feat: add plugin JWT Decoder

plugins/jwt_decoder.lua

@@ -0,0 +1,80 @@
+Plugin = {
+	name = "JWT Decoder",
+	description = [[
+Decodes JWTs found in request headers and exposes the decoded payload inline.
+
+For every request header whose value looks like a JWT (three dot-separated
+base64url segments), the base64url-encoded payload is decoded and added back to
+the request as a new header:
+
+```
+Authorization: Bearer eyJ...   ->   X-JWT-Decoded-Authorization: {"sub":"123",...}
+X-Auth-Token:  eyJ...          ->   X-JWT-Decoded-X-Auth-Token:  {"sub":"123",...}
+```
+
+A leading `Bearer ` prefix is stripped from any header value before decoding,
+so custom authorization headers (`Authorization-Test`, `X-Auth-Token`, ...)
+are handled too. The decoded header is added to the outbound request, so it is visible
+in the intercept, history and replay views (and forwarded upstream).
+
+Pure Lua, no external dependencies.
+  ]],
+	on_request = { sync = true },
+}
+
+-- Reverse lookup table: base64url character -> 6-bit value (built once at load time).
+local B64_ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
+local B64_DEC = {}
+for i = 1, #B64_ALPHABET do
+	B64_DEC[B64_ALPHABET:sub(i, i)] = i - 1
+end
+
+-- Decode an unpadded base64url string. Returns the decoded string, or nil on invalid input.
+local function b64url_decode(s)
+	if #s % 4 == 1 then
+		return nil
+	end
+	local out, acc, bits = {}, 0, 0
+	for c in s:gmatch(".") do
+		local v = B64_DEC[c]
+		if not v then
+			return nil
+		end
+		acc, bits = acc * 64 + v, bits + 6
+		if bits >= 8 then
+			bits = bits - 8
+			out[#out + 1] = string.char(math.floor(acc / 2 ^ bits))
+			acc = acc % 2 ^ bits
+		end
+	end
+	return table.concat(out)
+end
+
+-- If value is a JWT, return its decoded payload
+local function decode_jwt_payload(value)
+	-- Three dot-separated base64url segments; the signature may be empty (alg:none).
+	local payload = value:match("^[A-Za-z0-9_-]+%.([A-Za-z0-9_-]+)%.[A-Za-z0-9_-]*$")
+	if not payload or #payload > 8192 then
+		return nil
+	end
+	local decoded = b64url_decode(payload)
+	if not decoded then
+		return nil
+	end
+	-- Only accept plausible JSON payloads.
+	if decoded:match("^%s*{") == nil then
+		return nil
+	end
+	decoded = decoded:gsub("[%c]+", " ")
+	return decoded
+end
+
+function on_request(req)
+	for name, value in pairs(req.headers) do
+		local token = value:gsub("^[Bb][Ee][Aa][Rr][Ee][Rr]%s+", "")
+		local payload = decode_jwt_payload(token)
+		if payload then
+			req:set_header("X-JWT-Decoded-" .. name, payload)
+		end
+	end
+end