rand: convenience API (#28)

Helpers to generate random numbers, including simple Nim types

* disable C++ tests
  * there are too many `const` vs `non-const` issues to run these tests -
C++ never actually worked
* add generateBytes to make fresh seqs

Co-authored-by: Mamy Ratsimbazafy <mamy_github@numforge.co>
Co-authored-by: Etan Kissling <etan@status.im>

.github/workflows/ci.yml

@@ -163,4 +163,3 @@ jobs:
           nimble --version
           nimble install -y --depsOnly
           env TEST_LANG="c" nimble test
-          env TEST_LANG="cpp" nimble test

README.md

@@ -36,6 +36,11 @@ In general, the mappings follow the conventions of the original BearSSL library
 * `uint` used instead of `csize_t` - these are the same type in Nim, but spelled more conveniently
   * Canonical nim code will have to be careful when converting existing `int` lengths, looking out for out-of-range values
 
+In addition to the raw `C`-like api, convenience functions are added where applicable - these follow a similar set of conventions:
+
+* named after the function they simplify, but take advantage of types and overload support in Nim
+* help turn pointers and bytes into Nim types
+
 ## Installation
 
 You can install the developement version of the library through nimble with the following command:

bearssl/rand.nim

@@ -1,8 +1,100 @@
 import
-  ./abi/bearssl_rand
+  typetraits,
+  ./abi/[bearssl_hash, bearssl_rand]
 
 export bearssl_rand
 
-func hmacDrbgGenerate*(ctx: var HmacDrbgContext, output: var openArray[byte]) =
-  if output.len > 0:
-    hmacDrbgGenerate(ctx, addr output[0], uint output.len)
+# About types used in helpers:
+# `bool` types are problematic because because they only use one bit of the
+# entire byte - a similar problem occurs with `object` types with alignment
+# gaps - `supportsCopyMem` is wrong here, we should be using `supportsMemCmp` or
+# something similar that takes into account these issues, but alas, there's no
+# such trait as of now
+
+proc init*[S](T: type HmacDrbgContext, seed: openArray[S]): HmacDrbgContext =
+  ## Create a new randomness context with the given seed - typically, a single
+  ## instance per thread should be created.
+  ##
+  ## The seed can later be topped up with `update`.
+  static: doAssert supportsCopyMem(S) and sizeof(S) > 0 and S isnot bool
+
+  if seed.len == 0:
+    hmacDrbgInit(result, addr bearssl_hash.sha256Vtable, nil, 0)
+  else:
+    # In theory the multiplication can overflow, but practically we can't
+    # allocate that much memory, so it won't
+    hmacDrbgInit(
+      result, addr sha256Vtable, unsafeAddr seed[0], uint seed.len * sizeof(S))
+
+proc new*(T: type HmacDrbgContext): ref HmacDrbgContext =
+  ## Create a new randomness context intended to be shared between randomness
+  ## consumers - typically, a single instance per thread should be created.
+  ##
+  ## The context is seeded with randomness from the OS / system.
+  ## Returns `nil` if the OS / system has no randomness API.
+  let seeder = prngSeederSystem(nil)
+  if seeder == nil:
+    return nil
+
+  let rng = (ref HmacDrbgContext)()
+  hmacDrbgInit(rng[], addr sha256Vtable, nil, 0)
+
+  if seeder(addr rng.vtable) == 0:
+    return nil
+
+  rng
+
+func generate*(ctx: var HmacDrbgContext, v: var auto) =
+  ## Fill `v` with random data - `v` must be a simple type
+  static: doAssert supportsCopyMem(type v)
+
+  when sizeof(v) > 0:
+    when v is bool:
+      # `bool` would result in a heavily biased value because >0 == true
+      var tmp: byte
+      hmacDrbgGenerate(ctx, addr tmp, uint sizeof(tmp))
+      v = (tmp and 1'u8) == 1
+    else:
+      hmacDrbgGenerate(ctx, addr v, uint sizeof(v))
+
+func generate*[V](ctx: var HmacDrbgContext, v: var openArray[V]) =
+  ## Fill `v` with random data - `T` must be a simple type
+  static: doAssert supportsCopyMem(V) and sizeof(V) > 0
+
+  when V is bool:
+    for b in v.mitems:
+      ctx.generate(b)
+  else:
+    if v.len > 0:
+      # In theory the multiplication can overflow, but practically we can't
+      # allocate that much memory, so it won't
+      hmacDrbgGenerate(ctx, addr v[0], uint v.len * sizeof(V))
+
+template generate*[V](ctx: var HmacDrbgContext, v: var seq[V]) =
+  generate(ctx, v.toOpenArray(0, v.high()))
+
+func generateBytes*(ctx: var HmacDrbgContext, n: int): seq[byte] =
+  # https://github.com/nim-lang/Nim/issues/19357
+  if n > 0:
+    result = newSeqUninitialized[byte](n)
+    ctx.generate(result)
+
+func generate*(ctx: var HmacDrbgContext, T: type): T {.noinit.} =
+  ## Create a new instance of `T` filled with random data - `T` must be
+  ## a simple type
+  ctx.generate(result)
+
+func update*[S](ctx: var HmacDrbgContext, seed: openArray[S]) =
+  ## Update context with additional seed data
+  static: doAssert supportsCopyMem(S) and sizeof(S) > 0 and S isnot bool
+
+  if seed.len > 0:
+    # In theory the multiplication can overflow, but practically we can't
+    # allocate that much memory, so it won't
+    hmacDrbgUpdate(ctx, unsafeAddr seed[0], uint seed.len * sizeof(S))
+
+# Convenience helpers using bearssl naming
+
+template hmacDrbgGenerate*(
+    ctx: var HmacDrbgContext, output: var openArray[byte]) =
+  generate(ctx, output)

tests/test_rand.nim

@@ -0,0 +1,62 @@
+import
+  unittest2,
+  ../bearssl/rand
+
+{.used.}
+
+suite "random":
+  test "simple random ops":
+    # Some of these tests may end up triggering false fails, but given their
+    # probability, should be fine
+
+    let rng = HmacDrbgContext.new()
+
+    var v: array[1024, byte]
+    rng[].generate(v)
+
+    let v2 = rng[].generate(array[1024, byte])
+    check:
+      v != default(array[1024, byte]) # probable
+      v2 != default(array[1024, byte]) # probable
+
+    for i in 0..<1000:
+      doAssert cast[int](rng[].generate(bool)) in [0, 1]
+
+    var bools: array[64 * 1024, bool]
+    rng[].generate(bools)
+
+    check:
+      true in bools # probable
+      false in bools # probable
+
+    var
+      xxx = newSeq[int](1024)
+      yyy = xxx
+    rng[].generate(xxx)
+    check:
+      xxx != yyy # probable
+
+  test "seed":
+    for seed in [@[byte 0], @[byte 1], @[byte 1, 1], @[byte 42, 13, 37]]:
+      var
+        rng = HmacDrbgContext.init(seed)
+        rng2 = HmacDrbgContext.init(seed)
+
+      check:
+        rng.generate(uint64) == rng2.generate(uint64)
+
+    for seed in [@[0], @[1], @[1, 1], @[42, 1337, -5]]:
+      var
+        rng = HmacDrbgContext.init(seed)
+        rng2 = HmacDrbgContext.init(seed)
+
+      check:
+        rng.generate(uint64) == rng2.generate(uint64)
+
+  test "antiseed":
+    var
+      rng = HmacDrbgContext.init([0])
+      rng2 = HmacDrbgContext.init([1])
+
+    check:
+      rng.generate(array[1024, byte]) != rng2.generate(array[1024, byte])