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 non-representable 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");
// convert x to a String
byte[] byteArray = x.toByteArray();
String s = new String(byteArray);
// convert s back to a BigInteger
byteArray = s.getBytes();
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

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");
  String ns = new String(byteArray, "UTF8");  // ns prints as "530500452766"

  BigInteger x1 = new BigInteger(ns);        // construct the original BigInteger
} 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 which, 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 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)