OP_CAT was originally an opcode in Bitcoin. It was disabled in 2010 but slight variations on it are frequently proposed to be added to Bitcoin using a soft fork.

Both the original OP_CAT and the new proposals for it concatenate two elements on the stack into a single element. For example, the following script:

<0xB10C> <0xCAFE> OP_CAT

Would become:


The primary expected use for OP_CAT is for data provided by the creator of a script to be concatenated with data provided by someone spending from that script. For example, Alice wants to create an equivocation bond that she can’t create competing spends for without putting her funds at risk. She generates a private key in the normal way, derives a public key from it in the normal way, chooses a random private nonce the same way she usually would for a schnorr signature, and derives the public nonce also in the normal way. She then pays to the following script:

<public nonce> OP_CAT <pubkey> OP_CHECKSIG

Later, when she signs, instead of providing a complete schnorr signature—which includes both a public nonce and a scalar—she’s forced to use the public nonce from her script. In her witness field, she only provides the scalar. The scalar and the public nonce are concatenated together to produce a BIP340 schnorr signature, which is then verified against Alice’s public key like normal using the OP_CHECKSIG opcode.

If Alice later tries to sign a different version of the transaction, she’s forced to reuse the same public nonce but must (because of the BIP340 equation) generate a different scalar. This reuse of the same nonce in different signatures from the same private key allows anyone to derive her private key. They can then create their own signatures for Alice’s private key, potentially spending her funds if they haven’t been spent already.

There are many other proposed applications of OP_CAT, see BIN24-1 for one list. Some applications, such as the example above, are possible with just OP_CAT and other features that are already part of Bitcoin script; other applications require additional new opcodes or other changes to Bitcoin.

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