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 hence 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 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?ž" -
                                     // the fourth character is invalid

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

Compliant Solution

This compliant solution first produces a String representation of the BigInteger, then converts the String to a byte array. This process is reversed on input. Because the textual representation in the String was generated by the BigInteger class, it contains valid characters.

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

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

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

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: Untrusted binary data that is expected to be a valid string may be read and converted to a string. Doing so safely is explained in IDS17-J. Specify the character encoding while performing file or network IO. Also see guideline FIO02-J. Keep track of bytes read and account for character encoding while reading data.

Risk Assessment

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

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

[[API 2006]] class String

FIO10-J. Do not let Runtime.exec() fail or block indefinitely      12. Input Output (FIO)