fixes #17

src/main/java/org/cryptomator/siv/SivMode.java

@@ -51,7 +51,7 @@ public SivMode() {
 	 * @see #SivMode(BlockCipherFactory)
 	 */
 	public SivMode(final Provider jceSecurityProvider) {
-		this(ThreadLocal.withInitial(() -> new JceAesBlockCipher(jceSecurityProvider)), new JceAesCtrComputer(jceSecurityProvider));
+		this(ThreadLocals.withInitial(() -> new JceAesBlockCipher(jceSecurityProvider)), new JceAesCtrComputer(jceSecurityProvider));
 	}
 
 	/**
@@ -60,7 +60,7 @@ public SivMode(final Provider jceSecurityProvider) {
 	 * @param cipherFactory A factory method creating a Blockcipher.get(). Must use a block size of 128 bits (16 bytes).
 	 */
 	public SivMode(final BlockCipherFactory cipherFactory) {
-		this(ThreadLocal.withInitial(() -> cipherFactory.create()));
+		this(ThreadLocals.withInitial(() -> cipherFactory.create()));
 	}
 
 	private SivMode(final ThreadLocal<BlockCipher> threadLocalCipher) {

src/main/java/org/cryptomator/siv/ThreadLocals.java

@@ -0,0 +1,17 @@
+package org.cryptomator.siv;
+
+import java.util.function.Supplier;
+
+class ThreadLocals {
+
+	static <S> ThreadLocal<S> withInitial(Supplier<S> supplier) {
+		// ThreadLocal.withInitial is unavailable on Android 7.x
+		return new ThreadLocal<S>() {
+			@Override
+			protected S initialValue() {
+				return supplier.get();
+			}
+		};
+	}
+
+}

src/main/java9/org.cryptomator.siv/ThreadLocals.java

@@ -0,0 +1,11 @@
+package org.cryptomator.siv;
+
+import java.util.function.Supplier;
+
+class ThreadLocals {
+
+	static <S> ThreadLocal<S> withInitial(Supplier<S> supplier) {
+		return ThreadLocal.withInitial(supplier);
+	}
+
+}