50 lines
976 B
Go
50 lines
976 B
Go
// Common block cipher functionality shared across this library
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package block
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import (
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"crypto/cipher"
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"crypto/subtle"
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)
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const (
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// Size of an AES block in bytes
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Size = 16
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// R is the minimal irreducible polynomial for a 128-bit block size
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R = 0x87
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)
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// Block is a 128-bit array used by certain block ciphers (i.e. AES)
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type Block [Size]byte
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// Clear zeroes out the contents of the block
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func (b *Block) Clear() {
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// TODO: use a more secure zeroing method that won't be optimized away
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for i := range b {
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b[i] = 0
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}
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}
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// Dbl performs a doubling of a block over GF(2^128):
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//
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// a<<1 if firstbit(a)=0
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// (a<<1) ⊕ 0¹²⁰10000111 if firstbit(a)=1
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//
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func (b *Block) Dbl() {
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var z byte
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for i := Size - 1; i >= 0; i-- {
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zz := b[i] >> 7
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b[i] = b[i]<<1 | z
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z = zz
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}
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b[Size-1] ^= byte(subtle.ConstantTimeSelect(int(z), R, 0))
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}
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// Encrypt a block with the given block cipher
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func (b *Block) Encrypt(c cipher.Block) {
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c.Encrypt(b[:], b[:])
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}
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