diff --git a/src/crypto.ml b/src/crypto.ml
index 4ce582f9..dd701491 100644
--- a/src/crypto.ml
+++ b/src/crypto.ml
@@ -275,7 +275,8 @@ type rsa_pub = RsaPublicKey.t
 type rsa_sig = RsaSignature.t
 type denom_hash = Hash.DenominationHash.t
 
-(* WIP *)
+(* WIP
+   https://www.taler.net/papers/cs-thesis.pdf *)
 module FDH_RSA = struct
   open Mirage_crypto_pk