Fix sha2 update() function crashes on uninitialized context.

nimcrypto/sha2/sha2.nim

@@ -81,6 +81,10 @@ func reset*(ctx: var Sha2Context) {.noinit.} =
     ctx.state[6] = 0x2B0199FC2C85B8AA'u64
     ctx.state[7] = 0x0EB72DDC81C52CA2'u64
 
+template checkContext(ctx: var Sha2Context) =
+  doAssert(not(isNil(ctx.compressFunc)),
+    "Context should be initialized first!")
+
 func getCompressFunction(
     Sha2ContextType: typedesc[sha224|sha256],
     implementation: Sha2Implementation,
@@ -226,6 +230,8 @@ func clear*(ctx: var Sha2Context) {.noinit.} =
     burnMem(ctx)
 
 func update*[T: bchar](ctx: var Sha2Context, data: openArray[T]) {.noinit.} =
+  ctx.checkContext()
+
   var
     pos = 0
     bytesLeft = len(data)
@@ -313,6 +319,7 @@ func finalize512(ctx: var Sha2Context) {.inline, noinit.} =
 
 func finish*(ctx: var Sha2Context,
              data: var openArray[byte]): uint {.noinit, discardable.} =
+  ctx.checkContext()
   when ctx.bits == 224 and ctx.bsize == 64:
     if len(data) >= 28:
       finalize256(ctx)

tests/testsha2.nim

@@ -915,3 +915,50 @@ suite "SHA2 Tests":
         skip()
       else:
         check sha512.millionAtest2() == stripSpaces(digest1ma512)
+
+    test "Uninitialized context update()/finish() crash test":
+      var
+        ctx1: sha224
+        ctx2: sha256
+        ctx3: sha384
+        ctx4: sha512
+        ctx5: sha512_224
+        ctx6: sha512_256
+
+      template doCheckUpdate(a: untyped): untyped =
+        try:
+          a.update([0x00'u8])
+          false
+        except AssertionDefect:
+          true
+        except CatchableError:
+          false
+        except Defect:
+          false
+
+      template doCheckFinish(a: untyped): untyped =
+        try:
+          var digest: array[64, byte]
+          let count {.used.} = a.finish(digest)
+          false
+        except AssertionDefect:
+          true
+        except CatchableError:
+          false
+        except Defect:
+          false
+
+      check:
+        ctx1.doCheckUpdate() == true
+        ctx2.doCheckUpdate() == true
+        ctx3.doCheckUpdate() == true
+        ctx4.doCheckUpdate() == true
+        ctx5.doCheckUpdate() == true
+        ctx6.doCheckUpdate() == true
+      check:
+        ctx1.doCheckFinish() == true
+        ctx2.doCheckFinish() == true
+        ctx3.doCheckFinish() == true
+        ctx4.doCheckFinish() == true
+        ctx5.doCheckFinish() == true
+        ctx6.doCheckFinish() == true