diff --git a/photo21/hashers.py b/photo21/hashers.py
index e94397e..0c3305a 100644
--- a/photo21/hashers.py
+++ b/photo21/hashers.py
@@ -17,23 +17,26 @@ class SHA512PasswordHasher(BasePasswordHasher):
     """
     algorithm = "sha512"
 
-    def encode(self, password, salt):
+    def encode(self, password, iteration, salt):
         assert password is not None
         assert salt and '$' not in salt
-        hash = hashlib.sha512(force_bytes(password + "{" + salt + "}"))
-        hash = base64.b64encode(hash)
-        encoded = "%s$%s$%s" % (self.algorithm, salt, hash)
-        encoded = encoded[:128]
-        return encoded
+        salted = force_bytes(password + "{" + salt + "}")
+        digest = hashlib.sha512(salted).digest()
+        # "stretch" hash
+        for _i in range(1, int(iteration)):
+            digest = hashlib.sha512(digest + salted).digest()
+        digest = base64.b64encode(digest).decode()
+        encoded = "%s$%s$%s$%s" % (self.algorithm, iteration, salt, digest)
+        return encoded[:128]
 
     def verify(self, password, encoded):
-        algorithm, salt, hash = encoded.split('$', 2)
+        algorithm, iteration, salt, hash = encoded.split('$', 3)
         assert algorithm == self.algorithm
-        encoded_2 = self.encode(password, salt)
+        encoded_2 = self.encode(password, iteration, salt)
         return constant_time_compare(encoded, encoded_2)
 
     def safe_summary(self, encoded):
-        algorithm, salt, hash = encoded.split('$', 2)
+        algorithm, iteration, salt, hash = encoded.split('$', 3)
         assert algorithm == self.algorithm
         return OrderedDict([
             (_('algorithm'), algorithm),