Package com.yahoo.security

Class BaseNCodec

java.lang.Object

com.yahoo.security.BaseNCodec

public class BaseNCodec
extends Object

Codec that enables easy conversion from an array of bytes to any numeric base in [2, 256) and back again, using a supplied custom alphabet.

Implemented by treating the input byte sequence to encode verbatim as a big-endian BigInteger and iteratively doing a divmod operation until the quotient is zero, emitting the modulus mapped onto the alphabet for each iteration.

Decoding reverses this process, ending up with the same BigInteger as in the initial encoding step.

Note that BigIntegers represent the canonical form of any given integer, which means that leading zero bytes are implicitly ignored. We therefore special-case this by unary-coding the number of leading zeroes in the encoded form, where a leading zero byte is mapped to the first character of the alphabet.

Example for Base58, which starts its alphabet with 1 (0 is not present):

   "Hello World!"     = "2NEpo7TZRRrLZSi2U"
   "\0\0Hello World!" = "112NEpo7TZRRrLZSi2U" (note leading 1s)
 

Example for Base62, which starts its alphabet with 0:

   "Hello World!"     = "T8dgcjRGkZ3aysdN"
   "\0\0Hello World!" = "00T8dgcjRGkZ3aysdN" (node leading 0s)
 

Important: runtime complexity is O(n²) for both encoding and decoding, so this should only be used to encode/decode relatively short byte sequences. This is not a replacement for Base64 etc. encoding that runs in linear time! In addition, a BaseNCodec with a Base64 alphabet encodes to a completely different output than a regular Base64 encoder when the input is not evenly divisible by three. This is due to regular Base64 explicitly handling padding, while this codec does not.

Author:

vekterli

Field Summary

Fields

Modifier and Type	Field	Description
static final int	MAX_BASE

Method Summary

Modifier and Type	Method	Description
int	base()
byte[]	decode(String input)
String	encode(byte[] input)
static BaseNCodec	of(String alphabet)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Details

MAX_BASE

public static final int MAX_BASE

See Also:

• Constant Field Values

Method Details

of

public static BaseNCodec of(String alphabet)

base

public int base()

encode

public String encode(byte[] input)

decode

public byte[] decode(String input)