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Script debugger

The btcdeb command can step through a Bitcoin Script and show stack content and operations on a per op level.

btcdeb> help
step     Execute one instruction and iterate in the script.
rewind   Go back in time one instruction.
stack    Print stack content.
altstack Print altstack content.
vfexec   Print vfexec content.
exec     Execute command.
tf       Transform a value using a given function.
print    Print script.
help     Show help information.

(note this example will fail on the OP_CHECKSIG part; see "Signature checking" below).

$ btcdeb '[OP_DUP OP_HASH160 897c81ac37ae36f7bc5b91356cfb0138bfacb3c1 OP_EQUALVERIFY OP_CHECKSIG]' 3045022100c7d8e302908fdc601b125c2734de63ed3bf54353e13a835313c2a2aa5e8f21810220131fad73787989d7fbbdbbd8420674f56bdf61fed5dc2653c826a4789c68501101 03b05bdbdf395e495a61add92442071e32703518b8fca3fc34149db4b56c93be42
valid script
5 op script loaded. type `help` for usage information
script                                                             |                                                             stack
-------------------------------------------------------------------+-------------------------------------------------------------------
OP_DUP                                                             | 03b05bdbdf395e495a61add92442071e32703518b8fca3fc34149db4b56c93be42
OP_HASH160                                                         | 3045022100c7d8e302908fdc601b125c2734de63ed3bf54353e13a835313c2a...
897c81ac37ae36f7bc5b91356cfb0138bfacb3c1                           |
OP_EQUALVERIFY                                                     |
OP_CHECKSIG                                                        |
#0001 OP_DUP
btcdeb>

Signature checking

In order to run an OP_CHECKSIG command, the debugger needs to know about the transaction being checked, since it creates the signature hash from the transaction content. You can pass the transaction to btcdeb when you run it, using the --tx=amount1,amount2:hexdata, where amountN is the amount of the inputs of the transaction, and hexdata is the hexadecimal representation of the entire transaction (not just the transaction ID). For example, to verify transaction ID c2fdfbcbef9acb6107eb5d18c172f234ee694254be1128d29b85b80b9bad9b3a, the following will produce an output of TRUE.

> $ btcdeb --tx=0.3315983:02000000013a9bad9b0bb8859bd22811be544269ee34f272c1185deb0761cb9aefcbfbfdc2000000006a47304402200cc8b0471a38edad2ff9f9799521b7d948054817793c980eaf3a6637ddfb939702201c1a801461d4c3cf4de4e7336454dba0dd70b89d71f221e991cb6a79df1a860d012102ce9f5972fe1473c9b6948949f676bbf7893a03c5b4420826711ef518ceefd8dcfeffffff0226f20b00000000001976a914d138551aa10d1f891ba02689390f32ce09b71c1788ac28b0ed01000000001976a914870c7d8085e1712539d8d78363865c42d2b5f75a88ac5b880800 '[OP_DUP OP_HASH160 1290b657a78e201967c22d8022b348bd5e23ce17 OP_EQUALVERIFY OP_CHECKSIG ]' 304402200cc8b0471a38edad2ff9f9799521b7d948054817793c980eaf3a6637ddfb939702201c1a801461d4c3cf4de4e7336454dba0dd70b89d71f221e991cb6a79df1a860d01 02ce9f5972fe1473c9b6948949f676bbf7893a03c5b4420826711ef518ceefd8dc
btcdeb -- type `btcdeb -h` for start up options
got transaction 7aba9a51fa8d8441c3e46b76e7dfeef2363d89e1b64fed142632e043cef7e24f:
CTransaction(hash=7aba9a51fa, ver=2, vin.size=1, vout.size=2, nLockTime=559195)
    CTxIn(COutPoint(c2fdfbcbef, 0), scriptSig=47304402200cc8b0471a38ed, nSequence=4294967294)
    CScriptWitness()
    CTxOut(nValue=0.00782886, scriptPubKey=76a914d138551aa10d1f891ba02689)
    CTxOut(nValue=0.32354344, scriptPubKey=76a914870c7d8085e1712539d8d783)

valid script
5 op script loaded. type `help` for usage information
script                                   |                                                             stack
-----------------------------------------+-------------------------------------------------------------------
OP_DUP                                   | 02ce9f5972fe1473c9b6948949f676bbf7893a03c5b4420826711ef518ceefd8dc
OP_HASH160                               | 304402200cc8b0471a38edad2ff9f9799521b7d948054817793c980eaf3a663...
1290b657a78e201967c22d8022b348bd5e23ce17 |
OP_EQUALVERIFY                           |
OP_CHECKSIG                              |
#0000 OP_DUP
btcdeb>

Alternatively, you can pass both the transaction (using --tx=) and the input transaction (using --txin=). In this case, if you do not provide any other parameters (script/stack data), btcdeb will automatically figure out the script and stack content for verifying the input specified by the txin hex. You also do not need to include the amounts when passing the txin explicitly.

$ btcdeb --tx=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 \
--txin=0100000000010170a6ee35199eae2d8ea659561374fa704f8fd95188ff5931157e4598dd0c44020100000000ffffffff0280f0fa02000000001976a914eec426a744f7a3b2ffd346925ac832e248834dd788ac4013543500000000220020701a8d401c84fb13e6baf169d59684e17abd9fa216c8cc5b9fc63d622ff8c58d04004730440220054c0b331a31496d9123aeabe8415b8d2f877f1cf67709120af4eb1e09de59e002206cdf84e733e23be531aff202f868d200773e22aa0037033a74fc6752df2fd19601483045022100b54fa12828d13b58cb654dd910b9e8b36d471d644d8f66516577990ca099ee19022048ea2ac78f964d1b823af70c13c5607a29b14bb2348022190b3c280f51ec5df2016952210375e00eb72e29da82b89367947f29ef34afb75e8654f6ea368e0acdfd92976b7c2103a1b26313f430c4b15bb1fdce663207659d8cac749a0e53d70eff01874496feff2103c96d495bfdd5ba4145e3e046fee45e84a8a48ad05bd8dbb395c011a32cf9f88053ae00000000
btcdeb -- type `./btcdeb -h` for start up options
got segwit transaction:
CTransaction(hash=bf19bb6924, ver=1, vin.size=1, vout.size=2, nLockTime=0)
    CTxIn(COutPoint(aa15935e03, 1), scriptSig=)
    CScriptWitness(, 304402207f874ef00f11dcc9a621acad9354f3fca1bf90c43878f607b7e2d358088487e7022052a01b47b8eef5e1c96a6affdc3dac46fdc11b60612464dc8c5921a852090d2701, 3045022100c56ab2abb17fdf565417228763bc9f2940a6465042fd62fbd9f4c7406345d7f702201cb1a56b45181f8347713627b325ec5df48fc1aee6bdaf937cbb804d7409b10c01, 52210375e00eb72e29da82b89367947f29ef34afb75e8654f6ea368e0acdfd92976b7c2103a1b26313f430c4b15bb1fdce663207659d8cac749a0e53d70eff01874496feff2103c96d495bfdd5ba4145e3e046fee45e84a8a48ad05bd8dbb395c011a32cf9f88053ae)
    CTxOut(nValue=0.62477075, scriptPubKey=a914c3f413d0918853a8e23766678d)
    CTxOut(nValue=8.32037925, scriptPubKey=0020701a8d401c84fb13e6baf169d5)

got input tx #0:
CTransaction(hash=aa15935e03, ver=1, vin.size=1, vout.size=2, nLockTime=0)
    CTxIn(COutPoint(02440cdd98, 1), scriptSig=)
    CScriptWitness(, 30440220054c0b331a31496d9123aeabe8415b8d2f877f1cf67709120af4eb1e09de59e002206cdf84e733e23be531aff202f868d200773e22aa0037033a74fc6752df2fd19601, 3045022100b54fa12828d13b58cb654dd910b9e8b36d471d644d8f66516577990ca099ee19022048ea2ac78f964d1b823af70c13c5607a29b14bb2348022190b3c280f51ec5df201, 52210375e00eb72e29da82b89367947f29ef34afb75e8654f6ea368e0acdfd92976b7c2103a1b26313f430c4b15bb1fdce663207659d8cac749a0e53d70eff01874496feff2103c96d495bfdd5ba4145e3e046fee45e84a8a48ad05bd8dbb395c011a32cf9f88053ae)
    CTxOut(nValue=0.50000000, scriptPubKey=76a914eec426a744f7a3b2ffd34692)
    CTxOut(nValue=8.94702400, scriptPubKey=0020701a8d401c84fb13e6baf169d5)

input tx index = 0; tx input vout = 1; value = 894702400
valid script
6 op script loaded. type `help` for usage information
script                                                             |                                                             stack
-------------------------------------------------------------------+-------------------------------------------------------------------
2                                                                  | 3045022100c56ab2abb17fdf565417228763bc9f2940a6465042fd62fbd9f4c...
0375e00eb72e29da82b89367947f29ef34afb75e8654f6ea368e0acdfd92976b7c | 304402207f874ef00f11dcc9a621acad9354f3fca1bf90c43878f607b7e2d35...
03a1b26313f430c4b15bb1fdce663207659d8cac749a0e53d70eff01874496feff |                                                                 0x
03c96d495bfdd5ba4145e3e046fee45e84a8a48ad05bd8dbb395c011a32cf9f880 |
3                                                                  |
OP_CHECKMULTISIG                                                   |
#0001 2
btcdeb>

Transforms

btcdeb has a built-in toolkit for transforming values. To see all currently available transform operations type tf -h.

If you compile with the --enable-dangerous flag set, you can also manipulate private keys. E.g.

btcdeb> tf sha256 alice
2bd806c97f0e00af1a1fc3328fa763a9269723c8db8fac4f93af71db186d6e90
btcdeb> tf get-xpubkey 2bd806c97f0e00af1a1fc3328fa763a9269723c8db8fac4f93af71db186d6e90
9997a497d964fc1a62885b05a51166a65a90df00492c8d7cf61d6accf54803be
btcdeb> tf sha256 message
ab530a13e45914982b79f9b7e3fba994cfd1f3fb22f71cea1afbf02b460c6d1d
btcdeb> tf sign ab530a13e45914982b79f9b7e3fba994cfd1f3fb22f71cea1afbf02b460c6d1d 2bd806c97f0e00af1a1fc3328fa763a9269723c8db8fac4f93af71db186d6e90
bd910c7f1340c525384c739bbde410e7d37701fdfacbec0a0d6d1d360381a4c01e85aee61eb30de834f111630b6c47803e0f17bf6397573f28508168531019ec
btcdeb> tf verify-sig ab530a13e45914982b79f9b7e3fba994cfd1f3fb22f71cea1afbf02b460c6d1d 9997a497d964fc1a62885b05a51166a65a90df00492c8d7cf61d6accf54803be bd910c7f1340c525384c739bbde410e7d37701fdfacbec0a0d6d1d360381a4c01e85aee61eb30de834f111630b6c47803e0f17bf6397573f28508168531019ec
1

There is also built-in support for inline operations; these mostly look the same as the transform operations with the minus signs converted to underscore signs, except for a few exceptions; typing tf -h should list the inline operation names.

Either way, we can re-do the above using inline operations like this:

btcdeb> tf get-xpubkey sha256(alice)
9997a497d964fc1a62885b05a51166a65a90df00492c8d7cf61d6accf54803be
btcdeb> tf sign sha256(message) sha256(alice)
bd910c7f1340c525384c739bbde410e7d37701fdfacbec0a0d6d1d360381a4c01e85aee61eb30de834f111630b6c47803e0f17bf6397573f28508168531019ec
tf verify-sig sha256(message) get_xpubkey(sha256(alice)) bd910c7f1340c525384c739bbde410e7d37701fdfacbec0a0d6d1d360381a4c01e85aee61eb30de834f111630b6c47803e0f17bf6397573f28508168531019ec
1