Page History

String objects in Java are encoded in UTF-16. Java Platform is required to support other character encodings or charsets such as US-ASCII, ISO-8859-1, and UTF-8.  Errors may occur when converting between differently coded character data.  There are two general types of encoding errors. If the byte sequence is not valid for the specified charset then the input is considered malformed. If the byte sequence cannot be mapped to an equivalent character sequence then an unmappable character has been encountered.

According to the Java API  [API 2014] for the String constructors:

The behavior of this constructor when the given bytes are not valid in the given charset is unspecified.

Similarly, the description of the String.getBytes(Charset) method states:

This method always replaces malformed-input and unmappable-character sequences with this charset's default replacement byte array.

The CharsetEncoder class is used to transform character data into a sequence of bytes in a specific charset.   The input character sequence is provided in a character buffer or a series of such buffers. The output byte sequence is written to a byte buffer or a series of such buffers.  The CharsetDecoder class reverses this process by transforming a sequence of bytes in a specific charset into character data.  The input byte sequence is provided in a byte buffer or a series of such buffers, while the output character sequence is written to a character buffer or a series of such buffers.

Special care should be taken when decoding untrusted byte data to ensure that malformed input or unmappable character errors do not result in defects and vulnerabilities.  Encoding errors can also occur, for example, encoding a cryptographic key containing malformed input for transmission will result in an error. Encoding and decoding errors typically result in data corruption. In Java, byte arrays are often used to transmit raw binary data as well as character-encoded data. Attempts to read raw binary data as if it were character-encoded data often fail because some of the bytes fall outside the default or specified encoding scheme and for that reason fail to denote valid characters. For example, converting a cryptographic key containing nonrepresentable bytes to character-encoded data for transmission may result in an error.

Noncompliant Code Example

This noncompliant code example is similar to the one used in STR03-J. Do not represent numeric data as strings in that it attempts to convert the a byte array representing a BigInteger into containing the two's-complement representation of this BigInteger value to a String. Because some of the bytes do not denote valid characters, the resulting String representation loses information. Converting the String back to a BigInteger produces a different valuethe byte array contains malformed-input sequences, the behavior of the String constructor is unspecified.

import java.math.BigInteger;
import java.nio.CharBuffer;

public class CharsetConversion {
  public static void main(String[] args) {
    BigInteger x = new BigInteger("530500452766");
// convert x to a String
byte[] byteArray = x.toByteArray();
    String s = new String(byteArray);
// convert s back to a BigInteger
byteArray = s.getBytes( System.out.println(s);
x = new BigInteger(byteArray);

When this program was run on a Linux platform where the default character encoding is US-ASCII, the string s got the value {?J??, because some of the characters were unprintable. When converted back to a BigInteger, x got the value 149830058370101340468658109.

Compliant Solution

 }
}

Compliant Solution

The java.nio.charset.CharsetEncoder and java.nio.charset.CharacterDecoder provide greater control over the process.  In this compliant solution, the CharsetDecode.decode() method is used to convert the byte array containing the two's-complement representation of this BigInteger value to a CharBuffer.  Because the bytes do not represent a valid UTF-16, the input is considered malformed, and a MalformedInputException is thrownThis compliant solution first produces a String representation of the BigInteger object and then converts the String object to a byte array. This process is reversed on input. Because the textual representation in the String object was generated by the BigInteger class, it contains valid characters.

import java.math.BigInteger;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.CharacterCodingException;
import java.nio.charset.CharsetDecoder;
import java.nio.charset.MalformedInputException;
import java.nio.charset.StandardCharsets;
import java.nio.charset.UnmappableCharacterException;

public class CharsetConversion {
  public static void main(String[] args) {
    CharBuffer charBuffer;
    CharsetDecoder decoder = StandardCharsets.UTF_16.newDecoder();
    BigInteger x = new BigInteger("530500452766");
String s    byte[] byteArray = x.toStringtoByteArray();
  // valid character data
  ByteBuffer byteBuffer = ByteBuffer.wrap(byteArray);
    try {
  byte[] byteArray    charBuffer = decoder.decode(byteBuffer);
      s = charBuffer.getBytestoString("UTF8");
  // ns prints as "530500452766" System.out.println(s);
  String ns =} newcatch String(byteArray, "UTF8");  
  // construct the original BigInteger
  BigInteger x1 = new BigInteger(ns); 
(IllegalStateException e) {
      e.printStackTrace();
    } catch (MalformedInputException e) {
      e.printStackTrace();
    } catch (UnmappableCharacterException e) {
      e.printStackTrace();
    } catch (UnsupportedEncodingExceptionCharacterCodingException exe) {
  // handle error
}

Do not try to convert the String object to a byte array to obtain the original BigInteger. Character encoded data may yield a byte array that, when converted to a BigInteger, results in a completely different value.

Exceptions

FIO11-EX0: Binary data that is expected to be a valid string may be read and converted to a string. How to perform this operation securely is explained in rule IDS13-J. Use compatible encodings on both sides of file or network IO. Also see rule IDS10-J. Don't form strings containing partial characters.

Risk Assessment

    e.printStackTrace();
    }
  }
}

Risk Assessment

Malformed input or unmappable character errors can result in a loss of data integrityAttempting to read a byte array containing binary data as if it were character data can produce erroneous results.

Related Guidelines

Bibliography

Image Modified Image Modified Image Modified