Java Base64编码提供与C base64编码不同的结果

Question

I try to encode this byte array:

[237, 217, 204, 218, 109, 227, 157, 145, 35, 152, 85, 142, 182, 180, 120, 8]

Using Java library org.apache.commons.codec.binary.Base64.encodeBase64 and org.bouncycastle.util.encoders.Base64.encode this is the results:

[55, 100, 110, 77, 50, 109, 51, 106, 110, 90, 69, 106, 109, 70, 87, 79, 116, 114, 82, 52, 67, 65, 61, 61]

(note the double '=' padding character at the end)

Using base64.c Copyright (c) 1995-2001 Kungliga Tekniska Högskolan (Royal Institute of Technology, Stockholm, Sweden) this is the output:

[55, 100, 110, 77, 50, 109, 51, 106, 110, 90, 69, 106, 109, 70, 87, 79, 116, 114, 82, 52, 67, 66, 72, 114]

Can anyone explain why? How can I make the Java/C library works the same way?

Answer 1

Every Base64 ASCII character holds 6 bits information (2 ⁶ = 64), so 4 Base64 characters hold 3 bytes information. You have 16 bytes, so one byte remains at the end, needing 2 Base64 characters, and to make the group up to 4 chars, two padding = s are added.

Mind: with JavaSE 8 came a class Base64 to replace several older class around.

Base64 has several fields of application, with various little changes: padding can be left out, line breaks added to limit line length, and so on. Java 8's Base64 even has an option for a non-compatible URL and file name safeversion, where + and / are replaced.

Answer 2

Base64适用于3个字节的块， = padding可以使输出大小达到3的倍数。这个填充是可选的，如果不存在那么你可以通过在尝试之前检查数组长度来手动添加它使用Java代码进行解码。