Magic Bytes

Magic bytes are used as a way to identify the separate messages sent between nodes on the bitcoin network.

For example, when connecting to a node with your own code, every message you receive from that node will start with f9beb4d9, and every message you send should begin with the same magic bytes.

Bitcoin

What are the magic bytes in Bitcoin?

The magic bytes used in Bitcoin are 4 bytes in length, and they are different for each network:

Examples

Where can you find magic bytes?

This is a raw "version" message, which is the first message you'll receive when connecting to a node:

f9beb4d976657273696f6e00000000006f0000004aae42a47c11010005000000000000001436396400000000010000000000000000000000000000000000ffffc1207f8db0d0050000000000000000000000000000000000ffff8a4414c5208df66af23ecba5bd68192f5361746f7368693a302e31322e3128626974636f7265292fc8fb0b0001

And this is a raw "tx" message containing a single transaction:

f9beb4d9747800000000000000000000e00000006a86deb701000000015ac5ae0a2ba96622c9b79de2c339084c8b1d30f63bb55a315f354db4d9a6abcf010000006b4830450221009ad52459e1e8bd5e758399cc0be963c75726c5089499465d9aa79ffb304ecd3802207d73ea58047f4d1f857b400cbff725ef562b7ada1c26e763c5a1aa6d29d2fdf401210234b7b614fcc0e4d926747d491992d8cc133f076bd79095eddf60c34b0e3fef4affffffff02390205000000000017a914ea3b6d7e92e05370bc8a61d3f05dbfdc90bb1d9587d1df3000000000001976a91425f0800454530549ed93747a6449aefe2618203988ac00000000

If you print out the genesis block from the raw blockchain files of your local node, you'll see that it gets stored on disk along with the magic bytes too:

$ hexdump -C -n 293 blk00000.dat

00000000  f9 be b4 d9 1d 01 00 00  01 00 00 00 00 00 00 00  |................|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000020  00 00 00 00 00 00 00 00  00 00 00 00 3b a3 ed fd  |............;...|
00000030  7a 7b 12 b2 7a c7 2c 3e  67 76 8f 61 7f c8 1b c3  |z{..z.,>gv.a....|
00000040  88 8a 51 32 3a 9f b8 aa  4b 1e 5e 4a 29 ab 5f 49  |..Q2:...K.^J)._I|
00000050  ff ff 00 1d 1d ac 2b 7c  01 01 00 00 00 01 00 00  |......+|........|
00000060  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000070  00 00 00 00 00 00 00 00  00 00 00 00 00 00 ff ff  |................|
00000080  ff ff 4d 04 ff ff 00 1d  01 04 45 54 68 65 20 54  |..M.......EThe T|
00000090  69 6d 65 73 20 30 33 2f  4a 61 6e 2f 32 30 30 39  |imes 03/Jan/2009|
000000a0  20 43 68 61 6e 63 65 6c  6c 6f 72 20 6f 6e 20 62  | Chancellor on b|
000000b0  72 69 6e 6b 20 6f 66 20  73 65 63 6f 6e 64 20 62  |rink of second b|
000000c0  61 69 6c 6f 75 74 20 66  6f 72 20 62 61 6e 6b 73  |ailout for banks|
000000d0  ff ff ff ff 01 00 f2 05  2a 01 00 00 00 43 41 04  |........*....CA.|
000000e0  67 8a fd b0 fe 55 48 27  19 67 f1 a6 71 30 b7 10  |g....UH'.g..q0..|
000000f0  5c d6 a8 28 e0 39 09 a6  79 62 e0 ea 1f 61 de b6  |\..(.9..yb...a..|
00000100  49 f6 bc 3f 4c ef 38 c4  f3 55 04 e5 1e c1 12 de  |I..?L.8..U......|
00000110  5c 38 4d f7 ba 0b 8d 57  8a 4c 70 2b 6b f1 1d 5f  |\8M....W.Lp+k.._|
00000120  ac 00 00 00 00                                    |.....|
00000125

Here's another "version" message, but this time it's on the Testnet3 network, so the magic bytes are different:

0b11090776657273696f6e000000000066000000c0094f817e1101000d000000000000004659775800000000000000000000000000000000000000000000ffff0000000000000d000000000000000000000000000000000000000000000000003d2324b2fc764108102f5361746f7368693a302e31332e312fab3d100001

Purpose

Why do we use magic bytes?

If you connect to a bitcoin node, the messages you get are in a continual stream of data.

If you are trying to read this data, it's good to have a way of figuring out when a new message may be starting. This is why a specific set of magic bytes are used as a marker so that you can more easily identify the start of a new message.

So there's nothing actually magical about magic bytes; they're just used to help with delimiting a stream of data.

Origin

Why were these specific bytes chosen?

So they could be different, but these are just 4 bytes that have the properties that make for good-enough magic bytes on the Bitcoin network.

• ASCII. If you convert the bytes f9beb4d9 to Extended ASCII you get ù¾´Ù, which makes for an unlikely string of characters to be unintentionally placed inside the scriptsig of a coinbase transaction by a miner, or as a text string in an OP_RETURN output.
• UTF-8. The basic Latin UTF-8 character set does not go above 7e, so if you're encoding some text using basic UTF-8, you're not going to collide with any of the magic bytes (as they're all greater than 7e).
• Integers. If you convert f9beb4d9 to an integer you get 4190024921. If you also reverse the byte order to d9b4bef9 and convert to an integer you get 3652501241. Both of these are very large numbers, so it's less likely they're going to be used within one of the fields of raw transaction data (e.g. version, input count, vout, output count, amount, script size, etc.).

It's not impossible for this particular set of bytes to appear within a block or transaction, but it's less likely that they're going to occur naturally, which is the next-best thing.

So you wouldn't rely solely on the magic bytes to identify the start of each message, but it's useful to help identify where a message likely begins and also to identify what network you're working with (i.e. mainnet, testnet, or regtest).

This particular set of magic bytes is also unique to the Bitcoin protocol.

Resources

• https://en.bitcoin.it/wiki/Protocol_documentation#Message_structure
• https://en.wikipedia.org/wiki/Magic_number_(programming)