Throw exception when DigestState consumed more than once (#79)

Author: 05nelsonm

library/digest/src/commonMain/kotlin/org/kotlincrypto/core/digest/Digest.kt

@@ -70,6 +70,8 @@ public expect abstract class Digest: Algorithm, Copyable<Digest>, Resettable, Up
      *     }
      *
      * @see [DigestState]
+     * @throws [IllegalStateException] If [DigestState] has already been used to instantiate
+     *   another instance of [Digest]
      * */
     protected constructor(state: DigestState)
 
@@ -123,8 +125,12 @@ public expect abstract class Digest: Algorithm, Copyable<Digest>, Resettable, Up
     protected fun compressions(): Long
 
     /**
-     * Called by the public [copy] function which produces the
-     * [DigestState] needed to create a wholly new instance.
+     * Called by the public [copy] function which produces the [DigestState]
+     * needed to create a wholly new instance.
+     *
+     * **NOTE:** [DigestState] can only be consumed once and should **NOT**
+     * be held on to. Attempting to instantiate multiple [Digest] instances
+     * with a single [DigestState] will raise an [IllegalStateException].
      * */
     protected abstract fun copy(state: DigestState): Digest
 

library/digest/src/commonMain/kotlin/org/kotlincrypto/core/digest/internal/-Buffer.kt

@@ -53,13 +53,23 @@ internal value class Buffer private constructor(internal val value: ByteArray) {
         compressCount,
         compressCountMultiplier,
     ) {
-        val buf = Buffer(buf.value.copyOf())
+        @Volatile
+        var buf: Buffer? = Buffer(buf.value.copyOf())
     }
 
     internal companion object {
 
         @JvmSynthetic
-        internal fun DigestState.buf(): Buffer = Buffer((this as State).buf.value.copyOf())
+        @Throws(IllegalStateException::class)
+        internal fun DigestState.buf(): Buffer {
+            val state = this as State
+            val buf = state.buf
+            state.buf = null
+            check(buf != null) {
+                "DigestState cannot be consumed more than once. Call copy again."
+            }
+            return buf
+        }
 
         @JvmSynthetic
         @Throws(IllegalArgumentException::class)

library/digest/src/jvmMain/kotlin/org/kotlincrypto/core/digest/Digest.kt

@@ -89,6 +89,8 @@ public actual abstract class Digest: MessageDigest, Algorithm, Cloneable, Copyab
      *     }
      *
      * @see [DigestState]
+     * @throws [IllegalStateException] If [DigestState] has already been used to instantiate
+     *   another instance of [Digest]
      * */
     protected actual constructor(state: DigestState): super(state.algorithm) {
         this.digestLength = state.digestLength
@@ -175,8 +177,12 @@ public actual abstract class Digest: MessageDigest, Algorithm, Cloneable, Copyab
     protected actual fun compressions(): Long = compressCount.commonCalculateCompressions(compressCountMultiplier)
 
     /**
-     * Called by the public [copy] function which produces the
-     * [DigestState] needed to create a wholly new instance.
+     * Called by the public [copy] function which produces the [DigestState]
+     * needed to create a wholly new instance.
+     *
+     * **NOTE:** [DigestState] can only be consumed once and should **NOT**
+     * be held on to. Attempting to instantiate multiple [Digest] instances
+     * with a single [DigestState] will raise an [IllegalStateException].
      * */
     protected actual abstract fun copy(state: DigestState): Digest
 

library/digest/src/jvmTest/kotlin/org/kotlincrypto/core/digest/internal/BufferUnitTest.kt

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2025 Matthew Nelson
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ **/
+package org.kotlincrypto.core.digest.internal
+
+import org.kotlincrypto.core.digest.internal.Buffer.Companion.buf
+import kotlin.test.Test
+import kotlin.test.assertFailsWith
+
+class BufferUnitTest {
+
+    @Test
+    fun givenBufferDigestState_whenAlreadyConsumed_thenFails() {
+        val buf = Buffer.initialize("test", 8, 0)
+        val state = buf.toState("test", 0, 0, 0, 0)
+        state.buf()
+        assertFailsWith<IllegalStateException> { state.buf() }
+    }
+}

library/digest/src/nonJvmMain/kotlin/org/kotlincrypto/core/digest/Digest.kt

@@ -87,6 +87,8 @@ public actual abstract class Digest: Algorithm, Copyable<Digest>, Resettable, Up
      *     }
      *
      * @see [DigestState]
+     * @throws [IllegalStateException] If [DigestState] has already been used to instantiate
+     *   another instance of [Digest]
      * */
     protected actual constructor(state: DigestState) {
         this.algorithm = state.algorithm
@@ -174,8 +176,12 @@ public actual abstract class Digest: Algorithm, Copyable<Digest>, Resettable, Up
     protected actual fun compressions(): Long = compressCount.commonCalculateCompressions(compressCountMultiplier)
 
     /**
-     * Called by the public [copy] function which produces the
-     * [DigestState] needed to create a wholly new instance.
+     * Called by the public [copy] function which produces the [DigestState]
+     * needed to create a wholly new instance.
+     *
+     * **NOTE:** [DigestState] can only be consumed once and should **NOT**
+     * be held on to. Attempting to instantiate multiple [Digest] instances
+     * with a single [DigestState] will raise an [IllegalStateException].
      * */
     protected actual abstract fun copy(state: DigestState): Digest