!!! Overview
The [{$pagename}] is specified in the [Proof Key for Code Exchange by OAuth Public Clients]


The [OAuth Client] first creates a [code_verifier], "code_verifier", for each  [OAuth 2.0] [RFC 6749] [Authorization Request], in the following manner:

[code_verifier] = high [entropy] [cryptographic|Cryptography] [random] [STRING|String] using the  Unreserved Characters [[A-Z] / [[a-z] / [[0-9] / "-" / "." / "_" / "~"  from Sec 2.3 of [RFC 3986], with a minimum length of 43 characters and a maximum length of 128 characters.

ABNF for "[{$pagename}]" is as follows.
%%prettify 
{{{
code-verifier = 43*128unreserved
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
ALPHA = %x41-5A / %x61-7A
DIGIT = %x30-39
}}} /%

NOTE: [{$pagename}] __[SHOULD]__ have enough [entropy] to make it impractical to guess the value.  It is __[RECOMMENDED]__ that the output of a suitable [random number generator|Pseudorandom generators] be used to create a 32-octet sequence.  The Octet sequence is then base64url encoded to produce a 43-octet [URL] safe string to use as the [{$pagename}].

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]