lib/aes.go: add KeyExpansion

Written while listening to Neil Young's Decade.

1 files changed, 97 insertions, 0 deletions

diff --git a/lib/aes.go b/lib/aes.go
new file mode 100644
index 0000000..ecc7479
--- /dev/null
+++ b/lib/aes.go
@@ -0,0 +1,97 @@
+// Copyright © 2020 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+// Returns a key schedule (44 bytes, 11 4-byte words) given a key 'k'
+// (16 bytes, 4 4-byte words).
+func KeyExpansion(k []byte) [][]byte {
+	ks := make([][]byte, 44) // key schedule
+	nk := 4
+	nb := 4
+	nr := 10
+
+	// Generate first 4 (Nk) words of the key schedule from the
+	// key 'k'
+	for i := 0; i < nk; i++ {
+		ks[i] = make([]byte, 4)
+
+		ks[i][0] = k[(4*i)+0]
+		ks[i][1] = k[(4*i)+1]
+		ks[i][2] = k[(4*i)+2]
+		ks[i][3] = k[(4*i)+3]
+	}
+
+	// Generate the rest of the key schedule.
+	for i := 4; i < (nb * (nr + 1)); i++ {
+		tmp := make([]byte, 4)
+		copy(tmp, ks[i-1])
+
+		if i%nk == 0 {
+			tmp = FixedXORBytes(SubWord(RotWord(tmp)), rcon[i/nk])
+		}
+		ks[i] = make([]byte, 4)
+		ks[i] = FixedXORBytes(ks[i-nk], tmp)
+	}
+
+	return ks
+}
+
+// Performs a cyclic permutation to the left on the 4-byte word.
+func RotWord(w []byte) []byte {
+	for i := 1; i < 4; i++ {
+		t := w[i-1]
+		w[i-1] = w[i]
+		w[i] = t
+	}
+	return w
+}
+
+// Performs S-Box transformation on the 4-byte word.
+func SubWord(w []byte) []byte {
+	sw := make([]byte, 4)
+
+	for i := 0; i < 4; i++ {
+		r := w[i] >> 4
+		c := w[i] & 0x0f
+
+		sw[i] = sbox[r][c]
+	}
+	return sw
+}
+
+// Generated using https://github.com/mvaneerde/blog/blob/061f/rijndael/s-box.pl
+var sbox [16][16]byte = [16][16]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},
+}
+
+// Round constants for 1 <= i <= 10
+// From https://en.wikipedia.org/wiki/AES_key_schedule#Rcon
+var rcon [11][]byte = [11][]byte{
+	{0x00, 0x00, 0x00, 0x00}, // i = 0; dummy
+	{0x01, 0x00, 0x00, 0x00},
+	{0x02, 0x00, 0x00, 0x00},
+	{0x04, 0x00, 0x00, 0x00},
+	{0x08, 0x00, 0x00, 0x00},
+	{0x10, 0x00, 0x00, 0x00},
+	{0x20, 0x00, 0x00, 0x00},
+	{0x40, 0x00, 0x00, 0x00},
+	{0x80, 0x00, 0x00, 0x00},
+	{0x1b, 0x00, 0x00, 0x00},
+	{0x36, 0x00, 0x00, 0x00},
+}