adam bien's blog

Decoding a String with invalid Base64 '_' character:

import java.nio.charset.StandardCharsets;
import java.util.Base64;
import org.junit.jupiter.api.Test;

public class Base64EncodeTest {

    @Test
    public void decodeUnderscode() {
        byte[] bytes = "CAFEHELLO_BABEDUKE".getBytes(StandardCharsets.UTF_8);
        Base64.getDecoder().decode(bytes);
    }
}

raises the java.lang.IllegalArgumentException: Illegal base64 character 5f

@Test
public void decodeMinus() {
    byte[] bytes = "CAFEHELLO-BABEDUKE".getBytes(StandardCharsets.UTF_8);
    Base64.getDecoder().decode(bytes);
}    

...leads to: java.lang.IllegalArgumentException: Illegal base64 character 2d

Both characters can be decoded with Base64.getUrlDecoder:

@Test
public void urlDecode() {
    byte[] underscored = "CAFEHELLO_BABEDUKE".getBytes(StandardCharsets.UTF_8);
    Base64.getUrlDecoder().decode(underscored);

    byte[] dashed = "CAFEHELLO-BABEDUKE".getBytes(StandardCharsets.UTF_8);
    Base64.getUrlDecoder().decode(dashed);

}