AES encryption implementation in Golang

encryption.go

package encryption

import (
	"fmt"
	"math"
)

// Rijndael S-box as a 256-byte array
var sBox = [256]byte{
	0x63, 0x7c, 0x77, 0x7b, 0xf2, 0x6b, 0x6f, 0xc5, 0x30, 0x01, 0x67, 0x2b, 0xfe, 0xd7, 0xab, 0x76,
	0xca, 0x82, 0xc9, 0x7d, 0xfa, 0x59, 0x47, 0xf0, 0xad, 0xd4, 0xa2, 0xaf, 0x9c, 0xa4, 0x72, 0xc0,
	0xb7, 0xfd, 0x93, 0x26, 0x36, 0x3f, 0xf7, 0xcc, 0x34, 0xa5, 0xe5, 0xf1, 0x71, 0xd8, 0x31, 0x15,
	0x04, 0xc7, 0x23, 0xc3, 0x18, 0x96, 0x05, 0x9a, 0x07, 0x12, 0x80, 0xe2, 0xeb, 0x27, 0xb2, 0x75,
	0x09, 0x83, 0x2c, 0x1a, 0x1b, 0x6e, 0x5a, 0xa0, 0x52, 0x3b, 0xd6, 0xb3, 0x29, 0xe3, 0x2f, 0x84,
	0x53, 0xd1, 0x00, 0xed, 0x20, 0xfc, 0xb1, 0x5b, 0x6a, 0xcb, 0xbe, 0x39, 0x4a, 0x4c, 0x58, 0xcf,
	0xd0, 0xef, 0xaa, 0xfb, 0x43, 0x4d, 0x33, 0x85, 0x45, 0xf9, 0x02, 0x7f, 0x50, 0x3c, 0x9f, 0xa8,
	0x51, 0xa3, 0x40, 0x8f, 0x92, 0x9d, 0x38, 0xf5, 0xbc, 0xb6, 0xda, 0x21, 0x10, 0xff, 0xf3, 0xd2,
	0xcd, 0x0c, 0x13, 0xec, 0x5f, 0x97, 0x44, 0x17, 0xc4, 0xa7, 0x7e, 0x3d, 0x64, 0x5d, 0x19, 0x73,
	0x60, 0x81, 0x4f, 0xdc, 0x22, 0x2a, 0x90, 0x88, 0x46, 0xee, 0xb8, 0x14, 0xde, 0x5e, 0x0b, 0xdb,
	0xe0, 0x32, 0x3a, 0x0a, 0x49, 0x06, 0x24, 0x5c, 0xc2, 0xd3, 0xac, 0x62, 0x91, 0x95, 0xe4, 0x79,
	0xe7, 0xc8, 0x37, 0x6d, 0x8d, 0xd5, 0x4e, 0xa9, 0x6c, 0x56, 0xf4, 0xea, 0x65, 0x7a, 0xae, 0x08,
	0xba, 0x78, 0x25, 0x2e, 0x1c, 0xa6, 0xb4, 0xc6, 0xe8, 0xdd, 0x74, 0x1f, 0x4b, 0xbd, 0x8b, 0x8a,
	0x70, 0x3e, 0xb5, 0x66, 0x48, 0x03, 0xf6, 0x0e, 0x61, 0x35, 0x57, 0xb9, 0x86, 0xc1, 0x1d, 0x9e,
	0xe1, 0xf8, 0x98, 0x11, 0x69, 0xd9, 0x8e, 0x94, 0x9b, 0x1e, 0x87, 0xe9, 0xce, 0x55, 0x28, 0xdf,
	0x8c, 0xa1, 0x89, 0x0d, 0xbf, 0xe6, 0x42, 0x68, 0x41, 0x99, 0x2d, 0x0f, 0xb0, 0x54, 0xbb, 0x16,
}

func AesEncrypt(data, iv, key []byte) ([]byte, error) {
	if len(iv) != 16 {
		return nil, fmt.Errorf("iv must be of length 16, got %d", len(iv))
	}

	// Determine number of rounds based on KeySize
	numOfRounds, keys, err := expandKey(key)
	if err != nil {
		return nil, err
	}
	if len(keys) != numOfRounds+1 {
		panic(fmt.Sprintf("Recevied %d round keys for %d number of rounds", len(keys), numOfRounds))
	}

	// Convert the input data into blocks of 4x4
	blocks := convert2blocks(data, 4)

	// Add IV
	for i := range blocks {
		blocks[i] = addRoundKey(blocks[i], iv)
	}

	for i, block := range blocks {
		// Perform the rounds for each block
		for j := range numOfRounds {
			// Add round key, only on the initial round
			if j == 0 {
				block = addRoundKey(block, keys[0])
			}

			block = subBytes(block)
			block = shiftRows(block)

			// don't execute mixColumns on last round
			if j < numOfRounds-1 {
				block = mixColumns(block)
			}

			block = addRoundKey(block, keys[j+1])
		}

		blocks[i] = block
	}

	return flattenBlocks(blocks), nil
}

func convert2blocks(data []byte, blockSideLen int) [][][]byte {
	blockSize := blockSideLen * blockSideLen

	// Calculate into how many 16 byte blocks the data can be divided
	numOfBlocks := len(data) / blockSize
	if len(data)%blockSize > 0 {
		numOfBlocks++
	}

	// reserve memory for the array of blocks
	blocks := make([][][]byte, numOfBlocks)

	// Looping through each blockIdx
	for blockIdx := 0; blockIdx < numOfBlocks; blockIdx++ {
		// Arrange in column-major grid
		startIndex := blockIdx * blockSize
		endIndex := min(startIndex+blockSize, len(data))
		bytes := data[startIndex:endIndex]
		block := make([][]byte, blockSideLen)

		for i := 0; i < len(bytes); i++ {
			// initialize the row
			rowNum := i % blockSideLen
			colNum := i / blockSideLen

			if block[rowNum] == nil {
				block[rowNum] = make([]byte, blockSideLen)
			}

			block[rowNum][colNum] = bytes[i]
		}

		blocks[blockIdx] = block
	}

	return blocks
}

func min(nums ...int) int {
	min := math.MaxInt
	for _, n := range nums {
		if n < min {
			min = n
		}
	}
	return min
}

func expandKey(key []byte) (int, [][]byte, error) {
	// Calculate number of rounds
	numOfRounds, err := func(keySize int) (int, error) {
		switch keySize {
		case 16:
			return 10, nil
		case 24:
			return 12, nil
		case 32:
			return 14, nil
		default:
			return 0, fmt.Errorf("key length %d is not supported", keySize)
		}
	}(len(key))
	if err != nil {
		return 0, nil, err
	}

	wordsInEachRound := len(key) / 4
	words := make([][]byte, (numOfRounds+1)*wordsInEachRound)

	// Generate inital words
	for i := 0; i < wordsInEachRound; i++ {
		start := i * wordsInEachRound
		end := start + wordsInEachRound
		words[i] = key[start:end]
	}

	// Generate round constants
	roundConstants := generateRcons(numOfRounds)

	// Perform the rounds
	for i := range numOfRounds {
		for j := range wordsInEachRound {
			idx := ((i + 1) * wordsInEachRound) + j

			// if word index is multiple of key length, then perform special transformation
			if idx%wordsInEachRound == 0 {
				words[idx] = xorBytes(g(words[idx-1], roundConstants[i]), words[idx-wordsInEachRound])
			} else {
				words[idx] = xorBytes(words[idx-1], words[idx-wordsInEachRound])
			}
		}
	}

	// Group words into 4 to make keys
	keys := make([][]byte, numOfRounds+1)
	for i := range keys {
		rkey := make([]byte, 16)
		wordsSlice := words[i*4 : i*4+4]
		for j := range rkey {
			rkey[j] = wordsSlice[j/4][j%4]
		}
		keys[i] = rkey
	}

	return numOfRounds, keys, nil
}

func subBytes(block [][]byte) [][]byte {
	for i := range block {
		for j := range block[i] {
			block[i][j] = substitute(block[i][j])
		}
	}

	return block
}

func shiftRows(block [][]byte) [][]byte {
	for i := range block {
		block[i] = shiftRow(block[i], i)
	}
	return block
}

func shiftRow(row []byte, extent int) []byte {
	for i := 0; i < extent; i++ {
		b0 := row[0]
		for i, b := range row {
			if i == 0 {
				continue
			} else {
				row[i-1] = b
			}

			if i == len(row)-1 {
				row[i] = b0
			}
		}
	}

	return row
}

func mixColumns(block [][]byte) [][]byte {
	for i := 0; i < 4; i++ {
		col := make([]byte, 4)
		for j := range block {
			col[j] = block[j][i]
		}
		col = mixColumn(col)
		for j := range block {
			block[j][i] = col[j]
		}
	}

	return block
}

// MixColumn applies the MixColumns transformation to a single column
func mixColumn(column []byte) []byte {
	return []byte{
		gfMul(column[0], 0x02) ^ gfMul(column[1], 0x03) ^ column[2] ^ column[3],
		column[0] ^ gfMul(column[1], 0x02) ^ gfMul(column[2], 0x03) ^ column[3],
		column[0] ^ column[1] ^ gfMul(column[2], 0x02) ^ gfMul(column[3], 0x03),
		gfMul(column[0], 0x03) ^ column[1] ^ column[2] ^ gfMul(column[3], 0x02),
	}
}

func addRoundKey(block [][]byte, key []byte) [][]byte {
	if len(key) != 16 {
		panic(fmt.Sprintf("Key must of of 16 bytes, got %d bytes", len(key)))
	}

	if len(block) != 4 {
		panic(fmt.Sprintf("block must have 4 rows, got %d", len(block)))
	}

	for i := range block {
		if len(block[i]) != 4 {
			panic(fmt.Sprintf("block must have 4 columns, got %d", len(block[i])))
		}

		for j := range block[i] {

			idx := i + j*4
			block[i][j] ^= key[idx]
		}
	}

	return block
}

func substitute(b byte) byte {
	return sBox[b]
}

func g(word []byte, rcon byte) []byte {
	if len(word) != 4 {
		panic(fmt.Sprintf("word must be of 4 bytes length, give length %d", len(word)))
	}

	// Shift left
	word = shiftRow(word, 1)

	// Substitue
	for i := range word {
		word[i] = substitute(word[i])
	}

	// Xor with RCon
	word[0] ^= rcon

	return word
}

// generateRcons generates Rcon values for the specified number of rounds in AES
func generateRcons(rounds int) []byte {
	rcon := make([]byte, rounds)
	rcon[0] = 0x01 // The first Rcon value

	for i := 1; i < rounds; i++ {
		// Double the previous Rcon value
		rcon[i] = rcon[i-1] << 1

		// If the value is greater than 0xFF, apply modular reduction
		if rcon[i] > 0xFF {
			rcon[i] ^= 0x1b // XOR with 0x1b (0x11b minus 0x100) for reduction in GF(2^8)
		}
	}

	return rcon
}

func xorBytes(b1, b2 []byte) []byte {
	if len(b1) != len(b2) {
		panic(fmt.Sprintf("When xor'ing two array of bytes, both arrays should be of the same length. Got lengths: (%d, %d)", len(b1), len(b2)))
	}

	res := make([]byte, len(b1))
	for i := range b1 {
		res[i] = b1[i] ^ b2[i]
	}

	return res
}

// Galois Field Multiply a byte by 2 in GF(2^8)
func gfMul(a byte, multiplier byte) byte {
	switch multiplier {
	case 0x01:
		return a
	case 0x02:
		if a&0x80 != 0 {
			return (a << 1) ^ 0x1b
		}
		return a << 1
	case 0x03:
		return gfMul(a, 0x02) ^ a
	}
	return 0
}

func flattenBlocks(blocks [][][]byte) []byte {
	var combined []byte

	for _, block := range blocks {
		for _, word := range block {
			combined = append(combined, word...) // Flatten each word into the combined array
		}
	}

	return combined
}

Note: It doesn't work :(