diff --git a/poseidon2/compress.nim b/poseidon2/compress.nim
index b0ce10e..85c5d8c 100644
--- a/poseidon2/compress.nim
+++ b/poseidon2/compress.nim
@@ -2,9 +2,9 @@ import ./types
 import ./permutation
 
 # 2-to-1 compression
-func compress*(a, b : F) : F =
+func compress*(a, b : F, key = zero) : F =
   var x = a
   var y = b
-  var z : F = zero
+  var z = key
   permInplace(x, y, z)
   return x
diff --git a/tests/poseidon2/testCompress.nim b/tests/poseidon2/testCompress.nim
new file mode 100644
index 0000000..9f51492
--- /dev/null
+++ b/tests/poseidon2/testCompress.nim
@@ -0,0 +1,15 @@
+import std/unittest
+import constantine/math/arithmetic
+import poseidon2/types
+import poseidon2/permutation
+import poseidon2/compress
+
+suite "compress":
+
+  test "uses permutation to compress two elements":
+    check bool(compress(1.toF, 2.toF) == (1.toF, 2.toF, 0.toF).perm[0])
+    check bool(compress(3.toF, 4.toF) == (3.toF, 4.toF, 0.toF).perm[0])
+
+  test "allows for keyed compression":
+    check bool(compress(1.toF, 2.toF, key=3.toF) == (1.toF, 2.toF, 3.toF).perm[0])
+    check bool(compress(4.toF, 5.toF, key=6.toF) == (4.toF, 5.toF, 6.toF).perm[0])
diff --git a/tests/test.nim b/tests/test.nim
index 2313cae..da47d22 100644
--- a/tests/test.nim
+++ b/tests/test.nim
@@ -1,5 +1,6 @@
 import ./poseidon2/testPermutation
 import ./poseidon2/testSponge
+import ./poseidon2/testCompress
 import ./poseidon2/testPoseidon2
 import ./poseidon2/testIo
 import ./poseidon2/testReadme