IdentityServer4 PKCE error: "Transformed code verifier does not match code challenge"

Question

I cannot get IdentityServer4 PKCE authorization to work using Postman.

Using online tools I create the necessary parts:

Choose a random string:

1234567890

Get its SHA-256 hash:

c775e7b757ede630cd0aa1113bd102661ab38829ca52a6422ab782862f268646

Base64 encode the hash to get the code challenge:

Yzc3NWU3Yjc1N2VkZTYzMGNkMGFhMTExM2JkMTAyNjYxYWIzODgyOWNhNTJhNjQyMmFiNzgyODYyZjI2ODY0Ng==

In the browser I navigate to the following URL, fill in my credentials and retrieve the code from the fragmented redirect URL.

GET https://localhost:5000/connect/authorize
?client_id=pkceclient
&scope=openid
&response_type=code
&redirect_uri=https://jwt.ms
&state=abc
&nonce=xyz  
&code_challenge=Yzc3NWU3Yjc1N2VkZTYzMGNkMGFhMTExM2JkMTAyNjYxYWIzODgyOWNhNTJhNjQyMmFiNzgyODYyZjI2ODY0Ng==
&code_challenge_method=S256

When redeeming the code for a token I pass the code_verifier (SHA-256 hash) but my IdentityServer logs the following error:

"Transformed code verifier does not match code challenge".

POST https://localhost:5000/connect/token
client_id=pkceclient
grant_type=authorization_code
code:-CesrmjPYjdLdDd5AviOZpR6GdjjkZia_ZapoJdGUZI
redirect_uri=https://jwt.ms
code_verifier=c775e7b757ede630cd0aa1113bd102661ab38829ca52a6422ab782862f268646

In his blog post, the author uses the following code to generate the parts.

var verifier = CryptoRandom.CreateRandomKeyString(64);
var challenge = verifier.ToCodeChallenge();

but I cannot find the code in the repositories for the ToCodeChallenge method.

Why doesn't my manually generated challenge match the one used in the verification process, what am I missing?

Answer 1

While putting this question together I came across the specification document for PKCE and found the following line:

code_challenge = BASE64URL-ENCODE(SHA256(ASCII(code_verifier)))

It turns out the ASCII part is not carried out by the online tools that I used.

Implementing the steps in code I get the following which, when substituting the values from before, passes the verification in the second step of the process.

var codeVerifier = "c775e7b757ede630cd0aa1113bd102661ab38829ca52a6422ab782862f268646";
var codeVerifierBytes = Encoding.ASCII.GetBytes(codeVerifier);
var hashedBytes = codeVerifierBytes.Sha256();
var transformedCodeVerifier = Base64Url.Encode(hashedBytes);

code_challenge: 51FaJvQFsiNdiFWIq2EMWUKeAqD47dqU_cHzJpfHl-Q

code_verifier: c775e7b757ede630cd0aa1113bd102661ab38829ca52a6422ab782862f268646

Answer 2

Following links helps to achieve the PKCE-AuthZ-Code flow.

https://auth0.com/docs/api-auth/tutorials/authorization-code-grant-pkce

https://github.com/gilbert-fernandes/S256Code/blob/master/src/S256Code.java