diff --git a/cmd/taler-auditor-offline-signing/main.go b/cmd/taler-auditor-offline-signing/main.go
index dfbfd0c..6662f44 100644
--- a/cmd/taler-auditor-offline-signing/main.go
+++ b/cmd/taler-auditor-offline-signing/main.go
@@ -5,14 +5,15 @@ https://git.taler.net/exchange.git/tree/src/exchange-tools/taler-auditor-offline
 in order to simplify portability (to ARM f.e.).
 
 TODOs:
-  - [ ] implement TALER_exchange_offline_denom_validity_verify
-  - [ ] implement TALER_auditor_denom_validity_sign
+  - [x] implement TALER_exchange_offline_denom_validity_verify
+  - [x] implement TALER_auditor_denom_validity_sign
   - [x] implement JSON-encoding of
      - [x] hashes
      - [x] keys
      - [x] signatures
-  - factor out types and helper functions to own package codeblau.de/taler
-  - implement full functionality from origin
+  - [ ] implement a robust and zero-cost marshalling abstraction
+  - [ ] factor out types and helper functions to own package codeblau.de/taler
+  - [ ] implement full functionality from origin
 
 */
 package main
@@ -355,6 +356,13 @@ func (es *EdDSASignature) MarshalJSON() (b []byte, e error) {
 	return b, nil
 }
 
+func (es *EdDSASignature) BinaryMarshal() []byte {
+	var buf = make([]byte, len(es.R)+len(es.S))
+	copy(buf, es.R)
+	copy(buf[len(es.R):], es.S)
+	return buf
+}
+
 type RSAPublicKey rsa.PublicKey
 
 // following gnunet/src/json/json_helper.c and gnunet/src/util/crypto_rsa.c
@@ -445,7 +453,71 @@ func (h *SHA512Hash) MarshalJSON() ([]byte, error) {
 	return b, nil
 }
 
-func auditorSignDenom(denom *DenomKey, ahash SHA512Hash, master *EdDSAPublicKey, pk *ed25519.PrivateKey) (SHA512Hash, EdDSASignature) {
+func Verify(denom *DenomKey, master *EdDSAPublicKey, sig []byte) bool {
+	const TALER_SIGNATURE_MASTER_DENOMINATION_KEY_VALIDITY = 1025
+	/*
+		data := struct {
+			size            uint32
+			purpose         uint32
+			master          EdDSAPublicKey
+			start           AbsoluteTime
+			expire_withdraw AbsoluteTime
+			expire_deposit  AbsoluteTime
+			expire_legal    AbsoluteTime
+			value           Amount
+			fee_withdraw    Amount
+			fee_deposit     Amount
+			fee_refresh     Amount
+			fee_refund      Amount
+			denom_hash      SHA512Hash
+		}{
+			size:    4 + 4 + ed25519.PublicKeySize + 4*8 + 5*(8+4+len(denom.Value.Currency)) + sha512.Size,
+			purpose: TALER_SIGNATURE_MASTER_DENOMINATION_KEY_VALIDITY,
+			master:  input.Arguments.MasterPublicKey,
+		}
+	*/
+	size := 4 + // size
+		4 + // purpose
+		ed25519.PublicKeySize + // master
+		4*8 + // start and expire*
+		5*(8+4+len(denom.Value.Currency)) + // value and fee*
+		sha512.Size // denomHash
+	buf := make([]byte, size)
+
+	n := 0
+	be.PutUint32(buf[n:], uint32(size))
+	n += 4
+	be.PutUint32(buf[n:], TALER_SIGNATURE_MASTER_DENOMINATION_KEY_VALIDITY)
+	n += 4
+	copy(buf[n:], *master)
+	n += len(*master)
+	for _, v := range []uint64{
+		denom.StampStart.TMs,
+		denom.StampExpireWithdraw.TMs,
+		denom.StampExpireDeposit.TMs,
+		denom.StampExpireLegal.TMs,
+	} {
+		be.PutUint64(buf[n:], v*1000) // milli -> micro
+		n += 8
+	}
+	for _, v := range [][]byte{
+		denom.Value.Binary(),
+		denom.FeeWithdraw.Binary(),
+		denom.FeeDeposit.Binary(),
+		denom.FeeRefresh.Binary(),
+		denom.FeeRefund.Binary(),
+	} {
+		copy(buf[n:], v)
+		n += len(v)
+	}
+	bin := denom.DenomPub.Binary()
+	hash := sha512.Sum512(bin)
+	copy(buf[n:], hash[:])
+
+	return ed25519.Verify(ed25519.PublicKey(*master), buf, sig)
+}
+
+func SignDenom(denom *DenomKey, ahash SHA512Hash, master *EdDSAPublicKey, pk *ed25519.PrivateKey) (SHA512Hash, EdDSASignature) {
 
 	const TALER_SIGNATURE_AUDITOR_EXCHANGE_KEYS = 1064
 
@@ -516,18 +588,21 @@ func auditorSignDenom(denom *DenomKey, ahash SHA512Hash, master *EdDSAPublicKey,
 	return hash, EdDSASignature{R: sig[:32], S: sig[32:]}
 }
 
-func auditorSign(input *Input, url string, pk ed25519.PrivateKey) []SignOperation {
+func Sign(input *Input, url string, pk ed25519.PrivateKey) ([]SignOperation, error) {
 
 	output := make([]SignOperation, len(input.Arguments.Denoms))
 	for i, denom := range input.Arguments.Denoms {
+		if !Verify(&denom, &input.Arguments.MasterPublicKey, denom.MasterSig.BinaryMarshal()) {
+			return nil, fmt.Errorf("couldn verify denomination no. %d: %v", i+1, denom.DenomPub)
+		}
 		output[i].Operation = "auditor-sign-denomination-0"
 
-		hash, sig := auditorSignDenom(&denom, sha512.Sum512(append([]byte(url), 0)), &input.Arguments.MasterPublicKey, &pk)
+		hash, sig := SignDenom(&denom, sha512.Sum512(append([]byte(url), 0)), &input.Arguments.MasterPublicKey, &pk)
 		output[i].Arguments.HDenumPub = hash
 		output[i].Arguments.AuditorSig = sig
 	}
 
-	return output
+	return output, nil
 }
 
 var (
@@ -558,8 +633,10 @@ func main() {
 		log.Fatal(e)
 	}
 
-	// TODO: here needs to go the call to TALER_exchange_offline_denom_validity_verify
-	output := auditorSign(input, *url, pk)
+	output, err := Sign(input, *url, pk)
+	if err != nil {
+		log.Fatalf("error signing: %v", err)
+	}
 
 	enc := json.NewEncoder(os.Stdout)
 	enc.SetIndent("", "  ")