It can use text mask to bounds of text certain begin and end indices. Enjoy your data driven test via spock framework :)

TextMasker.java

public class TextMasker {

    public static String mask(String text, int begin, int end) {
        if (Objects.isNull(text)) {
            return null;
        }

        if (areIndexesInBoundOfText(text, begin, end)) {
            String firstDisplayablePart = text.substring(0, begin);
            String secondDisplayablePart = text.substring(end);
            String maskedPart = "*".repeat(end - begin);
            return firstDisplayablePart + maskedPart + secondDisplayablePart;
        }

        return "*".repeat(text.length());
    }

    private static boolean areIndexesInBoundOfText(String text, int begin, int end) {
        return begin > 0 && begin <= end && end < text.length();
    }

}

TextMaskerSpec.groovy

import spock.lang.Specification
import spock.lang.Unroll

class TextMaskerSpec extends Specification {

    @Unroll
    def "should mask text from #begin index to #end index"() {
        when:
        def maskedText = StringUtils.mask(text, begin, end)

        then:
        maskedText == expectedResult

        where:
        text               | begin | end | expectedResult
        "1234123412341234" | 6     | 12  | "123412******1234"
        "1234123412341234" | 4     | 10  | "1234******341234"
        null               | 4     | 6   | null
        ""                 | 4     | 6   | ""
        "1234"             | 6     | 3   | "****"
        "1234"             | 1     | 6   | "****"
        "1234"             | 1     | 2   | "1*34"
        "1234"             | 6     | 10  | "****"
        "1234"             | 0     | 0   | "****"
        "1234"             | -1    | 0   | "****"
        "1234"             | 1    | -1   | "****"
        "1234"             | -1    | -1   | "****"
    }

}