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Two Feistel rounds (one cycle) of TEA 

General  

Designers  Roger Needham, David Wheeler 
First published  1994 
Successors  XTEA 
Cipher detail  
Key sizes  128 bits 
Block sizes  64 bits 
Structure  Feistel network 
Rounds  variable; recommended 64 Feistel rounds (32 cycles) 
Best public cryptanalysis  
TEA suffers from equivalent keys (Kelsey et al., 1996) and can be broken using a relatedkey attack requiring 2^{23} chosen plaintexts and a time complexity of 2^{32}.^{[1]} 
In cryptography, the Tiny Encryption Algorithm (TEA) is a block cipher notable for its simplicity of description and implementation, typically a few lines of code. It was designed by David Wheeler and Roger Needham of the Cambridge Computer Laboratory; it was first presented at the Fast Software Encryption workshop in Leuven in 1994, and first published in the proceedings of that workshop.^{[2]}
The cipher is not subject to any patents.
Contents 
TEA operates on two 32bit unsigned integers (could be derived from a 64bit data block) and uses a 128bit key. It has a Feistel structure with a suggested 64 rounds, typically implemented in pairs termed cycles. It has an extremely simple key schedule, mixing all of the key material in exactly the same way for each cycle. Different multiples of a magic constant are used to prevent simple attacks based on the symmetry of the rounds. The magic constant, 2654435769 or 9E3779B9_{16} is chosen to be 2^{32}/ϕ, where ϕ is the golden ratio.^{[2]}
TEA has a few weaknesses. Most notably, it suffers from equivalent keys—each key is equivalent to three others, which means that the effective key size is only 126 bits.^{[3]} As a result, TEA is especially bad as a cryptographic hash function. This weakness led to a method for hacking Microsoft's Xbox game console, where the cipher was used as a hash function.^{[4]} TEA is also susceptible to a relatedkey attack which requires 2^{23} chosen plaintexts under a relatedkey pair, with 2^{32} time complexity.^{[1]} Because of these weaknesses, the XTEA cipher was designed.
The first published version of TEA was supplemented by a second version that incorporated extensions to make it more secure. Block TEA (sometimes referred to as XTEA) operates on arbitrarysize blocks in place of the 64bit blocks of the original.
A third version (XXTEA), published in 1998, described further improvements for enhancing the security of the Block TEA algorithm.
Following is an adaptation of the reference encryption and decryption routines in C, released into the public domain by David Wheeler and Roger Needham^{[2]}:
#include <stdint.h> void encrypt (uint32_t* v, uint32_t* k) { uint32_t v0=v[0], v1=v[1], sum=0, i; /* set up */ uint32_t delta=0x9e3779b9; /* a key schedule constant */ uint32_t k0=k[0], k1=k[1], k2=k[2], k3=k[3]; /* cache key */ for (i=0; i < 32; i++) { /* basic cycle start */ sum += delta; v0 += ((v1<<4) + k0) ^ (v1 + sum) ^ ((v1>>5) + k1); v1 += ((v0<<4) + k2) ^ (v0 + sum) ^ ((v0>>5) + k3); } /* end cycle */ v[0]=v0; v[1]=v1; } void decrypt (uint32_t* v, uint32_t* k) { uint32_t v0=v[0], v1=v[1], sum=0xC6EF3720, i; /* set up */ uint32_t delta=0x9e3779b9; /* a key schedule constant */ uint32_t k0=k[0], k1=k[1], k2=k[2], k3=k[3]; /* cache key */ for (i=0; i<32; i++) { /* basic cycle start */ v1 = ((v0<<4) + k2) ^ (v0 + sum) ^ ((v0>>5) + k3); v0 = ((v1<<4) + k0) ^ (v1 + sum) ^ ((v1>>5) + k1); sum = delta; } /* end cycle */ v[0]=v0; v[1]=v1; }
Note that the reference implementation acts on multibyte numeric values. The original paper does not specify how to derive the numbers it acts on from binary or other content.

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