import nimcrypto

proc hkdf*(HashType: typedesc, secret, salt, info: openarray[byte], output: var openarray[byte]) =
  var ctx: HMAC[HashType]
  ctx.init(salt)
  ctx.update(secret)
  let prk = ctx.finish().data
  const hashLen = HashType.bits div 8

  var t: MDigest[HashType.bits]

  var numIters = output.len div hashLen
  if output.len mod hashLen != 0:
    inc numIters

  for i in 0 ..< numIters:
    ctx.init(prk)
    if i != 0:
      ctx.update(t.data)
    ctx.update(info)
    ctx.update([uint8(i + 1)])
    t = ctx.finish()
    let iStart = i * hashLen
    var sz = hashLen
    if iStart + sz >= output.len:
      sz = output.len - iStart
    copyMem(addr output[iStart], addr t.data, sz)

  ctx.clear()

when isMainModule:
  import stew/byteutils

  proc hextToBytes(s: string): seq[byte] =
    if s.len != 0: return hexToSeqByte(s)

  template test(constants: untyped) =
    block:
      constants

      let
        bikm = hextToBytes(IKM)
        bsalt = hextToBytes(salt)
        binfo = hextToBytes(info)
        bprk {.used.} = hextToBytes(PRK)
        bokm = hextToBytes(OKM)

      var output = newSeq[byte](L)
      hkdf(HashType, bikm, bsalt, binfo, output)
      doAssert(output == bokm)

  test: # 1
    type HashType = sha256
    const
      IKM  = "0x0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b"
      salt = "0x000102030405060708090a0b0c"
      info = "0xf0f1f2f3f4f5f6f7f8f9"
      L    = 42
   
      PRK  = "0x077709362c2e32df0ddc3f0dc47bba63" &
             "90b6c73bb50f9c3122ec844ad7c2b3e5"
      OKM  = "0x3cb25f25faacd57a90434f64d0362f2a" &
             "2d2d0a90cf1a5a4c5db02d56ecc4c5bf" &
             "34007208d5b887185865"

  test: # 2
    type HashType = sha256
    const
      IKM  =  "0x000102030405060708090a0b0c0d0e0f" &
              "101112131415161718191a1b1c1d1e1f" &
              "202122232425262728292a2b2c2d2e2f" &
              "303132333435363738393a3b3c3d3e3f" &
              "404142434445464748494a4b4c4d4e4f"
      salt =  "0x606162636465666768696a6b6c6d6e6f" &
              "707172737475767778797a7b7c7d7e7f" &
              "808182838485868788898a8b8c8d8e8f" &
              "909192939495969798999a9b9c9d9e9f" &
              "a0a1a2a3a4a5a6a7a8a9aaabacadaeaf"
      info =  "0xb0b1b2b3b4b5b6b7b8b9babbbcbdbebf" &
              "c0c1c2c3c4c5c6c7c8c9cacbcccdcecf" &
              "d0d1d2d3d4d5d6d7d8d9dadbdcdddedf" &
              "e0e1e2e3e4e5e6e7e8e9eaebecedeeef" &
              "f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff"
      L    = 82
   
      PRK  =  "0x06a6b88c5853361a06104c9ceb35b45c" &
              "ef760014904671014a193f40c15fc244"
      OKM  =  "0xb11e398dc80327a1c8e7f78c596a4934" &
              "4f012eda2d4efad8a050cc4c19afa97c" &
              "59045a99cac7827271cb41c65e590e09" &
              "da3275600c2f09b8367793a9aca3db71" &
              "cc30c58179ec3e87c14c01d5c1f3434f" &
              "1d87"

  test: # 3
    type HashType = sha256
    const
      IKM  = "0x0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b"
      salt = ""
      info = ""
      L    = 42
   
      PRK  = "0x19ef24a32c717b167f33a91d6f648bdf" &
             "96596776afdb6377ac434c1c293ccb04"
      OKM  = "0x8da4e775a563c18f715f802a063c5a31" &
             "b8a11f5c5ee1879ec3454e5f3c738d2d" &
             "9d201395faa4b61a96c8"

  test: # 4
    type HashType = sha1
    const
      IKM  = "0x0b0b0b0b0b0b0b0b0b0b0b"
      salt = "0x000102030405060708090a0b0c"
      info = "0xf0f1f2f3f4f5f6f7f8f9"
      L    = 42
   
      PRK  = "0x9b6c18c432a7bf8f0e71c8eb88f4b30baa2ba243"
      OKM  = "0x085a01ea1b10f36933068b56efa5ad81" &
             "a4f14b822f5b091568a9cdd4f155fda2" &
             "c22e422478d305f3f896"

  test: # 5
    type HashType = sha1
    const
      IKM  = "0x000102030405060708090a0b0c0d0e0f" &
             "101112131415161718191a1b1c1d1e1f" &
             "202122232425262728292a2b2c2d2e2f" &
             "303132333435363738393a3b3c3d3e3f" &
             "404142434445464748494a4b4c4d4e4f"
      salt = "0x606162636465666768696a6b6c6d6e6f" &
             "707172737475767778797a7b7c7d7e7f" &
             "808182838485868788898a8b8c8d8e8f" &
             "909192939495969798999a9b9c9d9e9f" &
             "a0a1a2a3a4a5a6a7a8a9aaabacadaeaf"
      info = "0xb0b1b2b3b4b5b6b7b8b9babbbcbdbebf" &
             "c0c1c2c3c4c5c6c7c8c9cacbcccdcecf" &
             "d0d1d2d3d4d5d6d7d8d9dadbdcdddedf" &
             "e0e1e2e3e4e5e6e7e8e9eaebecedeeef" &
             "f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff"
      L    = 82
   
      PRK  = "0x8adae09a2a307059478d309b26c4115a224cfaf6"
      OKM  = "0x0bd770a74d1160f7c9f12cd5912a06eb" &
             "ff6adcae899d92191fe4305673ba2ffe" &
             "8fa3f1a4e5ad79f3f334b3b202b2173c" &
             "486ea37ce3d397ed034c7f9dfeb15c5e" &
             "927336d0441f4c4300e2cff0d0900b52" &
             "d3b4"

  test: # 6
    type HashType = sha1
    const
      IKM  = "0x0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b"
      salt = ""
      info = ""
      L    = 42

      PRK  = "0xda8c8a73c7fa77288ec6f5e7c297786aa0d32d01"
      OKM  = "0x0ac1af7002b3d761d1e55298da9d0506" &
             "b9ae52057220a306e07b6b87e8df21d0" &
             "ea00033de03984d34918"

  test: # 7
    type HashType = sha1
    const
      IKM  = "0x0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c0c"
      salt = ""
      info = ""
      L    = 42
   
      PRK  = "0x2adccada18779e7c2077ad2eb19d3f3e731385dd"
      OKM  = "0x2c91117204d745f3500d636a62f64f0a" &
             "b3bae548aa53d423b0d1f27ebba6f5e5" &
             "673a081d70cce7acfc48"

  block:
    var output: array[5, byte]
    var secret = [0x01.byte, 0x02, 0x03]
    var salt = [0x04.byte, 0x05, 0x06]
    var info = [0x07.byte, 0x08, 0x09]
    hkdf(sha256, secret, salt, info, output)
    doAssert(@output == "D5CF839F63".hextToBytes)