FIO11-J. Do not attempt to read raw binary data as character data

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 attempts to convert the byte array representing a BigInteger into 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 value.

BigInteger x = new BigInteger("530500452766");
byte [] byteArray = x.toByteArray(); // convert to byte array
String s = new String(byteArray);    // s prints as "{,J?z" -
                                     // the fourth character is invalid

// convert s back to a BigInteger
byteArray = s.getBytes();            // convert to bytes
x = new BigInteger(byteArray);       // now x = 530500435870

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

This 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.

BigInteger x = new BigInteger("530500452766");
String s = x.toString();  // valid character data

try {
  byte[] byteArray = s.getBytes("UTF8");
  // ns prints as "530500452766"
  String ns = new String(byteArray, "UTF8");  

  // construct the original BigInteger
  BigInteger x1 = new BigInteger(ns); 
} catch (UnsupportedEncodingException ex) {
  // 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. Do not split characters between two data structures.

Risk Assessment

Attempting to read a byte array containing raw binary data as if it were character data may produce erroneous results.

Bibliography

      12. Input Output (FIO)