GCD Decoder

The string is always twelve characters: a format version, four pairs by consent type and three service characters. Here is the same example split into parts:

1 3t 3t 3t 3t 5 l 1version | ad_storage | analytics_storage | ad_user_data | ad_personalization | service

A letter is a number

Every character is a number from 0 to 63 - its position in Google's alphabet. Meaning is carried by the binary bits of that number, not by the letter itself.

0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_

State codes

• 0 - nothing is known
• 1 - not set
• 2 - denied
• 3 - granted

What a pair of characters holds

• first character - type delegation (bits 3-2) and the implicitly inferred state (bits 1-0)
• second character - declared before interaction (5-4), site default (3-2), the answer from the person (1-0)

The three service characters

• tenth - bit 3 - the GPC signal; bit 2 - whether a consent manager is active; bits 1-0 - the allow_ad_personalization_signals parameter
• eleventh - Tag Manager container defaults: ad_storage (5-4), analytics_storage (3-2), ad_user_data (1-0)
• twelfth - bit 2 - whether container defaults were applied; bits 1-0 - the container default for ad_personalization

Which one wins

The person's answer outranks the site default, and the default outranks the implicitly inferred state. That is the order the decoder picks the final state in.

The formula was read off the gtag.js code (function xu) and verified by reproduction: the constant 13t3t3t3t5l1 that an unconfigured gtag sends is rebuilt character for character.