diff --git a/src/signatures.ml b/src/signatures.ml
new file mode 100644
index 00000000..9db5dbb2
--- /dev/null
+++ b/src/signatures.ml
@@ -0,0 +1,23 @@
+(* https://docs.taler.net/core/api-common.html#id7 *)
+
+module Purpose = struct
+  (* defined in gnunet_crypto_lib.h *)
+  type t = {
+    (* This field equals the number of bytes being signed,
+       namely 'sizeof (struct Data)'. *)
+    size: Int32.t; (* = uint32_t *)
+    (* This field is used to express the context in
+       which the signature is made, ensuring that a
+       signature cannot be lifted from one part of the protocol
+       to another. See `src/include/taler/taler_signatures.h` within the
+       exchange's codebase (git://taler.net/exchange). *)
+    purpose: Int32.t; (* = uint32_t *)
+  }
+end
+
+module Data = struct
+  type t = {
+    purpose: Purpose.t;
+    payloads: string list;
+  }
+end