diff --git a/CHANGELOG.org b/CHANGELOG.org
index d3f4d19..996eab3 100644
--- a/CHANGELOG.org
+++ b/CHANGELOG.org
@@ -18,6 +18,7 @@ The format is based on [[https://keepachangelog.com/en/1.0.0/][Keep a Changelog]
 
    - Replace cats-effect with zio (#117)
    - Replace Monocle with local SimpleLens implementation (#121)
+   - Don't use String as key in Map for hashes (#124)
 
 ** Dependencies
 
diff --git a/core/src/main/scala/net/kemitix/thorp/core/SimpleHashService.scala b/core/src/main/scala/net/kemitix/thorp/core/SimpleHashService.scala
index 6991af6..16b4a6e 100644
--- a/core/src/main/scala/net/kemitix/thorp/core/SimpleHashService.scala
+++ b/core/src/main/scala/net/kemitix/thorp/core/SimpleHashService.scala
@@ -2,7 +2,8 @@ package net.kemitix.thorp.core
 
 import java.nio.file.Path
 
-import net.kemitix.thorp.domain.MD5Hash
+import net.kemitix.thorp.domain.HashType.MD5
+import net.kemitix.thorp.domain.{HashType, MD5Hash}
 import net.kemitix.thorp.storage.api.HashService
 import zio.Task
 
@@ -10,9 +11,9 @@ case class SimpleHashService() extends HashService {
 
   override def hashLocalObject(
       path: Path
-  ): Task[Map[String, MD5Hash]] =
+  ): Task[Map[HashType, MD5Hash]] =
     for {
       md5 <- MD5HashGenerator.md5File(path)
-    } yield Map("md5" -> md5)
+    } yield Map(MD5 -> md5)
 
 }
diff --git a/core/src/test/scala/net/kemitix/thorp/core/ActionGeneratorSuite.scala b/core/src/test/scala/net/kemitix/thorp/core/ActionGeneratorSuite.scala
index ea5172f..146c280 100644
--- a/core/src/test/scala/net/kemitix/thorp/core/ActionGeneratorSuite.scala
+++ b/core/src/test/scala/net/kemitix/thorp/core/ActionGeneratorSuite.scala
@@ -3,6 +3,7 @@ package net.kemitix.thorp.core
 import java.time.Instant
 
 import net.kemitix.thorp.core.Action.{DoNothing, ToCopy, ToUpload}
+import net.kemitix.thorp.domain.HashType.MD5
 import net.kemitix.thorp.domain._
 import org.scalatest.FunSpec
 
@@ -143,7 +144,7 @@ class ActionGeneratorSuite extends FunSpec {
     }
   }
 
-  private def md5HashMap(theHash: MD5Hash) = {
-    Map("md5" -> theHash)
+  private def md5HashMap(theHash: MD5Hash): Map[HashType, MD5Hash] = {
+    Map(MD5 -> theHash)
   }
 }
diff --git a/core/src/test/scala/net/kemitix/thorp/core/DummyHashService.scala b/core/src/test/scala/net/kemitix/thorp/core/DummyHashService.scala
index 31b7941..f2b2186 100644
--- a/core/src/test/scala/net/kemitix/thorp/core/DummyHashService.scala
+++ b/core/src/test/scala/net/kemitix/thorp/core/DummyHashService.scala
@@ -2,14 +2,14 @@ package net.kemitix.thorp.core
 
 import java.nio.file.Path
 
-import net.kemitix.thorp.domain.MD5Hash
+import net.kemitix.thorp.domain.{HashType, MD5Hash}
 import net.kemitix.thorp.storage.api.HashService
 import zio.Task
 
-case class DummyHashService(hashes: Map[Path, Map[String, MD5Hash]])
+case class DummyHashService(hashes: Map[Path, Map[HashType, MD5Hash]])
     extends HashService {
 
-  override def hashLocalObject(path: Path): Task[Map[String, MD5Hash]] =
+  override def hashLocalObject(path: Path): Task[Map[HashType, MD5Hash]] =
     Task(hashes(path))
 
 }
diff --git a/core/src/test/scala/net/kemitix/thorp/core/LocalFileStreamSuite.scala b/core/src/test/scala/net/kemitix/thorp/core/LocalFileStreamSuite.scala
index 9388a01..077f3aa 100644
--- a/core/src/test/scala/net/kemitix/thorp/core/LocalFileStreamSuite.scala
+++ b/core/src/test/scala/net/kemitix/thorp/core/LocalFileStreamSuite.scala
@@ -2,6 +2,7 @@ package net.kemitix.thorp.core
 
 import java.nio.file.Paths
 
+import net.kemitix.thorp.domain.HashType.MD5
 import net.kemitix.thorp.domain._
 import net.kemitix.thorp.storage.api.HashService
 import org.scalatest.FunSpec
@@ -13,8 +14,8 @@ class LocalFileStreamSuite extends FunSpec {
   private val sourcePath = source.toPath
   private val hashService: HashService = DummyHashService(
     Map(
-      file("root-file")        -> Map("md5" -> MD5HashData.Root.hash),
-      file("subdir/leaf-file") -> Map("md5" -> MD5HashData.Leaf.hash)
+      file("root-file")        -> Map(MD5 -> MD5HashData.Root.hash),
+      file("subdir/leaf-file") -> Map(MD5 -> MD5HashData.Leaf.hash)
     ))
 
   private def file(filename: String) =
diff --git a/core/src/test/scala/net/kemitix/thorp/core/PlanBuilderTest.scala b/core/src/test/scala/net/kemitix/thorp/core/PlanBuilderTest.scala
index e55d9e6..9fd0d75 100644
--- a/core/src/test/scala/net/kemitix/thorp/core/PlanBuilderTest.scala
+++ b/core/src/test/scala/net/kemitix/thorp/core/PlanBuilderTest.scala
@@ -5,6 +5,7 @@ import java.nio.file.Path
 
 import net.kemitix.thorp.console._
 import net.kemitix.thorp.core.Action.{DoNothing, ToCopy, ToDelete, ToUpload}
+import net.kemitix.thorp.domain.HashType.MD5
 import net.kemitix.thorp.domain._
 import net.kemitix.thorp.storage.api.{HashService, StorageService}
 import org.scalatest.FreeSpec
@@ -433,7 +434,7 @@ class PlanBuilderTest extends FreeSpec with TemporaryFolder {
     def md5Hash(file: File) = {
       runtime
         .unsafeRunSync {
-          hashService.hashLocalObject(file.toPath).map(_.get("md5"))
+          hashService.hashLocalObject(file.toPath).map(_.get(MD5))
         }
         .toEither
         .toOption
@@ -477,7 +478,7 @@ class PlanBuilderTest extends FreeSpec with TemporaryFolder {
         case ToUpload(_, lf, _) =>
           ("upload",
            lf.remoteKey.key,
-           lf.hashes("md5").hash,
+           lf.hashes(MD5).hash,
            lf.source.toString,
            lf.file.toString)
         case ToDelete(_, remoteKey, _) => ("delete", remoteKey.key, "", "", "")
diff --git a/core/src/test/scala/net/kemitix/thorp/core/S3MetaDataEnricherSuite.scala b/core/src/test/scala/net/kemitix/thorp/core/S3MetaDataEnricherSuite.scala
index abed045..3dc1a74 100644
--- a/core/src/test/scala/net/kemitix/thorp/core/S3MetaDataEnricherSuite.scala
+++ b/core/src/test/scala/net/kemitix/thorp/core/S3MetaDataEnricherSuite.scala
@@ -3,6 +3,7 @@ package net.kemitix.thorp.core
 import java.time.Instant
 
 import net.kemitix.thorp.core.S3MetaDataEnricher.{getMetadata, getS3Status}
+import net.kemitix.thorp.domain.HashType.MD5
 import net.kemitix.thorp.domain._
 import org.scalatest.FunSpec
 
@@ -171,8 +172,8 @@ class S3MetaDataEnricherSuite extends FunSpec {
     }
   }
 
-  private def md5HashMap(theHash: MD5Hash) = {
-    Map("md5" -> theHash)
+  private def md5HashMap(theHash: MD5Hash): Map[HashType, MD5Hash] = {
+    Map(MD5 -> theHash)
   }
 
   describe("getS3Status") {
diff --git a/core/src/test/scala/net/kemitix/thorp/core/SequencePlanTest.scala b/core/src/test/scala/net/kemitix/thorp/core/SequencePlanTest.scala
index 0f8df6f..c3363b7 100644
--- a/core/src/test/scala/net/kemitix/thorp/core/SequencePlanTest.scala
+++ b/core/src/test/scala/net/kemitix/thorp/core/SequencePlanTest.scala
@@ -3,22 +3,28 @@ package net.kemitix.thorp.core
 import java.io.File
 
 import net.kemitix.thorp.core.Action._
-import net.kemitix.thorp.domain.{Bucket, LocalFile, MD5Hash, RemoteKey}
+import net.kemitix.thorp.domain.{
+  Bucket,
+  HashType,
+  LocalFile,
+  MD5Hash,
+  RemoteKey
+}
 import org.scalatest.FreeSpec
 
 class SequencePlanTest extends FreeSpec {
 
   "sort" - {
     "a list of assorted actions" - {
-      val bucket                       = Bucket("aBucket")
-      val remoteKey1                   = RemoteKey("remoteKey1")
-      val remoteKey2                   = RemoteKey("targetHash")
-      val hash                         = MD5Hash("aHash")
-      val hashes: Map[String, MD5Hash] = Map()
-      val size                         = 1024
-      val file1                        = new File("aFile")
-      val file2                        = new File("aFile")
-      val source                       = new File("source")
+      val bucket     = Bucket("aBucket")
+      val remoteKey1 = RemoteKey("remoteKey1")
+      val remoteKey2 = RemoteKey("targetHash")
+      val hash       = MD5Hash("aHash")
+      val hashes     = Map[HashType, MD5Hash]()
+      val size       = 1024
+      val file1      = new File("aFile")
+      val file2      = new File("aFile")
+      val source     = new File("source")
       val localFile1 =
         LocalFile(file1, source, hashes, remoteKey1)
       val localFile2 =
diff --git a/core/src/test/scala/net/kemitix/thorp/core/SyncSuite.scala b/core/src/test/scala/net/kemitix/thorp/core/SyncSuite.scala
index f3d00e1..0b3f1ea 100644
--- a/core/src/test/scala/net/kemitix/thorp/core/SyncSuite.scala
+++ b/core/src/test/scala/net/kemitix/thorp/core/SyncSuite.scala
@@ -7,6 +7,7 @@ import java.time.Instant
 import net.kemitix.thorp.console
 import net.kemitix.thorp.console.MyConsole
 import net.kemitix.thorp.core.Action.{ToCopy, ToDelete, ToUpload}
+import net.kemitix.thorp.domain.HashType.MD5
 import net.kemitix.thorp.domain.MD5HashData.{Leaf, Root}
 import net.kemitix.thorp.domain.StorageQueueEvent.{
   CopyQueueEvent,
@@ -43,8 +44,8 @@ class SyncSuite extends FunSpec {
   private val hashService =
     DummyHashService(
       Map(
-        file("root-file")        -> Map("md5" -> MD5HashData.Root.hash),
-        file("subdir/leaf-file") -> Map("md5" -> MD5HashData.Leaf.hash)
+        file("root-file")        -> Map(MD5 -> MD5HashData.Root.hash),
+        file("subdir/leaf-file") -> Map(MD5 -> MD5HashData.Leaf.hash)
       ))
   private val configOptions =
     ConfigOptions(
@@ -62,8 +63,8 @@ class SyncSuite extends FunSpec {
                        localFile: LocalFile): (String, String, File) =
     (bucket.name, remoteKey.key, localFile.file)
 
-  private def md5HashMap(md5Hash: MD5Hash): Map[String, MD5Hash] =
-    Map("md5" -> md5Hash)
+  private def md5HashMap(md5Hash: MD5Hash): Map[HashType, MD5Hash] =
+    Map(MD5 -> md5Hash)
 
   private def file(filename: String) =
     sourcePath.resolve(Paths.get(filename))
@@ -207,7 +208,7 @@ class SyncSuite extends FunSpec {
                         batchMode: Boolean,
                         uploadEventListener: UploadEventListener,
                         tryCount: Int): Task[UploadQueueEvent] =
-      Task(UploadQueueEvent(localFile.remoteKey, localFile.hashes("md5")))
+      Task(UploadQueueEvent(localFile.remoteKey, localFile.hashes(MD5)))
 
     override def copy(bucket: Bucket,
                       sourceKey: RemoteKey,
diff --git a/domain/src/main/scala/net/kemitix/thorp/domain/HashType.scala b/domain/src/main/scala/net/kemitix/thorp/domain/HashType.scala
new file mode 100644
index 0000000..1c0d70b
--- /dev/null
+++ b/domain/src/main/scala/net/kemitix/thorp/domain/HashType.scala
@@ -0,0 +1,7 @@
+package net.kemitix.thorp.domain
+
+trait HashType
+
+object HashType {
+  case object MD5 extends HashType
+}
diff --git a/domain/src/main/scala/net/kemitix/thorp/domain/LocalFile.scala b/domain/src/main/scala/net/kemitix/thorp/domain/LocalFile.scala
index 8f91a4b..10d8be2 100644
--- a/domain/src/main/scala/net/kemitix/thorp/domain/LocalFile.scala
+++ b/domain/src/main/scala/net/kemitix/thorp/domain/LocalFile.scala
@@ -3,10 +3,12 @@ package net.kemitix.thorp.domain
 import java.io.File
 import java.nio.file.Path
 
+import net.kemitix.thorp.domain.HashType.MD5
+
 final case class LocalFile(
     file: File,
     source: File,
-    hashes: Map[String, MD5Hash],
+    hashes: Map[HashType, MD5Hash],
     remoteKey: RemoteKey
 ) {
 
@@ -19,7 +21,7 @@ final case class LocalFile(
 
   def matches(other: MD5Hash): Boolean = hashes.values.exists(other equals _)
 
-  def md5base64: Option[String] = hashes.get("md5").map(_.hash64)
+  def md5base64: Option[String] = hashes.get(MD5).map(_.hash64)
 
 }
 
@@ -27,7 +29,7 @@ object LocalFile {
 
   def resolve(
       path: String,
-      md5Hashes: Map[String, MD5Hash],
+      md5Hashes: Map[HashType, MD5Hash],
       source: Path,
       pathToKey: Path => RemoteKey
   ): LocalFile = {
diff --git a/storage-api/src/main/scala/net/kemitix/thorp/storage/api/HashService.scala b/storage-api/src/main/scala/net/kemitix/thorp/storage/api/HashService.scala
index 6b2c022..95d8b6d 100644
--- a/storage-api/src/main/scala/net/kemitix/thorp/storage/api/HashService.scala
+++ b/storage-api/src/main/scala/net/kemitix/thorp/storage/api/HashService.scala
@@ -2,7 +2,7 @@ package net.kemitix.thorp.storage.api
 
 import java.nio.file.Path
 
-import net.kemitix.thorp.domain.MD5Hash
+import net.kemitix.thorp.domain.{HashType, MD5Hash}
 import zio.Task
 
 /**
@@ -10,6 +10,6 @@ import zio.Task
   */
 trait HashService {
 
-  def hashLocalObject(path: Path): Task[Map[String, MD5Hash]]
+  def hashLocalObject(path: Path): Task[Map[HashType, MD5Hash]]
 
 }
diff --git a/storage-aws/src/main/scala/net/kemitix/thorp/storage/aws/ETag.scala b/storage-aws/src/main/scala/net/kemitix/thorp/storage/aws/ETag.scala
new file mode 100644
index 0000000..4b71bc9
--- /dev/null
+++ b/storage-aws/src/main/scala/net/kemitix/thorp/storage/aws/ETag.scala
@@ -0,0 +1,5 @@
+package net.kemitix.thorp.storage.aws
+
+import net.kemitix.thorp.domain.HashType
+
+object ETag extends HashType
diff --git a/storage-aws/src/main/scala/net/kemitix/thorp/storage/aws/S3HashService.scala b/storage-aws/src/main/scala/net/kemitix/thorp/storage/aws/S3HashService.scala
index 644ca33..665e8ac 100644
--- a/storage-aws/src/main/scala/net/kemitix/thorp/storage/aws/S3HashService.scala
+++ b/storage-aws/src/main/scala/net/kemitix/thorp/storage/aws/S3HashService.scala
@@ -3,7 +3,8 @@ package net.kemitix.thorp.storage.aws
 import java.nio.file.Path
 
 import net.kemitix.thorp.core.MD5HashGenerator
-import net.kemitix.thorp.domain.MD5Hash
+import net.kemitix.thorp.domain.HashType.MD5
+import net.kemitix.thorp.domain.{HashType, MD5Hash}
 import net.kemitix.thorp.storage.api.HashService
 import zio.Task
 
@@ -17,14 +18,14 @@ trait S3HashService extends HashService {
     */
   override def hashLocalObject(
       path: Path
-  ): Task[Map[String, MD5Hash]] =
+  ): Task[Map[HashType, MD5Hash]] =
     for {
       md5  <- MD5HashGenerator.md5File(path)
       etag <- ETagGenerator.eTag(path).map(MD5Hash(_))
     } yield
       Map(
-        "md5"  -> md5,
-        "etag" -> etag
+        MD5  -> md5,
+        ETag -> etag
       )
 
 }
diff --git a/storage-aws/src/test/scala/net/kemitix/thorp/storage/aws/StorageServiceSuite.scala b/storage-aws/src/test/scala/net/kemitix/thorp/storage/aws/StorageServiceSuite.scala
index 170ec8c..9722d30 100644
--- a/storage-aws/src/test/scala/net/kemitix/thorp/storage/aws/StorageServiceSuite.scala
+++ b/storage-aws/src/test/scala/net/kemitix/thorp/storage/aws/StorageServiceSuite.scala
@@ -5,6 +5,7 @@ import java.time.Instant
 import com.amazonaws.services.s3.model.PutObjectRequest
 import com.amazonaws.services.s3.transfer.model.UploadResult
 import net.kemitix.thorp.core.{KeyGenerator, Resource, S3MetaDataEnricher}
+import net.kemitix.thorp.domain.HashType.MD5
 import net.kemitix.thorp.domain.MD5HashData.Root
 import net.kemitix.thorp.domain.StorageQueueEvent.UploadQueueEvent
 import net.kemitix.thorp.domain._
@@ -113,8 +114,8 @@ class StorageServiceSuite extends FunSpec with MockFactory {
 
   }
 
-  private def md5HashMap(hash: MD5Hash) =
-    Map("md5" -> hash)
+  private def md5HashMap(hash: MD5Hash): Map[HashType, MD5Hash] =
+    Map(MD5 -> hash)
 
   val batchMode: Boolean = true