diff --git a/fints4k/src/main/kotlin/net/dankito/fints/response/ResponseParser.kt b/fints4k/src/main/kotlin/net/dankito/fints/response/ResponseParser.kt
index fc7bff78..ebe2f568 100644
--- a/fints4k/src/main/kotlin/net/dankito/fints/response/ResponseParser.kt
+++ b/fints4k/src/main/kotlin/net/dankito/fints/response/ResponseParser.kt
@@ -491,7 +491,7 @@ open class ResponseParser @JvmOverloads constructor(
 
     protected open fun parseTanResponse(segment: String, dataElementGroups: List<String>): TanResponse {
         val binaryJobHashValue = if (dataElementGroups.size > 2) parseStringToNullIfEmpty(dataElementGroups[2]) else null
-        val binaryChallengeHHD_UC = if (dataElementGroups.size > 5) parseStringToNullIfEmpty(dataElementGroups[5]) else null
+        val binaryChallengeHHD_UC = if (dataElementGroups.size > 5 && dataElementGroups[5].isNotEmpty()) dataElementGroups[5] else null
 
         return TanResponse(
             parseCodeEnum(dataElementGroups[1], TanProcess.values()),
diff --git a/fints4k/src/main/kotlin/net/dankito/fints/tan/TanImageDecoder.kt b/fints4k/src/main/kotlin/net/dankito/fints/tan/TanImageDecoder.kt
index 95ed120c..ebd9a128 100644
--- a/fints4k/src/main/kotlin/net/dankito/fints/tan/TanImageDecoder.kt
+++ b/fints4k/src/main/kotlin/net/dankito/fints/tan/TanImageDecoder.kt
@@ -20,11 +20,11 @@ open class TanImageDecoder {
 
             val mimeType = challengeHHD_UC.substring(2, mimeTypeEnd)
 
-            val imageLength = getLength(bytes[mimeTypeEnd], bytes[mimeTypeEnd + 1])
             val imageStart = mimeTypeEnd + 2
-            val imageString = challengeHHD_UC.substring(imageStart, imageStart + imageLength)
 
-            val imageBytes = imageString.toByteArray(HbciCharset.DefaultCharset)
+            // sometimes it happened that imageStart + getLength(bytes[mimeTypeEnd], bytes[mimeTypeEnd + 1])
+            // was greater than challengeHHD_UC.length + 1 -> ignore image length and simply return all bytes starting with imageStart
+            val imageBytes = bytes.copyOfRange(imageStart, bytes.size)
 
             return TanImage(mimeType, imageBytes)
         } catch (e: Exception) {
diff --git a/fints4k/src/test/kotlin/net/dankito/fints/FinTsTestBase.kt b/fints4k/src/test/kotlin/net/dankito/fints/FinTsTestBase.kt
index 32ca4318..323218c9 100644
--- a/fints4k/src/test/kotlin/net/dankito/fints/FinTsTestBase.kt
+++ b/fints4k/src/test/kotlin/net/dankito/fints/FinTsTestBase.kt
@@ -8,13 +8,21 @@ import net.dankito.fints.model.*
 import net.dankito.fints.response.segments.AccountType
 import net.dankito.fints.response.segments.ChangeTanMediaParameters
 import net.dankito.fints.response.segments.JobParameters
+import java.io.File
 import java.math.BigDecimal
+import java.nio.charset.Charset
 import java.util.*
 
 
 abstract class FinTsTestBase {
 
     companion object {
+
+        const val TestFilesFolderName = "test_files/"
+
+        const val TransactionsMt940Filename = "TransactionsMt940.txt"
+
+
         const val BankCode = "12345678"
 
         const val BankCountryCode = Laenderkennzeichen.Germany
@@ -87,4 +95,14 @@ abstract class FinTsTestBase {
         return JobParameters("", 1, 1, 1, ":0:0")
     }
 
+
+    /**
+     * testFilename has to be a file in src/test/resources/test_files/ folder
+     */
+    protected open fun loadTestFile(testFilename: String, charset: Charset = Charsets.UTF_8): String {
+        val fileStream = FinTsTestBase::class.java.classLoader.getResourceAsStream(File(TestFilesFolderName, testFilename).path)
+
+        return fileStream.reader(charset).readText()
+    }
+
 }
\ No newline at end of file
diff --git a/fints4k/src/test/kotlin/net/dankito/fints/response/ResponseParserTest.kt b/fints4k/src/test/kotlin/net/dankito/fints/response/ResponseParserTest.kt
index ea35dfdc..075c3a36 100644
--- a/fints4k/src/test/kotlin/net/dankito/fints/response/ResponseParserTest.kt
+++ b/fints4k/src/test/kotlin/net/dankito/fints/response/ResponseParserTest.kt
@@ -1,6 +1,7 @@
 package net.dankito.fints.response
 
 import net.dankito.fints.FinTsTestBase
+import net.dankito.fints.messages.HbciCharset
 import net.dankito.fints.messages.datenelemente.implementierte.Dialogsprache
 import net.dankito.fints.messages.datenelemente.implementierte.HbciVersion
 import net.dankito.fints.messages.datenelemente.implementierte.signatur.Sicherheitsfunktion
@@ -11,6 +12,7 @@ import net.dankito.fints.messages.datenelementgruppen.implementierte.signatur.Si
 import net.dankito.fints.messages.segmente.id.ISegmentId
 import net.dankito.fints.messages.segmente.id.MessageSegmentId
 import net.dankito.fints.response.segments.*
+import net.dankito.fints.tan.TanImageDecoder
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.Assert
 import org.junit.Test
@@ -102,6 +104,27 @@ class ResponseParserTest : FinTsTestBase() {
         assertThat(result.messageFeedback?.feedbacks?.map { it.message }).containsExactly("Nachricht nicht erwartet.", "Dialoginitialisierung abgebrochen.")
     }
 
+    @Test
+    fun `decode TanChallenge HHD_UC`() {
+
+        // given
+        val response = loadTestFile("Decode_TanChallengeHhdUc_WithMaskedCharacter.txt", HbciCharset.DefaultCharset)
+
+
+        // when
+        val result = underTest.parse(response)
+
+
+        // then
+        assertThat(result.successful).isTrue()
+
+        assertThat(result.tanResponse).isNotNull
+
+        val decodedChallengeHhdUc = TanImageDecoder().decodeChallenge(result.tanResponse?.challengeHHD_UC ?: "")
+        assertThat(decodedChallengeHhdUc.decodingSuccessful).isTrue()
+        assertThat(decodedChallengeHhdUc.imageBytes.size).isEqualTo(3664)
+    }
+
 
     @Test
     fun parseMessageHeader() {
diff --git a/fints4k/src/test/kotlin/net/dankito/fints/transactions/Mt940AccountTransactionsParserTest.kt b/fints4k/src/test/kotlin/net/dankito/fints/transactions/Mt940AccountTransactionsParserTest.kt
index 69fd83e2..efe03ea2 100644
--- a/fints4k/src/test/kotlin/net/dankito/fints/transactions/Mt940AccountTransactionsParserTest.kt
+++ b/fints4k/src/test/kotlin/net/dankito/fints/transactions/Mt940AccountTransactionsParserTest.kt
@@ -1,16 +1,12 @@
 package net.dankito.fints.transactions
 
+import net.dankito.fints.FinTsTestBase
 import net.dankito.fints.model.AccountData
 import org.assertj.core.api.Assertions.assertThat
 import org.junit.Test
 
-class Mt940AccountTransactionsParserTest {
 
-    companion object {
-        const val TestFilesFolderName = "test_files/"
-
-        const val TransactionsMt940FileRelativePath = TestFilesFolderName + "TransactionsMt940.txt" // TODO: place in common file
-    }
+class Mt940AccountTransactionsParserTest : FinTsTestBase() {
 
 
     private val underTest = Mt940AccountTransactionsParser()
@@ -20,8 +16,7 @@ class Mt940AccountTransactionsParserTest {
     fun parseTransactions() {
 
         // given
-        val fileStream = Mt940ParserTest::class.java.classLoader.getResourceAsStream(TransactionsMt940FileRelativePath)
-        val transactionsString = fileStream.reader().readText()
+        val transactionsString = loadTestFile(TransactionsMt940Filename)
 
 
         // when
diff --git a/fints4k/src/test/kotlin/net/dankito/fints/transactions/Mt940ParserTest.kt b/fints4k/src/test/kotlin/net/dankito/fints/transactions/Mt940ParserTest.kt
index a3563738..923b8c5f 100644
--- a/fints4k/src/test/kotlin/net/dankito/fints/transactions/Mt940ParserTest.kt
+++ b/fints4k/src/test/kotlin/net/dankito/fints/transactions/Mt940ParserTest.kt
@@ -15,9 +15,6 @@ import java.util.*
 class Mt940ParserTest : FinTsTestBase() {
 
     companion object {
-        const val TestFilesFolderName = "test_files/"
-
-        const val TransactionsMt940FileRelativePath = TestFilesFolderName + "TransactionsMt940.txt"
 
         const val Currency = "EUR"
 
@@ -134,8 +131,7 @@ class Mt940ParserTest : FinTsTestBase() {
     fun parseTransactions() {
 
         // given
-        val fileStream = Mt940ParserTest::class.java.classLoader.getResourceAsStream(TransactionsMt940FileRelativePath)
-        val transactionsString = fileStream.reader().readText()
+        val transactionsString = loadTestFile(TransactionsMt940Filename)
 
 
         // when
diff --git a/fints4k/src/test/resources/test_files/Decode_TanChallengeHhdUc_WithMaskedCharacter.txt b/fints4k/src/test/resources/test_files/Decode_TanChallengeHhdUc_WithMaskedCharacter.txt
new file mode 100644
index 00000000..5c15a027
Binary files /dev/null and b/fints4k/src/test/resources/test_files/Decode_TanChallengeHhdUc_WithMaskedCharacter.txt differ