The next helper function we will need for our key expansion is SubWord. SubWord takes an input of 4 bytes like the previous function, and returns an output of 4 bytes as well. SubWord is basically an Sbox, each bytes are checked against a look up table and replaced by their associated value. Here's the look up table:

	.0	.1	.2	.3	.4	.5	.6	.7	.8	.9	.A	.B	.C	.D	.E	.F
0.	63	7c	77	7b	f2	6b	6f	c5	30	01	67	2b	fe	d7	ab	76
1.	ca	82	c9	7d	fa	59	47	f0	ad	d4	a2	af	9c	a4	72	c0
2.	b7	fd	93	26	36	3f	f7	cc	34	a5	e5	f1	71	d8	31	15
3.	04	c7	23	c3	18	96	05	9a	07	12	80	e2	eb	27	b2	75
4.	09	83	2c	1a	1b	6e	5a	a0	52	3b	d6	b3	29	e3	2f	84
5.	53	d1	00	ed	20	fc	b1	5b	6a	cb	be	39	4a	4c	58	cf
6.	d0	ef	aa	fb	43	4d	33	85	45	f9	02	7f	50	3c	9f	a8
7.	51	a3	40	8f	92	9d	38	f5	bc	b6	da	21	10	ff	f3	d2
8.	cd	0c	13	ec	5f	97	44	17	c4	a7	7e	3d	64	5d	19	73
9.	60	81	4f	dc	22	2a	90	88	46	ee	b8	14	de	5e	0b	db
a.	e0	32	3a	0a	49	06	24	5c	c2	d3	ac	62	91	95	e4	79
b.	e7	c8	37	6d	8d	d5	4e	a9	6c	56	f4	ea	65	7a	ae	08
c.	ba	78	25	2e	1c	a6	b4	c6	e8	dd	74	1f	4b	bd	8b	8a
d.	70	3e	b5	66	48	03	f6	0e	61	35	57	b9	86	c1	1d	9e
e.	e1	f8	98	11	69	d9	8e	94	9b	1e	87	e9	ce	55	28	df
f.	8c	a1	89	0d	bf	e6	42	68	41	99	2d	0f	b0	54	bb	16

To read this table, split your input in two:

• the 4 most significant bits represent the row
• the 4 least significant bits represent the column

Examples:

• 01 → 7c
• c2 → 25
• 9e → 0b

Go ahead and implement this function. Below is the Sbox represented in Golang, that might be helpful.

var sbox_en = [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,
}