refine extension of table refresh_commitments

1 files changed, 17 insertions, 7 deletions

diff --git a/design-documents/024-age-restriction.rst b/design-documents/024-age-restriction.rst
index 061921bc..06b34c7e 100644
--- a/design-documents/024-age-restriction.rst
+++ b/design-documents/024-age-restriction.rst
@@ -168,20 +168,27 @@ The schema for the exchange is changed as follows:
    BEGIN;
    -- Check patch versioning is in place.
    SELECT _v.register_patch('exchange-TBD', NULL, NULL);
-   --
+
+   -- Support for age restriction is marked per denomination.
    ALTER TABLE denominations
      ADD COLUMN age_restricted BOOLEAN NOT NULL DEFAULT (false);
    COMMENT ON COLUMN denominations.age_restriced
      IS 'true if this denomination can be used for age restriction';
-   --
+
+   -- During the melting phase of the refresh, the wallet has to present the 
+   -- hash value of the age commitment (only for denominations with support 
+   -- for age restriction).
    ALTER TABLE refresh_commitments
-     ADD COLUMN age_commitment BYTEA ARRAY;
-   COMMENT ON COLUMN refresh_commitments.age_commitment
-     IS 'Vector of public keys that represents a commitment to an age. MUST be NULL if in the corresponding denomination .age_restricted equals 'false', MUST NOT be NULL otherwise.`
+     ADD COLUMN age_commitment_h BYTEA CHECK (LENGTH(age_commitment_h)=64);
+   COMMENT ON COLUMN refresh_commitments.age_commitment_h
+     IS 'SHA512 hash of the age commitment of the old coin, iff the corresponding
+         denomimination has support for age restriction, NULL otherwise.';
    COMMIT;
 
-**TODO**: can the contraint (NULL/NOT NULL) on column
-``refresh_commitments.age_commitment`` be made explicit in SQL?
+Note the constraint on ``refresh_commitments.age_commitment_h``:  It can be
+NULL, but only iff the corresponding denomination (indirectly referenced via
+table ``known_coins``) has ``.age_restricted`` set to true.  This constraint
+can not be expressed reliably with SQL.
 
 Protocol changes
 ----------------
@@ -189,6 +196,9 @@ Protocol changes
 Refresh
 ~~~~~~~
 
+During the melting phase of the refresh, the wallet has to present the hash
+value of the age commitment (for denominations with support for age restriction).
+
 TODO: Extension of the cut'n-choose-protocol.