refactor(cashu): move v4 (cashuB) token codec to quartz

The NUT-00 v4 `cashuB` wire-format encoder + CBOR models lived in
`amethyst/service/cashu/v4`, even though they are pure protocol code with
no app/UI dependencies. quartz had the proof/mint primitives but no
`cashuB` string codec at all, so this was a layering gap rather than a
duplicate.

Move `V4Encoder` and the `V4Token`/`V4T`/`V4Proof`/`V4DleqProof` wire
models into `quartz/nip60Cashu/token`, rewriting the JVM-only
`java.util.Base64` to the multiplatform `kotlin.io.encoding.Base64.UrlSafe`
so the codec lives in commonMain. This unblocks sharing CashuWalletOps /
CashuWalletState into commons (V4Encoder was one of the Android-only ties).

- add kotlinx-serialization-cbor (Apache-2.0) to quartz commonMain
- repoint CashuWalletOps (encoder) and V4Parser (V4Token) imports to quartz
- add V4EncoderTest round-trip coverage in quartz commonTest

amethyst's V4Parser stays put: it returns UI types (GenericLoadable +
commons CashuToken) and now just reuses the relocated quartz V4Token.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013UMNKix4qEfiAPP9s2a4gB

amethyst/src/main/java/com/vitorpamplona/amethyst/model/nip60Cashu/CashuWalletOps.kt

@@ -20,7 +20,6 @@
  */
 package com.vitorpamplona.amethyst.model.nip60Cashu
 
-import com.vitorpamplona.amethyst.service.cashu.v4.V4Encoder
 import com.vitorpamplona.quartz.nip01Core.core.Address
 import com.vitorpamplona.quartz.nip01Core.core.Event
 import com.vitorpamplona.quartz.nip01Core.core.HexKey
@@ -51,6 +50,7 @@ import com.vitorpamplona.quartz.nip60Cashu.quote.CashuMintQuoteEvent
 import com.vitorpamplona.quartz.nip60Cashu.token.CashuProof
 import com.vitorpamplona.quartz.nip60Cashu.token.CashuTokenEvent
 import com.vitorpamplona.quartz.nip60Cashu.token.TokenContent
+import com.vitorpamplona.quartz.nip60Cashu.token.V4Encoder
 import com.vitorpamplona.quartz.nip60Cashu.wallet.CashuWalletEvent
 import com.vitorpamplona.quartz.nip61Nutzaps.info.NutzapInfoEvent
 import com.vitorpamplona.quartz.nip61Nutzaps.info.tags.NutzapMintTag

amethyst/src/main/java/com/vitorpamplona/amethyst/service/cashu/v4/V4Parser.kt

@@ -24,6 +24,7 @@ import com.vitorpamplona.amethyst.commons.model.nip60Cashu.CashuToken
 import com.vitorpamplona.amethyst.commons.model.nip60Cashu.Proof
 import com.vitorpamplona.amethyst.ui.components.GenericLoadable
 import com.vitorpamplona.quartz.nip01Core.core.toHexKey
+import com.vitorpamplona.quartz.nip60Cashu.token.V4Token
 import kotlinx.collections.immutable.ImmutableList
 import kotlinx.collections.immutable.toImmutableList
 import kotlinx.serialization.ExperimentalSerializationApi

quartz/build.gradle.kts

@@ -127,6 +127,9 @@ kotlin {
                 // Kotlin serialization for the times where we need the Json tree and performance is not that important.
                 implementation(libs.kotlinx.serialization.json)
 
+                // CBOR serialization for the NUT-00 v4 (cashuB) token wire format.
+                implementation(libs.kotlinx.serialization.cbor)
+
                 // immutable collections to avoid recomposition
                 implementation(libs.kotlinx.collections.immutable)
 

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip60Cashu/token/V4Encoder.kt

@@ -18,24 +18,25 @@
  * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
-package com.vitorpamplona.amethyst.service.cashu.v4
+package com.vitorpamplona.quartz.nip60Cashu.token
 
 import com.vitorpamplona.quartz.nip01Core.core.hexToByteArray
-import com.vitorpamplona.quartz.nip60Cashu.token.CashuProof
 import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.cbor.Cbor
 import kotlinx.serialization.encodeToByteArray
-import java.util.Base64
+import kotlin.io.encoding.Base64
+import kotlin.io.encoding.ExperimentalEncodingApi
 
 /**
- * Inverse of [V4Parser]: encode a list of NIP-60 proofs into a `cashuB` token
- * string suitable for sending out-of-band.
+ * Inverse of [com.vitorpamplona.quartz.nip60Cashu.token.V4Token] parsing:
+ * encode a list of NIP-60 proofs into a `cashuB` token string suitable for
+ * sending out-of-band.
  *
  * V4 (NUT-00) wire format:
  *   "cashuB" + Base64URL(CBOR(V4Token { m, u, d?, t: [V4T { i, p: [V4Proof] }] }))
  */
 object V4Encoder {
-    @OptIn(ExperimentalSerializationApi::class)
+    @OptIn(ExperimentalSerializationApi::class, ExperimentalEncodingApi::class)
     fun encode(
         mintUrl: String,
         proofs: List<CashuProof>,
@@ -76,7 +77,7 @@ object V4Encoder {
                 ignoreUnknownKeys = true
             }
         val bytes = cbor.encodeToByteArray(token)
-        val base64 = Base64.getUrlEncoder().withoutPadding().encodeToString(bytes)
+        val base64 = Base64.UrlSafe.withPadding(Base64.PaddingOption.ABSENT).encode(bytes)
         return "cashuB$base64"
     }
 }

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip60Cashu/token/V4Models.kt

@@ -18,7 +18,7 @@
  * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
-package com.vitorpamplona.amethyst.service.cashu.v4
+package com.vitorpamplona.quartz.nip60Cashu.token
 
 import kotlinx.serialization.ExperimentalSerializationApi
 import kotlinx.serialization.Serializable

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nip60Cashu/token/V4EncoderTest.kt

@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip60Cashu.token
+
+import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.cbor.Cbor
+import kotlinx.serialization.decodeFromByteArray
+import kotlin.io.encoding.Base64
+import kotlin.io.encoding.ExperimentalEncodingApi
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertTrue
+
+class V4EncoderTest {
+    private val mintUrl = "http://localhost:3338"
+
+    // keyset id "009a1f293253e41e" + two proofs from the NUT-00 v4 example.
+    private val proofs =
+        listOf(
+            CashuProof(
+                id = "009a1f293253e41e",
+                amount = 1,
+                secret = "acc12435e7b8484c3cf1850149218af90f716a52bf4a5ed347e48ecc13f77388",
+                c = "0244538319d5485d55bed3b29a642bee587937dab9e7a620e11e48ba4824f1cf3f",
+            ),
+            CashuProof(
+                id = "009a1f293253e41e",
+                amount = 2,
+                secret = "1323d3d4707a58ad2e23ada4e9f1f49f5a5b4ac7b708eb0d61f738f48307e8ee",
+                c = "023456aa110d84b4ac747aebd82c3b005aca50bf457ebd5737a4414fac3ae7d8c5",
+            ),
+        )
+
+    @Test
+    fun encodesCashuBPrefixAndRoundTrips() {
+        val encoded = V4Encoder.encode(mintUrl, proofs, unit = "sat", memo = null)
+        assertTrue(encoded.startsWith("cashuB"), "must carry the v4 wire prefix")
+
+        val token = decode(encoded)
+        assertEquals(mintUrl, token.m)
+        assertEquals("sat", token.u)
+        assertEquals(null, token.d)
+
+        val groups = token.t!!
+        // both proofs share a keyset id, so they collapse into a single group.
+        assertEquals(1, groups.size)
+        val decodedProofs = groups[0].p
+        assertEquals(2, decodedProofs.size)
+        assertEquals(setOf(1, 2), decodedProofs.map { it.a }.toSet())
+        assertEquals(
+            proofs.map { it.secret }.toSet(),
+            decodedProofs.map { it.s }.toSet(),
+        )
+    }
+
+    @Test
+    fun carriesMemoWhenProvided() {
+        val token = decode(V4Encoder.encode(mintUrl, proofs, memo = "minibits rules!"))
+        assertEquals("minibits rules!", token.d)
+    }
+
+    @Test
+    fun rejectsEmptyProofList() {
+        assertFailsWith<IllegalArgumentException> {
+            V4Encoder.encode(mintUrl, emptyList())
+        }
+    }
+
+    @OptIn(ExperimentalSerializationApi::class, ExperimentalEncodingApi::class)
+    private fun decode(cashuB: String): V4Token {
+        val raw = cashuB.removePrefix("cashuB")
+        val bytes = Base64.UrlSafe.withPadding(Base64.PaddingOption.PRESENT_OPTIONAL).decode(raw)
+        return Cbor { ignoreUnknownKeys = true }.decodeFromByteArray<V4Token>(bytes)
+    }
+}