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Converting a byte array to a string has similar consequences if the byte array does not indicate a valid encoded string. 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.
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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.
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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.
Noncompliant Code Example
This noncompliant code example corrupts the data when string contains characters that are not representable in the specified charset.
// Corrupts data on errorspublic static byte[] toCodePage(String charset, String string) throws UnsupportedEncodingException { return string.getBytes(charset);}// Fails to detect corrupt datapublic static String fromCodePage(String charset, byte[] bytes) throws UnsupportedEncodingException { return new String(bytes, charset);} |
Compliant Solution
The java.nio.charset.CharsetEncoder class can transform a sequence of 16-bit Unicode characters into a sequence of bytes in a specific charset, while the java.nio.charset.CharacterDecoder class can reverse the procedure [API 2006].
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public static byte[] toCodePage(String charset, String string) throws IOException { Charset cs = Charset.forName(charset); CharsetEncoder coder = cs.newEncoder(); ByteBuffer bytebuf = coder.encode(CharBuffer.wrap(string)); byte[] bytes = new byte[bytebuf.limit()]; bytebuf.get(bytes); return bytes;}public static String fromCodePage(String charset,byte[] bytes) throws CharacterCodingException { Charset cs = Charset.forName(charset); CharsetDecoder coder = cs.newDecoder(); CharBuffer charbuf = coder.decode(ByteBuffer.wrap(bytes)); return charbuf.toString();} |
Noncompliant Code Example
This noncompliant code example attempts to append a string to a text file in the specified encoding. This is erroneous because the String may contain unrepresentable characters.
// Corrupts data on errorspublic static void toFile(String charset, String filename, String string) throws IOException { FileOutputStream stream = new FileOutputStream(filename, true); OutputStreamWriter writer = new OutputStreamWriter(stream, charset); writer.write(string, 0, string.length()); writer.close();} |
Compliant Solution
This compliant solution uses the CharsetEncoder class to perform the required function.
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Use the FileInputStream and InputStreamReader objects to read back the data from the file. InputStreamReader accepts a optional CharsetDecoder argument, which must be the same as that previously used for writing to the file.
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 IDS13STR04-J. Use compatible character encodings on both sides of file or network IO. Also see rule IDS10-J. Don't form strings containing partial characters.
Risk Assessment
Attempting to read a byte array containing binary data as if it were character data can produce erroneous results.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
FIO11STR03-J | low | unlikely | medium | P2 | L3 |
Related Guidelines
Bibliography