duanbanzhi4419
2018-04-22 11:14
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Go运行时中级AES加密如何工作?

I learn how AES`s implementation work in Go and I do not understand how middle rounds work while encrypt block in https://github.com/golang/go/blob/master/src/crypto/aes/block.go:

// Middle rounds shuffle using tables.
// Number of rounds is set by length of expanded key.
nr := len(xk)/4 - 2 // - 2: one above, one more below
k := 4
for r := 0; r < nr; r++ {
    t0 = xk[k+0] ^ te0[uint8(s0>>24)] ^ te1[uint8(s1>>16)] ^ te2[uint8(s2>>8)] ^ te3[uint8(s3)]
    t1 = xk[k+1] ^ te0[uint8(s1>>24)] ^ te1[uint8(s2>>16)] ^ te2[uint8(s3>>8)] ^ te3[uint8(s0)]
    t2 = xk[k+2] ^ te0[uint8(s2>>24)] ^ te1[uint8(s3>>16)] ^ te2[uint8(s0>>8)] ^ te3[uint8(s1)]
    t3 = xk[k+3] ^ te0[uint8(s3>>24)] ^ te1[uint8(s0>>16)] ^ te2[uint8(s1>>8)] ^ te3[uint8(s2)]
    k += 4
    s0, s1, s2, s3 = t0, t1, t2, t3
}

I understand that this code do SybButes, ShiftRows, MixColumns and AddRoundKey of AES, but I do not understand how this code do it by using "te0", "te1", "te2", "te3" arrays. It's precomputed arrays which are defined in https://github.com/golang/go/blob/master/src/crypto/aes/const.go.

May someone explain to me how these arrays was precomputed? Thank you so much for you help.

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