Java Glossary : base64

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base64

A way of encoding 8-bit characters using only ASCII printable characters similar to UUENCODE. UUENCODE embeds a filename where BASE64 does not. You will see BASE64 used in encoding digital certificates, in encoding user:password string in an Authorization: header for HTTP. The spec is described in RFC 2045.

Don't confuse it with x-www-form-urlencoded which is handled by java.net.URLEncoder.encode/decode.

This process of converting binary data to printable characters as sometimes called armouring because it protects the data from transport systems that meddle with, ignore or act on control characters embedded in the data.

Under the Hood

BASE64 is a scheme where 3 bytes are concatenated, then split to form 4 groups of 6-bits each; and each 6-bits gets translated to an encoded printable ASCII character, via a table lookup. An encoded string is therefore longer than the original by about 1/3. The "=" character is used to pad the end out to an even multiple of four.

Base 64 armouring uses only the characters A-Z, a-z, 0-9 and +/=. This makes it suitable for encoding binary data as SQL strings, that will work no matter what the encoding. Unfortunately + / and = all have special meaning in URLs.

Implementatinons

I have written source code for encoding/decoding BASE64 that you can download. Sun has an undocumented method called sun.misc.BASE64Encoder.encode. There is a non-public class in Java 1.4+ called java.util.prefs.Base64

armouring ¤ encoding ¤ printable ¤ Starlight Base64 ¤ UUENCODE ¤ yEnc

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