generate checksums without ivy

build.sbt

@@ -139,18 +139,15 @@ executableKey := {
   val outputFile    = workDir / warName
   IO jar (contentMappings, outputFile, manifest)
 
-  // patch ChecksumHelper to allow sha256
-  val hack  = classOf[ChecksumHelper] getDeclaredField "algorithms"
-  hack setAccessible true
-  val algos = (hack get null).asInstanceOf[java.util.Map[String,String]]
-  algos put ("sha256", "SHA-256")
-
   // generate checksums
-  Seq("md5", "sha1", "sha256") foreach { algorithm =>
-    IO.write(
-      workDir / (warName + "." + algorithm),
-      ChecksumHelper computeAsString (outputFile, algorithm)
+  Seq(
+    "md5"    -> "MD5",
+    "sha1"   -> "SHA-1",
+    "sha256" -> "SHA-256"
   )
+  .foreach { case (extension, algorithm) =>
+    val checksumFile = workDir / (warName + "." + extension)
+    Checksums generate (outputFile, checksumFile, algorithm)
   }
 
   // done

project/Checksums.scala

@@ -0,0 +1,34 @@
+import java.security.MessageDigest;
+import scala.annotation._
+import sbt._
+import sbt.Using._
+
+object Checksums {
+  private val bufferSize = 2048
+
+  def generate(source:File, target:File, algorithm:String):Unit =
+      IO write (target, compute(source, algorithm))
+
+  def compute(file:File, algorithm:String):String =
+      hex(raw(file, algorithm))
+
+  def raw(file:File, algorithm:String):Array[Byte]    =
+      (Using fileInputStream file) { is =>
+        val md  = MessageDigest getInstance algorithm
+        val buf = new Array[Byte](bufferSize)
+        md.reset()
+        @tailrec
+        def loop() {
+          val len = is read buf
+          if (len != -1) {
+            md update (buf, 0, len)
+            loop()
+          }
+        }
+        loop()
+        md.digest()
+      }
+
+  def hex(bytes:Array[Byte]):String =
+      bytes map { it => "%02x" format (it.toInt & 0xff) } mkString ""
+}