Why these 2 pubkeys start equal?

Question

I generated a seed into my coldcard, wrote down its pubkey:

xpubABCDEFGHIKL---------------------------------------------------

then I erased everything and generated a new seed, and wrote down its pubkey

xpubABCDEFGHIKL---------------------------------------------------

as you see, they both start with the first 15 letters the same. Then they differ.

How is that possible?

Answer 1

The encoding of an xpub contains multiple pieces of data:

• A 4-byte fixed prefix that results in the "xpub" prefix after encoding
• Depth in the tree (0 for all master xpubs, 1 for children thereof, ...)
• A 4-byte checksum of the parent key (0x00000000 for all master xpubs as they have no parent)
• The 4-byte index this node has in the derivation from its parent (0x00000000 for all master xpubs).
• The 32-byte chaincode
• The 33-byte public key

So, the first 13 bytes before encoding are identical for all master xpubs. This translates to roughly 17-18 characters after encoding.