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Coinspect OP_CHECKSIG challenge

Introduction

The challenge from Coinspect and Ekoparty conference consisted in taking the bitcoins from a multisig or p2sh (pay to script hash) wallet: https://blockchain.info/address/32GkPB9XjMAELR4Q2Hr31Jdz2tntY18zCe

I’m making this post as short as possible, and for understanding it, you may need previous knowledge about bitcoin internals.

Understanding OP_CHECKSIG opcode

This is a fuzzy and complicated part of validating bitcoin transactions. It’s explained here.

When OP_CHECKSIG is reached in a script it needs that the public key and the signature are in the stack. It takes the hash type from the signature’s last byte and depending on it, takes an specific approach to verifying the signature.

The internals of this opcode is, having the transaction serialized, strip parts of it depending on the hash type, hashing it and then comparing that the signature is valid for that hash and for the public key provided in the stack.

In this challenge a P2SH address is provided. It means that the scriptSig consists of many signatures and a redeem script.

This redeem script contains the public keys that can be used and how many of them are neccesary for validating the script. In this example needs 2 of 3:

{2 [pubkey1] [pubkey2] [pubkey3] 3 OP_CHECKMULTISIG}

The OP_CHECKMULTISIG opcode runs OP_CHECKSIG for every signature available. If those signatures are good, then it returns True. If any of them fail, it returns False.

Bug in SIGHASH_SINGLE

In case the signature ends with 03, then SIGHASH_SINGLE mode is choosen.

This mode is supposed to strip the other outputs that doesn’t correspond to the same index of the input before hashing the transaction and checking that the signature is valid.

The problem is that SIGHASH_SINGLE is supposed to sign only one output, the output that is in the same index as this input. In case there are less outputs than inputs, then it should probably fail. But because a bug in the initial implementation of bitcoind, it was validating it and signing a hardcoded hash.

From the bitcoin wiki, it describes that if there is no output in the same index of the input, then the hash of the transaction assumed to be signed is “0000000000000000000000000000000000000000000000000000000000000001”. (there is a whole thread about this behavior here). This invalidates the fact of a signature, because no inputs nor outputs are signed but instead, a hard-coded string is.

It means that this signature can be reused in case we need to sign 000..01 again.

The challenge

Going back to the Coinspect’s wallet 32GkPB9XjMAELR4Q2Hr31Jdz2tntY18zCe, we detect that there is already an outgoing transaction: 6102bfd4bad33443bcb99765c0751b6b8e4e65f4db4e3b65324c5e9e3dac8132.

The sigScript from the third input of the transaction:

0
3045022100dfcfafcea73d83e1c54d444a19fb30d17317f922c19e2ff92dcda65ad09cba24022001e7a805c5672c49b222c5f2f1e67bb01f87215fb69df184e7c16f66c1f87c2903
304402204a657ab8358a2edb8fd5ed8a45f846989a43655d2e8f80566b385b8f5a70dab402207362f870ce40f942437d43b6b99343419b14fb18fa69bee801d696a39b3410b803
5221023927b5cd7facefa7b85d02f73d1e1632b3aaf8dd15d4f9f359e37e39f05611962103d2c0e82979b8aba4591fe39cffbf255b3b9c67b3d24f94de79c5013420c67b802103ec010970aae2e3d75eef0b44eaa31d7a0d13392513cd0614ff1c136b3b1020df53ae

So we start analyzing the inputs and detect that these are multisigned (2 of 3) and as the signatures end with 03, correspond to SIGHASH_SINGLE working mode.

The last input doesn’t have a corresponding output, so it means that the last input’s signatures are signing the hash 000..01, so we can reuse those same signatures in case we need to sign the hardcoded hash again.

With that signature we can sign any input which index is bigger than the amount of outputs. In case we are using one output, that leaves us with the input at index 0 to be external to that address because we can not reuse the signatures for it. And we are adding an input from an address we control with the smallest amount possible.

Taking the bitcoins

I’m using sx for the solution.

Create a new transaction with the first input from an address I control and the other 2 with inputs from the coinspect’s wallet:

$ sx mktx txfile.tx -i 2073da15a26ac66043914f5d3936058565318b332e793719456cc0405c87b450:0 -i 969bfb1704f1dc8e6157bf56ea794e16e6d9b88ca5cbff6e12d0797400b8835c:1 -i 9bf39fbf0f89c869585fb59acb2bfec5e2f57069b81021b967a7269a2c873f63:0 -o 197iAesReT4Z6chRqnsficr3LQpVxBdv1J:3100000
Added input 2073da15a26ac66043914f5d3936058565318b332e793719456cc0405c87b450:0
Added input 969bfb1704f1dc8e6157bf56ea794e16e6d9b88ca5cbff6e12d0797400b8835c:1
Added input 9bf39fbf0f89c869585fb59acb2bfec5e2f57069b81021b967a7269a2c873f63:0
Added output sending 3100000 Satoshis to 197iAesReT4Z6chRqnsficr3LQpVxBdv1J.

Create the script for the first input signing it and adding my public key:

$ DECODED_ADDR=$(cat private.key | sx addr | sx decode-addr)
$ PREVOUT_SCRIPT=$(sx rawscript dup hash160 [ $DECODED_ADDR ] equalverify checksig)
$ SIGNATURE=$(cat private.key | sx sign-input txfile.tx 0 $PREVOUT_SCRIPT)
$ SCRIPT=$(sx rawscript [ $SIGNATURE ] [ $(cat private.key | sx pubkey) ])
$ sx set-input txfile.tx 0 $SCRIPT > signedtx

Now I will add the reused signature for the other two inputs, those inputs don’t have a corresponding output, so they only need to sign 000..01, which we already have the signature for (remember the third input from the previous transaction?):

$ SIGN_1=3045022100dfcfafcea73d83e1c54d444a19fb30d17317f922c19e2ff92dcda65ad09cba24022001e7a805c5672c49b222c5f2f1e67bb01f87215fb69df184e7c16f66c1f87c2903 
$ SIGN_2=304402204a657ab8358a2edb8fd5ed8a45f846989a43655d2e8f80566b385b8f5a70dab402207362f870ce40f942437d43b6b99343419b14fb18fa69bee801d696a39b3410b803
$ REDEEM_SCRIPT=5221023927b5cd7facefa7b85d02f73d1e1632b3aaf8dd15d4f9f359e37e39f05611962103d2c0e82979b8aba4591fe39cffbf255b3b9c67b3d24f94de79c5013420c67b802103ec010970aae2e3d75eef0b44eaa31d7a0d13392513cd0614ff1c136b3b1020df53ae

$ REUSED_SCRIPT=$(sx rawscript zero [ $SIGN_1 ] [ $SIGN_2 ] [ $REDEEM_SCRIPT ])

$ sx set-input signed-tx 1 $REUSED_SCRIPT > signed-tx2 
$ sx set-input signed-tx2 2 $REUSED_SCRIPT > signed-tx_final

So now we have the transaction complete and ready to broadcast. I did it on blockr.io because blockchain was complaining about the script containing 4 instructions instead of 2.

$ cat signed-tx_final
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

$ sx showtx signed-tx_final 
hash: 32aadcd309ea3fb30bd9490ee6417e378dad5111c21563ce532a6917b776051b
version: 1
locktime: 0
Input:
  previous output: 2073da15a26ac66043914f5d3936058565318b332e793719456cc0405c87b450:0
  script: [ 3044022043dfdc32cbe03f06200d4f1da806336cb79565c482432e60a6ad0c55dbb5947b02201cd22de62cf8ffc09288ff87ccac93038978e4d9d746b41bfb07614ce161f47201 ] [ 04e824c125eb482debade1f47f0225959b0e15fc667fd281b8e1cc99e08a5ab2285583d6fdcc6b61906d454249a76e666bd03e883df017054a04cc77d1b7dce92d ]
  sequence: 4294967295
  address: 16LvhDHHrcGXEeHtYu6W99kwptuS6Vp59B
Input:
  previous output: 969bfb1704f1dc8e6157bf56ea794e16e6d9b88ca5cbff6e12d0797400b8835c:1
  script: zero [ 3045022100dfcfafcea73d83e1c54d444a19fb30d17317f922c19e2ff92dcda65ad09cba24022001e7a805c5672c49b222c5f2f1e67bb01f87215fb69df184e7c16f66c1f87c2903 ] [ 304402204a657ab8358a2edb8fd5ed8a45f846989a43655d2e8f80566b385b8f5a70dab402207362f870ce40f942437d43b6b99343419b14fb18fa69bee801d696a39b3410b803 ] [ 5221023927b5cd7facefa7b85d02f73d1e1632b3aaf8dd15d4f9f359e37e39f05611962103d2c0e82979b8aba4591fe39cffbf255b3b9c67b3d24f94de79c5013420c67b802103ec010970aae2e3d75eef0b44eaa31d7a0d13392513cd0614ff1c136b3b1020df53ae ]
  sequence: 4294967295
  address: 32GkPB9XjMAELR4Q2Hr31Jdz2tntY18zCe
Input:
  previous output: 9bf39fbf0f89c869585fb59acb2bfec5e2f57069b81021b967a7269a2c873f63:0
  script: zero [ 3045022100dfcfafcea73d83e1c54d444a19fb30d17317f922c19e2ff92dcda65ad09cba24022001e7a805c5672c49b222c5f2f1e67bb01f87215fb69df184e7c16f66c1f87c2903 ] [ 304402204a657ab8358a2edb8fd5ed8a45f846989a43655d2e8f80566b385b8f5a70dab402207362f870ce40f942437d43b6b99343419b14fb18fa69bee801d696a39b3410b803 ] [ 5221023927b5cd7facefa7b85d02f73d1e1632b3aaf8dd15d4f9f359e37e39f05611962103d2c0e82979b8aba4591fe39cffbf255b3b9c67b3d24f94de79c5013420c67b802103ec010970aae2e3d75eef0b44eaa31d7a0d13392513cd0614ff1c136b3b1020df53ae ]
  sequence: 4294967295
  address: 32GkPB9XjMAELR4Q2Hr31Jdz2tntY18zCe
Output:
  value: 3100000
  script: dup hash160 [ 5905ddb52ed55abc9f8f4a58a8323296c642e932 ] equalverify checksig
  address: 197iAesReT4Z6chRqnsficr3LQpVxBdv1J

You can check the transaction and it’s details here .

Thanks for reading!