PSBT Binary Format and Magic Bytes

If you open a PSBT file in a text editor, it usually looks like a long string of random characters (Base64). However, underneath that encoding is a strictly structured binary format.

1. The Magic Bytes

Every PSBT begins with a specific "Magic" identifier to help software recognize the file type:

• Hex: 70 73 62 74 ff

• ASCII: psbt\u00ff

If a file does not start with these 5 bytes, it is not a valid BIP 174 PSBT.

2. Key-Value Architecture

The internal structure of a PSBT is a series of Key-Value maps. It is divided into three sections:

1. Global Map: Contains the unsigned raw transaction and overall metadata.

2. Input Maps: One map for every input being spent. Contains signatures, scripts, and the full previous transaction.

3. Output Maps: One map for every output being created. Contains information for the signer to verify "change" addresses.

3. Map Serialization

Each entry in a map follows a simple pattern:

• [Key Length] \u2192 [Key Type] \u2192 [Key Data] \u2192 [Value Length] \u2192 [Value Data]

• The map ends with a 0x00 byte (the separator).

4. Why Base64?

While the format is binary, it is almost always displayed in Base64 for humans. This makes it easy to:

• Copy and paste into a wallet.

• Convert into a QR code for "Air-Gapped" communication.

• Share via email or messaging apps without file corruption.

Section	Common Key Types	Purpose
Global	0x00 (Unsigned TX)	The foundation of the spend
Input	0x02 (Partial Sig), 0x04 (Sighash Type)	Collecting the math proof
Output	0x01 (Redeem Script)	Verifying the destination

In the next section, we will discuss Hardware Wallet Integration.