Cryptography Primitive II: Feistel Network

Feistel Networks are building blocks of many stream cipher notably DES, SwordFish etc, wherein one performs a fixed number of rounds of the feistel network, for both excryptopn and decrypton.

Feistel Network

The abbreviations are as follows,

F: Round function
K_i: Subkey for round i
L_i: Left half of the intermediate value at round i, L₀ would be the left half of clear text.
R_i: Right half of the intermediate value at round i, R₀ would be the right half of clear text.

Micheal Luby and Charles Rackoff proved that,

If the round function is a cryptographically secure pseudorandom function, then 3 rounds are sufficient to make the block cipher a pseudorandom permutation, while 4 rounds are sufficient to make it a "strong" pseudorandom permutation.

Strong psuedorandom permutation means that it remains pseudorandom even to an adversary who gets oracle access to its inverse permutation.

Note that in one round of a feistal network, only the left half of the cleartext is encrypted, while the right half is simply exchanged position with the encrypted left half.

Comments

Multimaster replication with Symmetric DS

Symmetric DS is an awesome tool for trigger based replication whcih works for all major database vendors, including but not limited to PostgreSQL, MySQL, MSSQL, Oracle and many others. Symmetric-DS is a java application and can execute on any platform on whcih JRE is available including Windows and Linux. Trigger based replication, in constrast to disk based (eg. DRBD ) or transaction log file shipping based or statement based , works by registering triggers on DMLs and sending the data thus generated to remote machines. Another very popular trigger based DB replication tool is Slony . Symmetric-DS in addition to being database agnostic also supports multi-master replication (MMR). MMR usecase involves multiple database nodes, connected in a pool with DML updates coming from any of them. This is different from the normal master-slave replication, where slaves are not expected to generate any data events, and the sole authority of database is the master. MMR requirement causes d...

RabbitMQ and SSL

RabbitMQ is an AMQP provider i.e. it can reliably queue, service and maintain messages according to a range of policies and parameters. By default, it listens to plain old TCP connections and sends and receives messages over plaintext. This feature just works "out of the box". For users who wish to use SSL over TCP aka TLS, it requires a bit more work on their part. First, let's create a bunch of certificates and sign them with our own CA. For this, we'll use easyrsa3 . Easyrsa is a CLI tool to create, sign and manage your own certification authorities. It's maintained by OpenVPN team. Download easyrsa using your native package manager i.e. yum or apt-get $cp -Rp /usr/share/easy-rsa ~/easy-rsa-3 $cd ~/easy-rsa-3 $./easyrsa init-pki $./easyrsa build-ca $./easyrsa build-server-full broker [nopass] $./easyrsa build-client-full client1 [nopass] This creates three entities (collection of private keys, public keys and certificates) for a CA, a s...

Motorola XT502 Custom ROMs

I purchased an Android Phone in the early days of it coming to Indian market and I able to afford it ( just got a job), during mid 2010. The recent and popular version was Éclairs. I just went up to the shop and bought a nice and shiny Motorola Quench XT3 (XT502). When in other places XT502 was having Andriod Donut, I got as a special offer Éclairs. I was happy as hell. The Droid A couple of year passes, and newer versions of Android came from Éclairs to Froyo to Gingerbread then to the bigger version upgrades like Honeycomb, Ice Cream Sandwich and then Jelly Bean. By the end of 2012, I was literally surviving on my Motorola with Éclairs. I had to upgrade, anyhow. Now more that Motorola denied any upgrades for Quench XT3 . A trivia, Android version names are taken from sugary desserts with lexicographic sequencing. So, I did up-gradation from Éclairs to Gingerbread using a custom ROM from Cyanogenmod . An excellent community of enthusiast who develop their own custom ROMs oft...

Cynic's Blog

Search This Blog