TCF Decoding Issues

[zbartholomew]

The example TCFv2 string in the documentation fails on most non IAB online decoders

COvFyGBOvFyGBAbAAAENAPCAAOAAAAAAAAAAAEEUACCKAAA.IFoEUQQgAIQwgIwQABAEAAAAOIAACAIAAAAQAIAgEAACEAAAAAgAQBAAAAAAAGBAAgAAAAAAAFAAECAAAgAAQARAEQAAAAAJAAIAAgAAAYQEAAAQmAgBC3ZAYzUw

However it "works" on the IAB decoder...BUT if you encode the decoded JSON you get a different string back.

COw12UAOw12UAAAAAAENAXCAAAAAAAAAAAAAAAAAAAAA.QFukWSAgAIQwgIUQEASFAAAAeIAACAIAQAAQAIAgEQACEABAAAgAQFAEAIAAAGBAAgAAAAQAIFAAECQAAAAAQiRAEQAAAAANAAIAAgAAAYQFAAARmggBCWQCYxUgyIAA.IFukWSAgAIQwgIUQEASFAAAAeIAACAIAQAAQAIAgEQACEABAAAgAQFAEAIAAAGBAAgAAAAQAIFAAECQAAAAAQiRAEQAAAAANAAIAAgAAAYQFAAARmggBCWQCYxUgyIAA

I've noticed that we have been receiving similar TCFv2 strings that will be decoded without error using the IAB GPP/TCFv2 decoders, but encoding them again returns a different string. Using our decoders built off the spec usually returns.

Another example using the https://www.uniconsent.com/tcf-decoder decoder with the other example string provided in the docs

COw4XqLOw4XqLAAAAAENAXCAAAAAAAAAAAAAAAAAAAAA.IFukWSQgAIQwgI0QEByFAAAAeIAACAIgSAAQAIAgEQACEABAAAgAQFAEAIAAAGBAAgAAAAQAIFAAMCQAAgAAQiRAEQAAAAANAAIAAggAIYQFAAARmggBC3ZCYzU2yIA.QFukWSQgAIQwgI0QEByFAAAAeIAACAIgSAAQAIAgEQACEABAAAgAQFAEAIAAAGBAAgAAAAQAIFAAMCQAAgAAQiRAEQAAAAANAAIAAggAIYQFAAARmggBC3ZCYzU2yIA.YAAAAAAAAAAAAAAAAAA

fails with

Danger
Decode failed with the following error:
invalid value 0 passed for cmpId

Am I missing something in the docs that could explain this?

[HeinzBaumann]

@zbartholomew Thank you for reporting this. It looks like we never update the actual TC string examples in the specification. We will need to this to correct the issues you listed.

[janwinkler]

@zbartholomew please note that strings can be encoded in multiple ways, e.g. the vendor consent section can be range or bitfield.

Without having a deeper look at the string itself you can already tell from decoding of the first string (COvFyGBOvFyGBAbAAAENAPCAAOAAAAAAAAAAAEEUACCKAAA.IFoEUQQgAIQwgIwQABAEAAAAOIAACAIAAAAQAIAgEAACEAAAAAgAQBAAAAAAAGBAAgAAAAAAAFAAECAAAgAAQARAEQAAAAAJAAIAAgAAAYQEAAAQmAgBC3ZAYzUw) that something is wrong:

• the string is from Feb 2020?
• policy version is 2, the string should have been updated
• string is not service specific (=global scope) which was removed in 2021

[janwinkler]

PS: the encoded string you sent also seems wrong: the second segment should never start with a Q

[zbartholomew]

These strings are directly from the IAB docs. The note about the second string bring wrong indicates the online encoder is incorrect since that is how that string was created.