diff --git a/ChangeLog b/ChangeLog
index 66905ed..e442ec6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,6 @@
+08/20/2020
+- added optional support for RFC 7636 "Proof Key for Code Exchange"
+
 04/08/2020
 - openidc.call_userinfo_endpoint will now invoke the discovery
   endpoint if this has not been done before in order to detect the URI
diff --git a/README.md b/README.md
index b09417d..5a5e83c 100644
--- a/README.md
+++ b/README.md
@@ -244,6 +244,11 @@ h2JHukolz9xf6qN61QMLSd83+kwoBr2drp6xg3eGDLIkQCQLrkY=
              --   return req
              -- end,
 
+             -- use_pkce = false,
+             -- when set to true the "Proof Key for Code Exchange" as
+             -- defined in RFC 7636 will be used. The code challenge
+             -- method will alwas be S256
+
           }
 
           -- call authenticate for OpenID Connect user authentication
diff --git a/lib/resty/openidc.lua b/lib/resty/openidc.lua
index c130be4..ddfa804 100644
--- a/lib/resty/openidc.lua
+++ b/lib/resty/openidc.lua
@@ -291,10 +291,16 @@ local function openidc_base64_url_decode(input)
     local padlen = 4 - reminder
     input = input .. string.rep('=', padlen)
   end
-  input = input:gsub('-', '+'):gsub('_', '/')
+  input = input:gsub('%-', '+'):gsub('_', '/')
   return unb64(input)
 end
 
+-- perform base64url encoding
+local function openidc_base64_url_encode(input)
+  local output = b64(input, true)
+  return output:gsub('%+', '-'):gsub('/', '_')
+end
+
 local function openidc_combine_uri(uri, params)
   if params == nil or next(params) == nil then
     return uri
@@ -310,6 +316,12 @@ local function decorate_request(http_request_decorator, req)
   return http_request_decorator and http_request_decorator(req) or req
 end
 
+local function openidc_s256(verifier)
+  local sha256 = (require 'resty.sha256'):new()
+  sha256:update(verifier)
+  return openidc_base64_url_encode(sha256:final())
+end
+
 -- send the browser of to the OP's authorization endpoint
 local function openidc_authorize(opts, session, target_url, prompt)
   local resty_random = require("resty.random")
@@ -319,6 +331,7 @@ local function openidc_authorize(opts, session, target_url, prompt)
   local state = resty_string.to_hex(resty_random.bytes(16))
   local nonce = (opts.use_nonce == nil or opts.use_nonce)
     and resty_string.to_hex(resty_random.bytes(16))
+  local code_verifier = opts.use_pkce and openidc_base64_url_encode(resty_random.bytes(32))
 
   -- assemble the parameters to the authentication request
   local params = {
@@ -341,6 +354,11 @@ local function openidc_authorize(opts, session, target_url, prompt)
     params.display = opts.display
   end
 
+  if code_verifier then
+    params.code_challenge_method = 'S256'
+    params.code_challenge = openidc_s256(code_verifier)
+  end
+
   -- merge any provided extra parameters
   if opts.authorization_params then
     for k, v in pairs(opts.authorization_params) do params[k] = v end
@@ -350,6 +368,7 @@ local function openidc_authorize(opts, session, target_url, prompt)
   session.data.original_url = target_url
   session.data.state = state
   session.data.nonce = nonce
+  session.data.code_verifier = code_verifier
   session.data.last_authenticated = ngx.time()
 
   if opts.lifecycle and opts.lifecycle.on_created then
@@ -1097,7 +1116,8 @@ local function openidc_authorization_response(opts, session)
     grant_type = "authorization_code",
     code = args.code,
     redirect_uri = openidc_get_redirect_uri(opts),
-    state = session.data.state
+    state = session.data.state,
+    code_verifier = session.data.code_verifier
   }
 
   log(DEBUG, "Authentication with OP done -> Calling OP Token Endpoint to obtain tokens")
@@ -1117,9 +1137,10 @@ local function openidc_authorization_response(opts, session)
 
   -- mark this sessions as authenticated
   session.data.authenticated = true
-  -- clear state and nonce to protect against potential misuse
+  -- clear state, nonce and code_verifier to protect against potential misuse
   session.data.nonce = nil
   session.data.state = nil
+  session.data.code_verifier = nil
   if store_in_session(opts, 'id_token') then
     session.data.id_token = id_token
   end
diff --git a/tests/spec/pkce_spec.lua b/tests/spec/pkce_spec.lua
new file mode 100644
index 0000000..be93a0c
--- /dev/null
+++ b/tests/spec/pkce_spec.lua
@@ -0,0 +1,90 @@
+local http = require('socket.http')
+local test_support = require('test_support')
+require 'busted.runner'()
+
+describe('when pkce is disabled in authorize request', function()
+  test_support.start_server()
+  teardown(test_support.stop_server)
+
+  local _, status, headers = http.request({
+    url = 'http://127.0.0.1/default/t',
+    redirect = false
+  })
+  it('there is no code_challenge parameter', function()
+    assert.falsy(string.match(headers['location'], '.*code_challenge=.*'))
+  end)
+  it('there is no code_challenge_method parameter', function()
+    assert.falsy(string.match(headers['location'], '.*code_challenge_method=.*'))
+  end)
+end)
+
+describe('when pkce is enabled in authorize request', function()
+  test_support.start_server({oidc_opts = { use_pkce = true } })
+  teardown(test_support.stop_server)
+
+  local _, status, headers = http.request({
+    url = 'http://127.0.0.1/default/t',
+    redirect = false
+  })
+  it('there is a code_challenge parameter', function()
+    assert.truthy(string.match(headers['location'], '.*code_challenge=.*'))
+  end)
+  it('there is a S256 code_challenge_method parameter', function()
+    assert.truthy(string.match(headers['location'], '.*code_challenge_method=S256.*'))
+  end)
+end)
+
+local function assert_token_endpoint_call_contains(s, case_insensitive)
+   assert.error_log_contains("Received token request: .*" ..  s .. ".*",
+                             case_insensitive)
+end
+
+local function assert_token_endpoint_call_doesnt_contain(s, case_insensitive)
+   assert.is_not.error_log_contains("Received token request: .*" ..  s .. ".*",
+                                    case_insensitive)
+end
+
+describe('when pkce is disabled and the token endpoint is invoked', function()
+  test_support.start_server()
+  teardown(test_support.stop_server)
+  test_support.login()
+  it("the request doesn't contain a code_verifier", function()
+    assert_token_endpoint_call_doesnt_contain('.*code_verifier=.*')
+  end)
+end)
+
+describe('when pkce is enabled and the token endpoint is invoked', function()
+  test_support.start_server({oidc_opts = { use_pkce = true } })
+  teardown(test_support.stop_server)
+
+  local _, _, headers = http.request({
+    url = "http://127.0.0.1/default/t",
+    redirect = false
+  })
+  local state = test_support.grab(headers, 'state')
+  local code_challenge = test_support.grab(headers, 'code_challenge')
+  test_support.register_nonce(headers)
+  http.request({
+        url = "http://127.0.0.1/default/redirect_uri?code=foo&state=" .. state,
+        headers = { cookie = test_support.extract_cookies(headers) },
+        redirect = false
+  })
+
+  local log = test_support.load("/tmp/server/logs/error.log")
+  local code_verifier = log:match('Received token request: .*code_verifier=([^&]-)[&,]')
+  it('the request contains a code_verifier', function()
+    assert.truthy(code_verifier)
+  end)
+  it('hashing the code verifier leads to the challenge', function()
+    local as_base64 = function(s)
+      local rem = #s % 4
+      if rem > 0 then
+        s = s .. string.rep('=', 4 - rem)
+      end
+      return s:gsub('%-', '+'):gsub('_', '/')
+    end
+    local challenge = as_base64(code_challenge)
+    local hashed_verifier = (require 'mime').b64((require 'sha2').bytes(code_verifier))
+    assert.are.equals(hashed_verifier, challenge)
+  end)
+end)
diff --git a/tests/spec/sha2.lua b/tests/spec/sha2.lua
new file mode 100644
index 0000000..4c84ccf
--- /dev/null
+++ b/tests/spec/sha2.lua
@@ -0,0 +1,133 @@
+-- Copy of https://github.com/catwell/cw-lua/blob/master/sha256/sha256.lua
+
+--- SHA-256 implementation by Pierre 'catwell' Chapuis
+--- MIT licensed (see LICENSE.txt)
+--- Only works on little endian platforms.
+
+local ffi = require "ffi"
+assert(ffi.abi("le"))
+
+local bit = require "bit"
+local band, bxor, bnot = bit.band, bit.bxor, bit.bnot
+local rshift, rrot = bit.rshift, bit.ror
+
+local ui32_8 = ffi.typeof("uint32_t[8]")
+local uchar_32 = ffi.typeof("unsigned char[32]")
+local uchar_256 = ffi.typeof("unsigned char[256]")
+local uchar_vla = ffi.typeof("unsigned char[?]")
+
+local H = ui32_8(
+  0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a,
+  0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19
+)
+
+local K = ffi.new("uint32_t[64]",
+  0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5,
+  0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
+  0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3,
+  0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
+  0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc,
+  0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
+  0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7,
+  0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
+  0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13,
+  0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
+  0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3,
+  0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
+  0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5,
+  0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
+  0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208,
+  0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2
+)
+
+local tohex = function(buf, n)
+  local x = ffi.cast("unsigned char*", buf)
+  local t = {}
+  for i=0,n-1 do t[i+1] = string.format("%02x", x[i]) end
+  return table.concat(t)
+end
+
+local write_i64 = function(buf, n)
+  local q, r = math.floor(n/2^32), n%(2^32)
+  local j = 0
+  for i=24,0,-8 do
+    buf[j] = band(rshift(q, i), 0xff)
+    j = j + 1
+  end
+  for i=24,0,-8 do
+    buf[j] = band(rshift(r, i), 0xff)
+    j = j + 1
+  end
+end
+
+local pad = function(msg) -- lua string -> buffer
+  local l0 = #msg
+  local n0 = ((56 - (l0 + 1)) % 64)
+  local l1 = l0 + 1 + n0 + 8
+  assert(l1%64 == 0, l1)
+  local nchunks = l1/64
+  local r = uchar_vla(l1)
+  ffi.copy(r, msg, l0)
+  r[l0] = 0x80
+  write_i64(r+l1-8, 8*l0)
+  return r, nchunks
+end
+
+local rcopy_32 = function(to, from, n) -- copy n 4B chunks, reversing them
+  local x = ffi.cast("unsigned char*", to)
+  local y = ffi.cast("unsigned char*", from)
+  for i=0,n-1 do
+    for j=0,3 do
+      x[4*i+j] = y[4*i+3-j]
+    end
+  end
+end
+
+local transform = function(chunk, res) -- works on a 64B chunk
+  local a, b, c, d = res[0], res[1], res[2], res[3]
+  local e, f, g, h = res[4], res[5], res[6], res[7]
+  local buf = uchar_256()
+  rcopy_32(buf, chunk,16)
+  local w = ffi.cast("uint32_t *", buf)
+  local t1, t2, s0, s1
+  for i=16,64-1 do
+    t1, t2 = w[i-15], w[i-2]
+    s0 = bxor( rrot(t1, 07), rrot(t1, 18), rshift(t1, 03) )
+    s1 = bxor( rrot(t2, 17), rrot(t2, 19), rshift(t2, 10) )
+    w[i] = w[i-16] + w[i-7] + s0 + s1 + 0LL -- 32 bit LuaJIT, see issue 1
+  end
+  for i=0,64-1 do
+    s0 = bxor( rrot(a, 02), rrot(a, 13), rrot(a, 22) )
+    s1 = bxor( rrot(e, 06), rrot(e, 11), rrot(e, 25) )
+    t1 = s1 + h + w[i] + K[i] + bxor( band(e, f), band(bnot(e), g) )
+    t2 = s0 + bxor( band(a ,b), band(a, c), band(b, c) )
+    a, b, c, d, e, f, g, h = t1+t2, a, b, c, d+t1, e, f, g
+  end
+  res[0], res[1], res[2], res[3] = res[0]+a, res[1]+b, res[2]+c, res[3]+d
+  res[4], res[5], res[6], res[7] = res[4]+e, res[5]+f, res[6]+g, res[7]+h
+end
+
+local sha256_calc = function(input)
+  local chunks, nchunks = pad(input)
+  local buf = ui32_8()
+  ffi.copy(buf, H, ffi.sizeof(ui32_8))
+  for i=0,nchunks-1 do transform(chunks+64*i, buf) end
+  local res = uchar_32()
+  rcopy_32(res, buf, 8)
+  return res
+end
+
+local sha256_bytes = function(input)
+  local r = sha256_calc(input)
+  return ffi.string(r, 32)
+end
+
+local sha256_hex = function(input)
+  local r = sha256_calc(input)
+  return tohex(r, 32)
+end
+
+return {
+  bytes = sha256_bytes,
+  hex = sha256_hex,
+}