Different base64 encoding validation in java.util and apach-commons

Question

Let's consider the following codebase:

val str = "contextId"
println(org.apache.commons.codec.binary.Base64.isBase64(str))
String(java.util.Base64.getDecoder().decode(str))

It prints:

true
Exception in thread "main" java.lang.IllegalArgumentException: Last unit does not have enough valid bits
    at java.base/java.util.Base64$Decoder.decode0(Base64.java:867)
    at java.base/java.util.Base64$Decoder.decode(Base64.java:566)
    at java.base/java.util.Base64$Decoder.decode(Base64.java:589)
     ...

It looks contradictory for me.

Could you please explain this behaviour?

Federico klez Culloca · Accepted Answer · 2024-02-09 10:53:33Z

1

The documentation for isBase64 states (emphasis mine)

Tests a given String to see if it contains only valid characters within the Base64 alphabet.

It says nothing about the fact that the string can be decoded or has an appropriate length. Hence it returns true.

On the other hand java.util.Base64.getDecoder().decode(str) actually tries to decode the string, but fails because the string doesn't have an appropriate length.

answered Feb 9, 2024 at 10:53

Federico klez Culloca

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5 Comments

gstackoverflow Over a year ago

Well, so Is there validation method which answers if it is possible to decode in boolean format without exception handling ?

Slaw Over a year ago

@gstackoverflow Not Java/Kotlin specific, but check out stackoverflow.com/q/8571501/6395627

gstackoverflow Over a year ago

@Slaw Yes, I've read exactly it. I've taken it from stackoverflow.com/a/14217969/2674303

Federico klez Culloca Over a year ago

@gstackoverflow I'd say go with John Skeet's answer instead.

gstackoverflow Over a year ago

@Federico klez Culloca I am surprised if there are no library for that

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