(Tcl) Implementing DPoP with OAuth2 Client Credentials

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Demonstrating Proof-of-Possession (DPoP) is an OAuth 2.0 extension (RFC 9449) that secures access tokens by binding them to a specific client’s private key, preventing stolen token misuse. It requires the client to sign a JWT (DPoP Proof) with every request, proving they hold the private key.

This example shows how to use Chilkat v11.4.0 or later to automatically add the following headers to HTTP requests:

Authorization: DPoP <access_token>
DPoP: <new JWT signed with same key>

Note: This example requires Chilkat v11.4.0 or greater.

Chilkat Tcl Extension Downloads

Chilkat Tcl Extension Downloads

load ./chilkat.dll

set success 0

set privKey [new_CkPrivateKey]

# Load your ECDSA private key.  (Keys uses with DPoP must always be ECDSA keys.  Not RSA.)
set success [CkPrivateKey_LoadAnyFormatFile $privKey "qa_data/pem/ecc_privKey.pem" ""]
if {$success == 0} then {
    puts [CkPrivateKey_lastErrorText $privKey]
    delete_CkPrivateKey $privKey
    exit
}

set tokenEndpoint "https://demo.duendesoftware.com/connect/token"
set apiEndpoint "https://demo.duendesoftware.com/api/dpop/test"

# Provide the details needed for Chilkat to automatically fetch the access token
# using the client_credentials OAuth2 flow.
set json [new_CkJsonObject]

CkJsonObject_UpdateString $json "client_secret" "secret"
CkJsonObject_UpdateString $json "client_id" "m2m.dpop"
CkJsonObject_UpdateString $json "token_endpoint" $tokenEndpoint
CkJsonObject_UpdateString $json "scope" "api"

set http [new_CkHttp]

CkHttp_put_SessionLogFilename $http "c:/aaworkarea/sessionLog.txt"

# Provide private key to be used with DPoP
set success [CkHttp_SetDPoPKey $http $privKey]
if {$success == 0} then {
    puts [CkHttp_lastErrorText $http]
    delete_CkPrivateKey $privKey
    delete_CkJsonObject $json
    delete_CkHttp $http
    exit
}

CkHttp_put_AuthToken $http [CkJsonObject_emit $json]

# When we make the first request to an API endpoint, we don't yet have
# an OAuth2 access token.  Chilkat (internally) will use the token endpoint
# to automatically get an access token.
# Chilkat will then add the following headers to the request sent to the API endpoint:

# Authorization: DPoP <access_token>
# DPoP: <new JWT signed with same key>

# For subsequent requests using the same Chilkat HTTP object instance,
# the same access token will be used with a newly generated DPoP header for each request.
# When the access_token is expired or about to expire, Chilkat will automatically (internally)
# refresh the access token.  Your application does not need to be concerned with the 
# complexities of fetching the access token or managing the refresh, or generating
# the DPoP header for each request.

set resp [CkHttp_quickGetStr $http $apiEndpoint]
if {[CkHttp_get_LastMethodSuccess $http] == 0} then {
    puts [CkHttp_lastErrorText $http]
    delete_CkPrivateKey $privKey
    delete_CkJsonObject $json
    delete_CkHttp $http
    exit
}

set statusCode [CkHttp_get_LastStatus $http]

puts "response status code = $statusCode"
puts "$resp"

# Here is a Session log showing the raw HTTP request(s) and response(s).
# Notice how Chilkat automatically fetched the access_token and then used it in the API request.

# ---- Sending Mon, 30 Mar 2026 20:09:44 GMT ----
# POST /connect/token HTTP/1.1
# Host: demo.duendesoftware.com
# Content-Type: application/x-www-form-urlencoded
# Content-Length: 79
# DPoP: ****
# 
# client_secret=secret&client_id=m2m.dpop&scope=api&grant_type=client_credentials
# ---- Received Mon, 30 Mar 2026 20:09:44 GMT ----
# HTTP/1.1 200 OK
# Date: Mon, 30 Mar 2026 20:09:43 GMT
# Content-Type: application/json; charset=UTF-8
# Transfer-Encoding: chunked
# Connection: keep-alive
# Server: Kestrel
# Cache-Control: no-store, no-cache, max-age=0
# Pragma: no-cache
# X-REVISION: 4f42f7e7
# 
# 374
# {"access_token":"eyJhbGciOiJSUzI1NiIsImtpZCI6IjE1OEUyRDQ1NzhFNDMyOUJBMzRGNzkxQTQ4Nzk5NzYwIiwieDV0IjoiczkyM2RnYnhWbnhoaTJSZmR1cWZyZXZIYUUwIi
# widHlwIjoiYXQrand0In0.eyJpc3MiOiJodHRwczovL2RlbW8uZHVlbmRlc29mdHdhcmUuY29tIiwibmJmIjoxNzc0OTAxMzgzLCJpYXQiOjE3NzQ5MDEzODMsImV4cCI6MTc3NDkwN
# Dk4MywiYXVkIjoiYXBpIiwiY25mIjp7ImprdCI6IlVMR2J4ZXdNOEZUNDhTd3hKeUY3YnJuMDZWMDdPZ3VpdW41d1ZzSVVENEUifSwic2NvcGUiOlsiYXBpIl0sImNsaWVudF9pZCI6
# Im0ybS5kcG9wIiwianRpIjoiQ0Q5MjBFQzQyOTRBOTA3QUUyRjMyQjlCREQzQTA3QzMifQ.bbq8tNMVtJp79Wc9xrAsE0h8oOfdxEKtGZ3qHpJX-mvSS4FVXLoO2bfpq1nhFryfbZkk
# ENfoIwSQjb_35aziq2NrnpWwQ4vDF1b9K-b7eJXPmJybm61DAP9HSE3ZI0jMrMTRFLtjwvE4WV6ZXNTwN0Pzn6teog1i09PZQkaGtlJEBjsBaN5N7OsQvtgFLD0sNkjy1JTw5LMNE7u
# 5-W32qv-XDk9KEGfoBkqyt-SR_A2wI7oZvl4OGnYyX8tUOFszlHZKy0_0V3Oc9wkHRM2JZDA_LfTqldJHR5CF5FuNiJ2hp-kioUA-i0gWxhqnVKVspJqQAN3Z95RDl9jSf-pYSg",
# "expires_in":3600,"token_type":"DPoP","scope":"api"}
# 0
# 
# 
# ---- Sending Mon, 30 Mar 2026 20:09:44 GMT ----
# GET /api/dpop/test HTTP/1.1
# Host: demo.duendesoftware.com
# Accept: */*
# Accept-Encoding: gzip
# Authorization: ****
# DPoP: ****
# 
# 
# ---- Received Mon, 30 Mar 2026 20:09:44 GMT ----
# HTTP/1.1 200 OK
# Date: Mon, 30 Mar 2026 20:09:44 GMT
# Content-Type: application/json; charset=utf-8
# Transfer-Encoding: chunked
# Connection: keep-alive
# Server: Kestrel
# X-REVISION: 4f42f7e7
# 
# 420
# [{"type":"iss","value":"https://demo.duendesoftware.com"},{"type":"nbf","value":"1774901383"},{"type":"iat","value":"1774901383"},
# {"type":"exp","value":"1774904983"},{"type":"aud","value":"api"},{"type":"cnf","value":"{\u0022jkt\u0022:\u0022ULGbxewM8FT48SwxJyF7brn06V07Oguiun5wVsIUD4E\u0022}"},
# {"type":"scope","value":"api"},{"type":"client_id","value":"m2m.dpop"},{"type":"jti","value":"CD920EC4294A907AE2F32B9BDD3A07C3"},
# {"type":"authorization_scheme","value":"DPoP"},{"type":"proof_token","value":"eyJhbGciOiJFU .... aGrGOgkRXLQ"}]
# 0
# 
# 

delete_CkPrivateKey $privKey
delete_CkJsonObject $json
delete_CkHttp $http