SegWit v0 outputs with a witness program length that is neither 20 nor 32 bytes are permanently unspendable by consensus rule, making their value provably lost.

Abstract

BIP 141 defines two valid SegWit v0 witness program lengths:

• 20 bytes — P2WPKH (Pay to Witness Public Key Hash)
• 32 bytes — P2WSH (Pay to Witness Script Hash)

The BIP 141 consensus rule states: "If the version byte is 0, but the witness program is neither 20 nor 32 bytes, the script MUST fail." This means any OP_0 <N-byte program> output where N ∈ [2, 40] and N ≠ 20 and N ≠ 32 can never be spent under current consensus rules, regardless of the witness data provided.

Script Format

OP_0 OP_PUSHBYTES_N <N-byte program>

Hex: 00 + push-byte + program bytes

Valid programs: 0014... (20 bytes, P2WPKH) and 0020... (32 bytes, P2WSH).

Detection

Parse the push byte and verify it falls in [2, 40] (BIP 141 witness program range) but is neither 20 nor 32, and that the total script length is consistent with the declared push length.

else if (hex.startsWith('00') && hex.length >= 8) {
    const pushByte = parseInt(hex.slice(2, 4), 16);
    if (pushByte >= 2 && pushByte <= 40 &&         // BIP141 valid range
        pushByte !== 20 && pushByte !== 32 &&       // not P2WPKH or P2WSH
        hex.length === 4 + pushByte * 2) {          // total length consistent
        return true;
    }
}

Scope Notes

• Programs of length 0 or 1 are also technically invalid but handled separately; only 2–40 byte programs are considered here as BIP 141 specifies this range for witness programs.
• SegWit v2–v16 are governed by different rules (currently "anyone can spend" under BIP 141 for unknown versions); only v0 has the strict length requirement enforced at consensus.