Code review feedback

- Rename sharedSecret variables to checkSharedSecret
- Don't perform the digest::digest(). This just means the timing could
  give away the length of the secret, but that's OK, there's enough
  entropy in the secret even if you know its length.

R/middleware.R

@@ -328,13 +328,13 @@ HandlerManager <- R6Class("HandlerManager",
         }
       )
     },
-    .httpServer = function(handler, sharedSecret) {
+    .httpServer = function(handler, checkSharedSecret) {
       filter <- getOption('shiny.http.response.filter')
       if (is.null(filter))
         filter <- function(req, response) response
 
       function(req) {
-        if (!sharedSecret(req$HTTP_SHINY_SHARED_SECRET)) {
+        if (!checkSharedSecret(req$HTTP_SHINY_SHARED_SECRET)) {
           return(list(status=403,
             body='<h1>403 Forbidden</h1><p>Shared secret mismatch</p>',
             headers=list('Content-Type' = 'text/html')))

R/server.R

@@ -161,7 +161,7 @@ createAppHandlers <- function(httpHandlers, serverFuncSource) {
   # This value, if non-NULL, must be present on all HTTP and WebSocket
   # requests as the Shiny-Shared-Secret header or else access will be
   # denied (403 response for HTTP, and instant close for websocket).
-  sharedSecret <- loadSharedSecret()
+  checkSharedSecret <- loadSharedSecret()
 
   appHandlers <- list(
     http = joinHandlers(c(
@@ -170,7 +170,7 @@ createAppHandlers <- function(httpHandlers, serverFuncSource) {
       reactLogHandler
     )),
     ws = function(ws) {
-      if (!sharedSecret(ws$request$HTTP_SHINY_SHARED_SECRET)) {
+      if (!checkSharedSecret(ws$request$HTTP_SHINY_SHARED_SECRET)) {
         ws$close()
         return(TRUE)
       }

R/utils.R

@@ -1744,14 +1744,25 @@ getSliderType <- function(min, max, value) {
 # Reads the `shiny.sharedSecret` global option, and returns a function that can
 # be used to test header values for a match.
 loadSharedSecret <- function() {
-  sharedSecret <- getOption("shiny.sharedSecret")
+  normalizeToRaw <- function(value, label = "value") {
+    if (is.null(value)) {
+      raw()
+    } else if (is.character(value)) {
+      charToRaw(paste(value, collapse = "\n"))
+    } else if (is.raw(value)) {
+      value
+    } else {
+      stop("Wrong type for ", label, "; character or raw expected")
+    }
+  }
+
+  sharedSecret <- normalizeToRaw(getOption("shiny.sharedSecret"))
   if (is.null(sharedSecret)) {
     function(x) TRUE
   } else {
     # We compare the digest of the two values so that their lengths are equalized
-    sharedSecret <- digest::digest(sharedSecret, "sha256", serialize = FALSE, raw = TRUE)
     function(x) {
-      x <- digest::digest(x, "sha256", serialize = FALSE, raw = TRUE)
+      x <- normalizeToRaw(x)
       # Constant time comparison to avoid timing attacks
       constantTimeEquals(sharedSecret, x)
     }
@@ -1762,7 +1773,9 @@ loadSharedSecret <- function() {
 constantTimeEquals <- function(raw1, raw2) {
   stopifnot(is.raw(raw1))
   stopifnot(is.raw(raw2))
-  stopifnot(length(raw1) == length(raw2))
+  if (length(raw1) != length(raw2)) {
+    return(FALSE)
+  }
 
   sum(as.integer(xor(raw1, raw2))) == 0
 }