block.go

Documentation: golang.org/x/crypto/xtea

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  /*
     6  	Implementation adapted from Needham and Wheeler's paper:
     7  	http://www.cix.co.uk/~klockstone/xtea.pdf
     8  
     9  	A precalculated look up table is used during encryption/decryption for values that are based purely on the key.
    10  */
    11  
    12  package xtea
    13  
    14  // XTEA is based on 64 rounds.
    15  const numRounds = 64
    16  
    17  // blockToUint32 reads an 8 byte slice into two uint32s.
    18  // The block is treated as big endian.
    19  func blockToUint32(src []byte) (uint32, uint32) {
    20  	r0 := uint32(src[0])<<24 | uint32(src[1])<<16 | uint32(src[2])<<8 | uint32(src[3])
    21  	r1 := uint32(src[4])<<24 | uint32(src[5])<<16 | uint32(src[6])<<8 | uint32(src[7])
    22  	return r0, r1
    23  }
    24  
    25  // uint32ToBlock writes two uint32s into an 8 byte data block.
    26  // Values are written as big endian.
    27  func uint32ToBlock(v0, v1 uint32, dst []byte) {
    28  	dst[0] = byte(v0 >> 24)
    29  	dst[1] = byte(v0 >> 16)
    30  	dst[2] = byte(v0 >> 8)
    31  	dst[3] = byte(v0)
    32  	dst[4] = byte(v1 >> 24)
    33  	dst[5] = byte(v1 >> 16)
    34  	dst[6] = byte(v1 >> 8)
    35  	dst[7] = byte(v1 >> 0)
    36  }
    37  
    38  // encryptBlock encrypts a single 8 byte block using XTEA.
    39  func encryptBlock(c *Cipher, dst, src []byte) {
    40  	v0, v1 := blockToUint32(src)
    41  
    42  	// Two rounds of XTEA applied per loop
    43  	for i := 0; i < numRounds; {
    44  		v0 += ((v1<<4 ^ v1>>5) + v1) ^ c.table[i]
    45  		i++
    46  		v1 += ((v0<<4 ^ v0>>5) + v0) ^ c.table[i]
    47  		i++
    48  	}
    49  
    50  	uint32ToBlock(v0, v1, dst)
    51  }
    52  
    53  // decryptBlock decrypts a single 8 byte block using XTEA.
    54  func decryptBlock(c *Cipher, dst, src []byte) {
    55  	v0, v1 := blockToUint32(src)
    56  
    57  	// Two rounds of XTEA applied per loop
    58  	for i := numRounds; i > 0; {
    59  		i--
    60  		v1 -= ((v0<<4 ^ v0>>5) + v0) ^ c.table[i]
    61  		i--
    62  		v0 -= ((v1<<4 ^ v1>>5) + v1) ^ c.table[i]
    63  	}
    64  
    65  	uint32ToBlock(v0, v1, dst)
    66  }
    67
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