func getBits_BE_vartime*(a: openArray[byte]): int {.inline.} =
   ## Returns the number of bits used by `a`
   ## Returns 0 for 0
-  1 + getMSB_vartime(a)
+  ##
+  ## Input MUST be ordered from least to most significant byte
+  1 + getMSB_BE_vartime(a)
+
+func getMSB_LE_vartime(a: openArray[SecretWord]): int =
+  ## Returns the position of the most significant bit
+  ## of `a`.
+  ## Returns -1 if a == 0
+  ##
+  ## Input MUST be ordered from least to most significant word
+  result = -1
+  for i in countdown(a.len-1, 0):
+    if bool a[i] != Zero:
+      return int(log2_vartime(uint64 a[i])) + 8*sizeof(SecretWord)*i
+
+func getBits_LE_vartime*(a: openArray[SecretWord]): int {.inline.} =
+  ## Returns the number of bits used by `a`
+  ## Returns 0 for 0
+  ##
+  ## Input MUST be ordered from least to most significant word
+  1 + getMSB_LE_vartime(a)
 
 {.pop.} # raises no exceptions

tests/math_arbitrary_precision/t_bigints_powmod_vs_gmp.nim

@@ -97,9 +97,9 @@ proc test(rng: var RngState) =
   mpz_clear(rr)
 
   let
-    aBits = a.getBits_vartime()
-    eBits = e.getBits_vartime()
-    mBits = M.getBits_vartime()
+    aBits = a.getBits_LE_vartime()
+    eBits = e.getBits_BE_vartime()
+    mBits = M.getBits_LE_vartime()
 
   rCtt.powMod_vartime(a, e, M, window = 4)
 

tests/t_ethereum_evm_modexp.nim

@@ -42,6 +42,37 @@ suite "EVM ModExp precompile (EIP-198)":
     doAssert status == cttEVM_Success
     doAssert r[0] == 0, ". Result was " & $r[0]
 
+  test "Audit #5-2 - Fuzz failure with even modulus strikes back":
+    let input = [
+
+        # Length of base (1)
+        uint8 0x00,
+              0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01,
+
+        # Length of exponent (5)
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x05,
+
+        # Length of modulus (1)
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01,
+
+        # Base
+        0x3a,
+
+        # Exponent
+        0x01, 0x00, 0x00, 0x00, 0x00,
+
+        # Modulus
+        0x08
+    ]
+
+    var r = newSeq[byte](1)
+    let status = r.eth_evm_modexp(input)
+    doAssert status == cttEVM_Success
+    doAssert r[0] == 0, ". Result was " & $r[0]
+
   test "Audit #8 - off-by-1 buffer overflow - ptr + length exclusive vs openArray(lo, hi) inclusive":
     let input = [
         # Length of base (24)