| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

Last 100 entries

Half of Americans Think Climate Change Is a Sign of the Apocalypse; Saturday Surf Sessions With Juvenile Delinquents; Ssh, tty, stdout and stderr; Feathers falling in a vacuum; Santiago 30m Bike Route; Mapa de Ciclovias en Santiago; How Unreliable is UDP?; SE Santiago 20m Bike Route; Cameron's Rap; Configuring libxml with Eclipse; Reducing Combinatorial Complexity With Occam - AI; Sentidos Comunes (Chilean Online Magazine); Hilary Mantel: The Assassination of Margaret Thatcher - August 6th 1983; NSA Interceptng Gmail During Delivery; General IIR Filters; What's happening with Scala?; Interesting (But Largely Illegible) Typeface; Retiring Essentialism; Poorest in UK, Poorest in N Europe; I Want To Be A Redneck!; Reverse Racism; The Lost Art Of Nomography; IBM Data Center (Photo); Interesting Account Of Gamma Hack; The Most Interesting Audiophile In The World; How did the first world war actually end?; Ky - Restaurant Santiago; The Black Dork Lives!; The UN Requires Unaninmous Decisions; LPIR - Steganography in Practice; How I Am 6; Clear Explanation of Verizon / Level 3 / Netflix; Teenage Girls; Formalising NSA Attacks; Switching Brakes (Tektro Hydraulic); Naim NAP 100 (Power Amp); AKG 550 First Impressions; Facebook manipulates emotions (no really); Map Reduce "No Longer Used" At Google; Removing RAID metadata; New Bike (Good Bike Shop, Santiago Chile); Removing APE Tags in Linux; Compiling Python 3.0 With GCC 4.8; Maven is Amazing; Generating Docs from a GitHub Wiki; Modular Shelves; Bash Best Practices; Good Emergency Gasfiter (Santiago, Chile); Readings in Recent Architecture; Roger Casement; Integrated Information Theory (Or Not); Possibly undefined macro AC_ENABLE_SHARED; Update on Charges; Sunburst Visualisation; Spectral Embeddings (Distances -> Coordinates); Introduction to Causality; Filtering To Help Colour-Blindness; ASUS 1015E-DS02 Too; Ready Player One; Writing Clear, Fast Julia Code; List of LatAm Novels; Running (for women); Building a Jenkins Plugin and a Jar (for Command Line use); Headphone Test Recordings; Causal Consistency; The Quest for Randomness; Chat Wars; Real-life Financial Co Without ACID Database...; Flexible Muscle-Based Locomotion for Bipedal Creatures; SQL Performance Explained; The Little Manual of API Design; Multiple Word Sizes; CRC - Next Steps; FizzBuzz; Update on CRCs; Decent Links / Discussion Community; Automated Reasoning About LLVM Optimizations and Undefined Behavior; A Painless Guide To CRC Error Detection Algorithms; Tests in Julia; Dave Eggers: what's so funny about peace, love and Starship?; Cello - High Level C Programming; autoreconf needs tar; Will Self Goes To Heathrow; Top 5 BioInformatics Papers; Vasovagal Response; Good Food in Vina; Chilean Drug Criminals Use Subsitution Cipher; Adrenaline; Stiglitz on the Impact of Technology; Why Not; How I Am 5; Lenovo X240 OpenSuse 13.1; NSA and GCHQ - Psychological Trolls; Finite Fields in Julia (Defining Your Own Number Type); Julian Assange; Starting Qemu on OpenSuse; Noisy GAs/TMs; Venezuela; Reinstalling GRUB with EFI; Instructions For Disabling KDE Indexing; Evolving Speakers

© 2006-2013 Andrew Cooke (site) / post authors (content).

Using AES CTR mode with Java / Clojure

From: andrew cooke <andrew@...>

Date: Sun, 27 May 2012 08:49:45 -0400

I needed to generate a sequence of "random" bytes and, for various reasons I
won't get into here, decided that running AES in CTR mode would be the best
solution.

Background:
https://en.wikipedia.org/wiki/Block_cipher_modes_of_operation#Counter_.28CTR.29

The general procedure is described in detail in RFC2686, along with some test
vectors (thanks!) - http://www.ietf.org/rfc/rfc3686.txt - but it wasn't clear
to me how this mapped to the Java Cipher API.

It turns out that you need to do the following:

 - create a cipher instance for AES/CTR/NoPadding
 - create the CTRBLK as described in the RFC
 - initialise the cipher with CTRBLK as the IV
 - encoding blocks of zero bytes then returns the key stream

The two main sources fo confusion are:

 - What the RFC calls the CTRBLK is what Java calls the IV (what RFC calls
   the IV is the central 8 bytes of the Java IV).

 - Java automatically increments CTRBLK on successive calls to update.

And I verified all this by testing against the 128-bit test vectors in the
RFC (that is the ONLY verification made - I have not looked at any source and
I am not an expert at this...).


Here is the equivalent Clojure code (cut+pasted from a more complete module
that I will be releasing soon - you can see the code at
https://github.com/andrewcooke/particl/blob/master/src/cl/parti/random.clj):

(ns cl.parti.random
  (:use (cl.parti utils))
  (:import javax.crypto.Cipher)
  (:import javax.crypto.spec.SecretKeySpec)
  (:import javax.crypto.spec.IvParameterSpec)
  (:import java.security.MessageDigest))


;; Constants for AES in counter mode.

(def ^:private CIPHER "AES")
(def ^:private CIPHER_SPEC (str CIPHER "/CTR/NoPadding"))
(def ^:private BLOCK_SIZE 16)
(def ^:private NONCE_SIZE 4)
(def ^:private IV_SIZE 8)
(def ^:private CTR_SIZE 4)
(def ^:private KEY_SIZE 16)

(def ^:private ^{:doc "The hash used for nonce and IV."} HASH "SHA-1")


;; ## Basic counter mode operation
;;
;; The general approach follows
;; [RFC3686](http://www.faqs.org/rfcs/rfc3686.html), except that Java
;; handles the increment of the counter.

(def ^:private ^{:doc "An array of zeroes; used as the 'plaintext' since we
want to access the key stream."}
  BLANK (byte-array BLOCK_SIZE (byte 0)))

(defn- stream-blocks
  "Run the given cipher, generating a lazy stream of blocks.  The underlying
  Java code increments the counter after each loop, generating a lazy stream
  of blocks."
  [cipher]
  (lazy-seq
    (let [block (.update cipher BLANK)]
      (cons block (stream-blocks cipher)))))

(defn- stream-bytes
  "Convert a stream of blocks to a stream of bytes.

  The first form re-calls with the head block and a zero offset.

  The second form recurses through the available bytes in the block and
  then re-calls with the remaining blocks."
  ([blocks] (stream-bytes (first blocks) 0 (rest blocks)))
  ([block i blocks]
    (lazy-seq
      (if (= i BLOCK_SIZE)
        (stream-bytes blocks)
        (cons (nth block i) (stream-bytes block (inc i) blocks))))))

(defn- init-ctrblk
  "Create a counter block with CTR set to 1 (lsb)."
  [nonce iv]
  (byte-array
    (for [i (range BLOCK_SIZE)]
      (let [j (- i NONCE_SIZE)]
        (cond
          (< i NONCE_SIZE) (nth nonce i)
          (< j IV_SIZE) (nth iv j)
          (not= i (dec BLOCK_SIZE)) (byte 0)
          :else (byte 1))))))

(defn stream-aes-ctr
  "Generate a stream of bytes from the initial data, using AES in counter
  mode.

  This is tested against the three 128-bit test vectors in
  [RFC3686](http://www.faqs.org/rfcs/rfc3686.html) - the bytes returned
  match those expected for the key stream."
  [key nonce iv]
  (let [cipher (Cipher/getInstance CIPHER_SPEC)
        key (SecretKeySpec. key CIPHER)
        ctrblk (init-ctrblk nonce iv)]
    (do (.init cipher Cipher/ENCRYPT_MODE key (IvParameterSpec. ctrblk))
      (stream-bytes (stream-blocks cipher)))))


Andrew

Comment on this post