diff --git a/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/Flickercode.kt b/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/Flickercode.kt
new file mode 100644
index 00000000..0401eeed
--- /dev/null
+++ b/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/Flickercode.kt
@@ -0,0 +1,14 @@
+package net.dankito.fints.tan
+
+
+open class Flickercode(
+    val challenge: String,
+    val challengeLength: Int,
+    val hasControlByte: Boolean,
+    val startCodeEncoding: FlickercodeEncoding,
+    val startCodeLength: Int,
+    val startCode: String,
+    val luhnChecksum: Int,
+    val xorChecksum: String,
+    val rendered: String
+)
\ No newline at end of file
diff --git a/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/FlickercodeDecoder.kt b/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/FlickercodeDecoder.kt
index 7f86e8aa..f5582327 100644
--- a/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/FlickercodeDecoder.kt
+++ b/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/FlickercodeDecoder.kt
@@ -5,9 +5,58 @@ import kotlin.math.floor
 
 open class FlickercodeDecoder {
 
-    open fun decodeChallenge(challenge: String): String {
+    open fun decodeChallenge(challenge: String): Flickercode {
         var code = challenge.toUpperCase().replace ("[^a-fA-F0-9]", "")
 
+        val challengeLength = parseIntToHex(challenge.substring(0, 2))
+
+        val startCodeLengthByte = parseIntToHex(challenge.substring(2, 4))
+        val hasControlByte = isBitSet(startCodeLengthByte, 7)
+        val startCodeEncoding = if (isBitNotSet(startCodeLengthByte, 6)) FlickercodeEncoding.BCD else FlickercodeEncoding.ASCII
+        val startCodeLength = startCodeLengthByte and 0b00011111 // TODO: is this correct?
+
+        val controlByte = "" // TODO (there can be multiple of them!)
+
+        val startCodeStartIndex = if (hasControlByte) 6 else 4
+        val startCodeEndIndex = startCodeStartIndex + startCodeLength
+        val startCode = code.substring(startCodeStartIndex, startCodeEndIndex)
+
+        val de1 = "" // TODO
+        val de2 = "" // TODO
+        val de3 = "" // TODO
+
+        var luhnData = controlByte + startCode + de1 + de2 + de3
+        if (luhnData.length % 2 != 0) {
+            luhnData = luhnData + "F" // for Luhn checksum it's required to have full bytes // TODO: should be incorrect. E.g. controlByte has to be checked / stuffed to full byte
+        }
+
+        val luhnSum = luhnData.mapIndexed { index, char ->
+            val asNumber = char.toString().toInt()
+
+            if (index % 2 == 1) {
+                val doubled = asNumber * 2
+                return@mapIndexed (doubled / 10) + (doubled % 10)
+            }
+
+            asNumber
+        }.sum()
+
+        val luhnChecksum = 10 - (luhnSum % 10)
+
+        val countStartCodeBytes = startCodeLength / 2
+        val dataWithoutLengthAndChecksum = toHex(countStartCodeBytes, 2) + controlByte + startCode + de1 + de2 + de3 // TODO add length of de1-3 (for controlByte as well?)
+        val dataLength = (dataWithoutLengthAndChecksum.length + 2) / 2 // + 2 for checksum
+        val dataWithoutChecksum = toHex(dataLength, 2) + dataWithoutLengthAndChecksum
+        val xorByteData = dataWithoutChecksum.map { parseIntToHex(it) }
+
+        var xorChecksum = 0
+        xorByteData.forEach { xorChecksum = xorChecksum xor it }
+
+        val xorChecksumString = toHex(xorChecksum, 1)
+
+        val rendered = dataWithoutChecksum + luhnChecksum + xorChecksumString
+
+
         /* length check: first byte */
         val len = code.length / 2 - 1
         code = toHex(len, 2) + code.substring(2)
@@ -35,7 +84,7 @@ open class FlickercodeDecoder {
 
         code = code.substring(0, code.length - 1) + toHex(xorsum, 1)
 
-        return code
+        return Flickercode(challenge, challengeLength, hasControlByte, startCodeEncoding, startCodeLength, startCode, luhnChecksum, toHex(xorChecksum, 1), rendered)
     }
 
     open fun toHex(number: Int, minLength: Int): String {
@@ -93,4 +142,12 @@ open class FlickercodeDecoder {
         return Integer.parseInt(string, 16)
     }
 
+    protected open fun isBitSet(num: Int, bit: Int): Boolean {
+        return num and (1 shl bit) != 0
+    }
+
+    protected open fun isBitNotSet(num: Int, bit: Int): Boolean {
+        return num and (1 shl bit) == 0
+    }
+
 }
\ No newline at end of file
diff --git a/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/FlickercodeEncoding.kt b/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/FlickercodeEncoding.kt
new file mode 100644
index 00000000..6ad84383
--- /dev/null
+++ b/fints4javaLib/src/main/kotlin/net/dankito/fints/tan/FlickercodeEncoding.kt
@@ -0,0 +1,10 @@
+package net.dankito.fints.tan
+
+
+enum class FlickercodeEncoding {
+
+    BCD,
+
+    ASCII
+
+}
\ No newline at end of file
diff --git a/fints4javaLib/src/test/kotlin/net/dankito/fints/tan/FlickercodeDecoderTest.kt b/fints4javaLib/src/test/kotlin/net/dankito/fints/tan/FlickercodeDecoderTest.kt
new file mode 100644
index 00000000..2fcaf342
--- /dev/null
+++ b/fints4javaLib/src/test/kotlin/net/dankito/fints/tan/FlickercodeDecoderTest.kt
@@ -0,0 +1,30 @@
+package net.dankito.fints.tan
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+
+
+class FlickercodeDecoderTest {
+
+    private val underTest = FlickercodeDecoder()
+
+
+    @Test
+    fun decodeChallenge_ExampleFromChapter_C_4_1_BCD_without_ControlByte() {
+
+        // given
+        val challenge = "070A2082901998"
+
+
+        // when
+        val response = underTest.decodeChallenge(challenge)
+
+
+        // then
+        assertThat(response.startCode).isEqualTo("2082901998")
+        assertThat(response.luhnChecksum).isEqualTo(1)
+        assertThat(response.xorChecksum).isEqualTo("A")
+        assertThat(response.rendered).isEqualTo("070520829019981A")
+    }
+
+}
\ No newline at end of file