GitHub 前幾天宣佈更換 API token 的格式：「Authentication token format updates are generally available」，在今年三月初的時候有先公告要換：「Authentication token format updates」。
另外昨天也解釋了換成這樣的優點：「Behind GitHub’s new authentication token formats」。
首先是 token 的字元集合變大了：
The character set changed from
[a-f0-9]to[A-Za-z0-9_]
另外是增加了 prefix 直接指出是什麼種類的 token：
The format now includes a prefix for each token type:
ghp_for Personal Access Tokensgho_for OAuth Access tokensghu_for GitHub App user-to-server tokensghs_for GitHub App server-to-server tokensghr_for GitHub App refresh tokens
另外官方目前先不會改變 token 長度 (透過字元變多增加 entropy)，但未來有打算要增加：
The length of our tokens is remaining the same for now. However, GitHub tokens will likely increase in length in future updates, so integrators should plan to support tokens up to 255 characters after June 1, 2021.
看起來當初當作 hex string 而轉成 binary 會有問題，不過就算這樣做應該也是轉的回來的。
回到好處的部份，這個作法跟 Slack 與 Stripe 類似，讓開發者或是管理者更容易辨識 token 的類型：
As we see across the industry from companies like Slack and Stripe, token prefixes are a clear way to make tokens identifiable. We are including specific 3 letter prefixes to represent each token, starting with a company signifier,
gh, and the first letter of the token type.
另外這也讓 secret scanning 的準確度更高，本來是 40 bytes 的 hex string，有機會撞到程式碼內的 SHA-1 string：
Many of our old authentication token formats are hex-encoded 40 character strings that are indistinguishable from other encoded data like SHA hashes. These have several limitations, such as inefficient or even inaccurate detection of compromised tokens for our secret scanning feature.
另外官方也建議現有的 token 換成新的格式，這樣如果真的發生洩漏，可以透過 secret scanning 偵測並通知：
We strongly encourage you to reset any personal access tokens and OAuth tokens you have. These improvements help secret scanning detection and will help you mitigate any risk to compromised tokens.
