The current recoup API is broken. I guess this is another example where "trivial" API changes turn out to have (multiple!) unexpected consequences.

The current "/recoup" API does not have clear idempotency semantics, as we've discussed on the phone.  This is already bad by itself, as it makes it hard to write down what the API does other than "whatever the implementation does".

However, it actually breaks correctness in this (admittedly kinda contrived, but not impossible) case:

Say that we have a coin A obtained via withdrawal and a coin B obtained via refreshing coin A. Now the denominations of A gets revoked..

The wallet does a recoup of A for EUR:1.

Now the denomination of B also gets revoked.  The wallet recoups B (incidentally also for EUR:1) and now A can be recouped again for EUR:1.  But now the exchange is in a state where it will refuse a legitimate recoup request for A because the detection for an idempotent request kicks in.

This is IMHO bad API design, and the exchange should simply always recoup the maximum amount.

Furthermore, we usually follow the principle of "API calls that take up DB space are paid".  With the current recoup API, I can do many tiny recoup requests which the exchange then has to store, right?

I guess it would not be a big change to remove the "amount" value from the recoup/recoup-refresh request bodies, right?

- Florian

1 files changed, 0 insertions, 6 deletions

diff --git a/src/include/taler_crypto_lib.h b/src/include/taler_crypto_lib.h
index e608effa6..f1fa0285d 100644
--- a/src/include/taler_crypto_lib.h
+++ b/src/include/taler_crypto_lib.h
@@ -1839,7 +1839,6 @@ TALER_wallet_link_sign (const struct TALER_DenominationHash *h_denom_pub,
  *
  * @param h_denom_pub hash of the denomiantion public key of the coin
  * @param coin_bks blinding factor used when withdrawing the coin
- * @param requested_amount amount that is left to be recouped
  * @param coin_pub coin key of the coin to be recouped
  * @param coin_sig resulting signature
  * @return #GNUNET_OK if the signature is valid
@@ -1848,7 +1847,6 @@ enum GNUNET_GenericReturnValue
 TALER_wallet_recoup_verify (
   const struct TALER_DenominationHash *h_denom_pub,
   const union TALER_DenominationBlindingKeyP *coin_bks,
-  const struct TALER_Amount *requested_amount,
   const struct TALER_CoinSpendPublicKeyP *coin_pub,
   const struct TALER_CoinSpendSignatureP *coin_sig);
 
@@ -1866,7 +1864,6 @@ void
 TALER_wallet_recoup_sign (
   const struct TALER_DenominationHash *h_denom_pub,
   const union TALER_DenominationBlindingKeyP *coin_bks,
-  const struct TALER_Amount *requested_amount,
   const struct TALER_CoinSpendPrivateKeyP *coin_priv,
   struct TALER_CoinSpendSignatureP *coin_sig);
 
@@ -1876,7 +1873,6 @@ TALER_wallet_recoup_sign (
  *
  * @param h_denom_pub hash of the denomiantion public key of the coin
  * @param coin_bks blinding factor used when withdrawing the coin
- * @param requested_amount amount that is left to be recouped
  * @param coin_pub coin key of the coin to be recouped
  * @param coin_sig resulting signature
  * @return #GNUNET_OK if the signature is valid
@@ -1885,7 +1881,6 @@ enum GNUNET_GenericReturnValue
 TALER_wallet_recoup_refresh_verify (
   const struct TALER_DenominationHash *h_denom_pub,
   const union TALER_DenominationBlindingKeyP *coin_bks,
-  const struct TALER_Amount *requested_amount,
   const struct TALER_CoinSpendPublicKeyP *coin_pub,
   const struct TALER_CoinSpendSignatureP *coin_sig);
 
@@ -1903,7 +1898,6 @@ void
 TALER_wallet_recoup_refresh_sign (
   const struct TALER_DenominationHash *h_denom_pub,
   const union TALER_DenominationBlindingKeyP *coin_bks,
-  const struct TALER_Amount *requested_amount,
   const struct TALER_CoinSpendPrivateKeyP *coin_priv,
   struct TALER_CoinSpendSignatureP *coin_sig);