OP CHECKSIG

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OP_CHECKSIG is script opcode used to verify that the signature for a tx input is valid. OP_CHECKSIG expects two values to be on the stack, these are, in order of stack depth, the public key and the signature of the script. These two values are normally obtained by running the scriptSig script of the transaction input we are attempting to validate. After the scriptSig script is run the script is deleted but the stack is left as is, and then then scriptPubKey script from the previous transaction output that is now being spent is run, generally concluding in an OP_CHECKSIG.

The standard scriptPubKey checks that the public key (actually a hash of) is a particular value, and that OP_CHECKSIG passes.

For normal transaction inputs if the creator of the current transaction can successfully create a ScriptSig signature that uses the right public key for the ScriptPubKey of the transaction output they are attempting to spend, that transaction input is considered valid.

Parameters

In addition to the script code itself and the stack parameters, to operate OP_CHECKSIG needs to know the current transaction, the current transaction input, and the current hashtype (discussed later)

How it works

  1. the public key and the signature are popped from the stack, in that order.
  2. A new subscript is created from the instruction from the most recently parsed OP_CODESEPARATOR (last one in script) to the end of the script. If there is no OP_CODESEPARATOR the entire script becomes the subscript (hereby referred to as subScript)
  3. The sig is deleted from subScript.
  4. The hashtype is removed from the last byte of the sig and stored
  5. A deep copy is made of the current transaction (hereby referred to txCopy)
  6. All OP_CODESEPARATORS are removed from subScript
  7. The scripts for all transaction inputs in txCopy are set to empty scripts
  8. The script for the current transaction input in txCopy is set to subScript

Now depending on the hashtype various things can happen to txCopy, these will be discussed individually


Hashtype Values (from script.h):

Name Value
SIGHASH_ALL 0x00000001
SIGHASH_NONE 0x00000002
SIGHASH_SINGLE 0x00000003
SIGHASH_ANYONECANPAY 0x00000080

Hashtype SIGHASH_ALL (default)

No special handling occurs in the default case

Hashtype SIGHASH_NONE

  1. The output of txCopy is set to a vector of zero size.
  2. All other inputs aside from the current input in txCopy have their nSequence index set to zero

Hashtype SIGHASH_SINGLE

  1. The output of txCopy is resized to the size of the current input index+1
  2. All other txCopy outputs aside from the output that is the same as the current input index are set to a blank script and a value of (long) -1;
  3. All other txCopy inputs aside from the current input are set to have an nSequence index of zero

Hashtype SIGHASH_ANYONECANPAY

  1. The txCopy input vector is resized to a length of one
  2. The current input is set as the first and only member of this vector

Final signature

An array of bytes is constructed from the serialized txCopy + four bytes for the hash type. This array is sha256 hashed twice, then the public key is used to to check the supplied signature against the hash.

Return values

OP_CHECKSIG will push true to the stack if the check passed, false otherwise. OP_CHECKSIG_VERIFY leaves nothing on the stack but will cause the script eval to fail immediately if the check does not pass.

Code samples and raw dumps

Taking the first transaction in Bitcoin which is in block number 170, we would get after serialising the transaction but before we hash+sign (or verify) it:

See also libbitcoin for code samples. 

01 00 00 00              version
01                       number of inputs (var_uint)

input 0:
c9 97 a5 e5 6e 10 41 02  input address hash
fa 20 9c 6a 85 2d d9 06 
60 a2 0b 2d 9c 35 24 23 
ed ce 25 85 7f cd 37 04
00 00 00 00              input index
??

43                       size of script (var_uint)
41                       push 65 bytes to stack
04 11 db 93 e1 dc db 8a 
01 6b 49 84 0f 8c 53 bc 
1e b6 8a 38 2e 97 b1 48 
2e ca d7 b1 48 a6 90 9a 
5c b2 e0 ea dd fb 84 cc 
f9 74 44 64 f8 2e 16 0b 
fa 9b 8b 64 f9 d4 c0 3f 
99 9b 86 43 f6 56 b4 12 
a3
ac                       OP_CHECKSIG

02                       number of outputs (var_uint)

output 0:
00 ca 9a 3b 00 00 00 00  amount = 10.00000000
43                       size of script (var_uint)
script for output 0:
41                       push 65 bytes to stack
04 ae 1a 62 fe 09 c5 f5 
1b 13 90 5f 07 f0 6b 99 
a2 f7 15 9b 22 25 f3 74 
cd 37 8d 71 30 2f a2 84 
14 e7 aa b3 73 97 f5 54 
a7 df 5f 14 2c 21 c1 b7 
30 3b 8a 06 26 f1 ba de 
d5 c7 2a 70 4f 7e 6c d8 
4c 
ac                       OP_CHECKSIG

output 1:
00 28 6b ee 00 00 00 00  amount = 40.00000000
43                       size of script (var_uint)
script for output 1:
41                       push 65 bytes to stack
04 11 db 93 e1 dc db 8a 
01 6b 49 84 0f 8c 53 bc 
1e b6 8a 38 2e 97 b1 48 
2e ca d7 b1 48 a6 90 9a
5c b2 e0 ea dd fb 84 cc 
f9 74 44 64 f8 2e 16 0b 
fa 9b 8b 64 f9 d4 c0 3f 
99 9b 86 43 f6 56 b4 12 
a3                       
ac                       OP_CHECKSIG

00 00 00 00              locktime
01 00 00 00              hash_code_type (added on)

result =
01 00 00 00 01 c9 97 a5 e5 6e 10 41 02 fa 20 9c 6a 85 2d d9 06 60 a2 0b 2d 9c 35
24 23 ed ce 25 85 7f cd 37 04 00 00 00 00 01 ac 02 00 ca 9a 3b 00 00 00 00 43 41
04 ae 1a 62 fe 09 c5 f5 1b 13 90 5f 07 f0 6b 99 a2 f7 15 9b 22 25 f3 74 cd 37 8d
71 30 2f a2 84 14 e7 aa b3 73 97 f5 54 a7 df 5f 14 2c 21 c1 b7 30 3b 8a 06 26 f1
ba de d5 c7 2a 70 4f 7e 6c d8 4c ac 00 28 6b ee 00 00 00 00 43 41 04 11 db 93 e1
dc db 8a 01 6b 49 84 0f 8c 53 bc 1e b6 8a 38 2e 97 b1 48 2e ca d7 b1 48 a6 90 9a
5c b2 e0 ea dd fb 84 cc f9 74 44 64 f8 2e 16 0b fa 9b 8b 64 f9 d4 c0 3f 99 9b 86
43 f6 56 b4 12 a3 ac 00 00 00 00 01 00 00 00

To understand where that raw dump has come from, it may be useful to examine tests/ec-key.cpp in libbitcoin,

libbitcoin has a unit test under tests/ec-key.cpp (make ec-key && ./bin/tests/ec-key). There is also a working OP_CHECKSIG implementation in src/script.cpp under script::op_checksig(). See also the unit test: tests/script-test.cpp 

#include <iostream>
#include <iomanip>
#include <bitcoin/util/serializer.hpp>
#include <bitcoin/util/elliptic_curve_key.hpp>
#include <bitcoin/util/sha256.hpp>
#include <bitcoin/util/assert.hpp>
#include <bitcoin/util/logger.hpp>
#include <bitcoin/types.hpp>
#include <openssl/ecdsa.h>
#include <openssl/obj_mac.h>
using libbitcoin::elliptic_curve_key;
using libbitcoin::serializer;
using libbitcoin::hash_digest;
using libbitcoin::data_chunk;
using libbitcoin::log_info;
using libbitcoin::log_fatal;

int main()
{
    serializer ss;
    // blk number 170, tx 1, input 0
    // version = 1
    ss.write_4_bytes(1);
    // 1 inputs
    ss.write_var_uint(1);

    // input 0
    // prevout hash
    ss.write_hash(hash_digest{0x04, 0x37, 0xcd, 0x7f, 0x85, 0x25, 0xce, 0xed, 0x23, 0x24, 0x35, 0x9c, 0x2d, 0x0b, 0xa2, 0x60, 0x06, 0xd9, 0x2d, 0x85, 0x6a, 0x9c, 0x20, 0xfa, 0x02, 0x41, 0x10, 0x6e, 0xe5, 0xa5, 0x97, 0xc9});
    // prevout index 
    ss.write_4_bytes(0);

    // input script after running OP_CHECKSIG for this tx is a single
    // OP_CHECKSIG opcode
    data_chunk raw_data;
    raw_data = {0x04, 0x11, 0xdb, 0x93, 0xe1, 0xdc, 0xdb, 0x8a, 0x01, 0x6b, 0x49, 0x84, 0x0f, 0x8c, 0x53, 0xbc, 0x1e, 0xb6, 0x8a, 0x38, 0x2e, 0x97, 0xb1, 0x48, 0x2e, 0xca, 0xd7, 0xb1, 0x48, 0xa6, 0x90, 0x9a, 0x5c, 0xb2, 0xe0, 0xea, 0xdd, 0xfb, 0x84, 0xcc, 0xf9, 0x74, 0x44, 0x64, 0xf8, 0x2e, 0x16, 0x0b, 0xfa, 0x9b, 0x8b, 0x64, 0xf9, 0xd4, 0xc0, 0x3f, 0x99, 0x9b, 0x86, 0x43, 0xf6, 0x56, 0xb4, 0x12, 0xa3};
    data_chunk raw_script;
    raw_script = data_chunk();
    raw_script.push_back(raw_data.size());
    libbitcoin::extend_data(raw_script, raw_data);
    raw_script.push_back(172);
    ss.write_var_uint(raw_script.size());
    ss.write_data(raw_script);
    // sequence
    ss.write_4_bytes(0xffffffff);

    // 2 outputs for this tx
    ss.write_var_uint(2);

    // output 0
    ss.write_8_bytes(1000000000);
    // script for output 0
    raw_data = {0x04, 0xae, 0x1a, 0x62, 0xfe, 0x09, 0xc5, 0xf5, 0x1b, 0x13, 0x90, 0x5f, 0x07, 0xf0, 0x6b, 0x99, 0xa2, 0xf7, 0x15, 0x9b, 0x22, 0x25, 0xf3, 0x74, 0xcd, 0x37, 0x8d, 0x71, 0x30, 0x2f, 0xa2, 0x84, 0x14, 0xe7, 0xaa, 0xb3, 0x73, 0x97, 0xf5, 0x54, 0xa7, 0xdf, 0x5f, 0x14, 0x2c, 0x21, 0xc1, 0xb7, 0x30, 0x3b, 0x8a, 0x06, 0x26, 0xf1, 0xba, 0xde, 0xd5, 0xc7, 0x2a, 0x70, 0x4f, 0x7e, 0x6c, 0xd8, 0x4c};
    // when data < 75, we can just write it's length as a single byte ('special'
    // opcodes)
    raw_script = data_chunk();
    raw_script.push_back(raw_data.size());
    libbitcoin::extend_data(raw_script, raw_data);
    // OP_CHECKSIG
    raw_script.push_back(172);
    // now actually write the script
    ss.write_var_uint(raw_script.size());
    ss.write_data(raw_script);

    // output 0
    ss.write_8_bytes(4000000000);
    // script for output 0
    raw_data = {0x04, 0x11, 0xdb, 0x93, 0xe1, 0xdc, 0xdb, 0x8a, 0x01, 0x6b, 0x49, 0x84, 0x0f, 0x8c, 0x53, 0xbc, 0x1e, 0xb6, 0x8a, 0x38, 0x2e, 0x97, 0xb1, 0x48, 0x2e, 0xca, 0xd7, 0xb1, 0x48, 0xa6, 0x90, 0x9a, 0x5c, 0xb2, 0xe0, 0xea, 0xdd, 0xfb, 0x84, 0xcc, 0xf9, 0x74, 0x44, 0x64, 0xf8, 0x2e, 0x16, 0x0b, 0xfa, 0x9b, 0x8b, 0x64, 0xf9, 0xd4, 0xc0, 0x3f, 0x99, 0x9b, 0x86, 0x43, 0xf6, 0x56, 0xb4, 0x12, 0xa3};
    // when data < 75, we can just write it's length as a single byte ('special'
    raw_script.push_back(raw_data.size());
    libbitcoin::extend_data(raw_script, raw_data);
    // OP_CHECKSIG
    raw_script.push_back(172);
    // now actually write the script
    ss.write_var_uint(raw_script.size());
    ss.write_data(raw_script);

    // End of 2 outputs

    // locktime
    ss.write_4_bytes(0);

    // write hash_type_code
    ss.write_4_bytes(1);

    // Dump hex to screen
    log_info() << "hashing:";
    {
        auto log_obj = log_info();
        log_obj << std::hex;
        for (int val: ss.get_data())
            log_obj << std::setfill('0') << std::setw(2) << val << ' ';
    }
    log_info();

    data_chunk raw_tx = {0x01, 0x00, 0x00, 0x00, 0x01, 0xc9, 0x97, 0xa5, 0xe5, 0x6e, 0x10, 0x41, 0x02, 0xfa, 0x20, 0x9c, 0x6a, 0x85, 0x2d, 0xd9, 0x06, 0x60, 0xa2, 0x0b, 0x2d, 0x9c, 0x35, 0x24, 0x23, 0xed, 0xce, 0x25, 0x85, 0x7f, 0xcd, 0x37, 0x04, 0x00, 0x00, 0x00, 0x00, 0x43, 0x41, 0x04, 0x11, 0xdb, 0x93, 0xe1, 0xdc, 0xdb, 0x8a, 0x01, 0x6b, 0x49, 0x84, 0x0f, 0x8c, 0x53, 0xbc, 0x1e, 0xb6, 0x8a, 0x38, 0x2e, 0x97, 0xb1, 0x48, 0x2e, 0xca, 0xd7, 0xb1, 0x48, 0xa6, 0x90, 0x9a, 0x5c, 0xb2, 0xe0, 0xea, 0xdd, 0xfb, 0x84, 0xcc, 0xf9, 0x74, 0x44, 0x64, 0xf8, 0x2e, 0x16, 0x0b, 0xfa, 0x9b, 0x8b, 0x64, 0xf9, 0xd4, 0xc0, 0x3f, 0x99, 0x9b, 0x86, 0x43, 0xf6, 0x56, 0xb4, 0x12, 0xa3, 0xac, 0xff, 0xff, 0xff, 0xff, 0x02, 0x00, 0xca, 0x9a, 0x3b, 0x00, 0x00, 0x00, 0x00, 0x43, 0x41, 0x04, 0xae, 0x1a, 0x62, 0xfe, 0x09, 0xc5, 0xf5, 0x1b, 0x13, 0x90, 0x5f, 0x07, 0xf0, 0x6b, 0x99, 0xa2, 0xf7, 0x15, 0x9b, 0x22, 0x25, 0xf3, 0x74, 0xcd, 0x37, 0x8d, 0x71, 0x30, 0x2f, 0xa2, 0x84, 0x14, 0xe7, 0xaa, 0xb3, 0x73, 0x97, 0xf5, 0x54, 0xa7, 0xdf, 0x5f, 0x14, 0x2c, 0x21, 0xc1, 0xb7, 0x30, 0x3b, 0x8a, 0x06, 0x26, 0xf1, 0xba, 0xde, 0xd5, 0xc7, 0x2a, 0x70, 0x4f, 0x7e, 0x6c, 0xd8, 0x4c, 0xac, 0x00, 0x28, 0x6b, 0xee, 0x00, 0x00, 0x00, 0x00, 0x43, 0x41, 0x04, 0x11, 0xdb, 0x93, 0xe1, 0xdc, 0xdb, 0x8a, 0x01, 0x6b, 0x49, 0x84, 0x0f, 0x8c, 0x53, 0xbc, 0x1e, 0xb6, 0x8a, 0x38, 0x2e, 0x97, 0xb1, 0x48, 0x2e, 0xca, 0xd7, 0xb1, 0x48, 0xa6, 0x90, 0x9a, 0x5c, 0xb2, 0xe0, 0xea, 0xdd, 0xfb, 0x84, 0xcc, 0xf9, 0x74, 0x44, 0x64, 0xf8, 0x2e, 0x16, 0x0b, 0xfa, 0x9b, 0x8b, 0x64, 0xf9, 0xd4, 0xc0, 0x3f, 0x99, 0x9b, 0x86, 0x43, 0xf6, 0x56, 0xb4, 0x12, 0xa3, 0xac, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00};
    BITCOIN_ASSERT(raw_tx == ss.get_data());

    hash_digest tx_hash = libbitcoin::generate_sha256_hash(ss.get_data());

    data_chunk pubkey{0x04, 0x11, 0xdb, 0x93, 0xe1, 0xdc, 0xdb, 0x8a, 0x01, 0x6b, 0x49, 0x84, 0x0f, 0x8c, 0x53, 0xbc, 0x1e, 0xb6, 0x8a, 0x38, 0x2e, 0x97, 0xb1, 0x48, 0x2e, 0xca, 0xd7, 0xb1, 0x48, 0xa6, 0x90, 0x9a, 0x5c, 0xb2, 0xe0, 0xea, 0xdd, 0xfb, 0x84, 0xcc, 0xf9, 0x74, 0x44, 0x64, 0xf8, 0x2e, 0x16, 0x0b, 0xfa, 0x9b, 0x8b, 0x64, 0xf9, 0xd4, 0xc0, 0x3f, 0x99, 0x9b, 0x86, 0x43, 0xf6, 0x56, 0xb4, 0x12, 0xa3};
    // Leave out last byte since that's the hash_type_code (SIGHASH_ALL in this
    // case)
    data_chunk signature{0x30, 0x44, 0x02, 0x20, 0x4e, 0x45, 0xe1, 0x69, 0x32, 0xb8, 0xaf, 0x51, 0x49, 0x61, 0xa1, 0xd3, 0xa1, 0xa2, 0x5f, 0xdf, 0x3f, 0x4f, 0x77, 0x32, 0xe9, 0xd6, 0x24, 0xc6, 0xc6, 0x15, 0x48, 0xab, 0x5f, 0xb8, 0xcd, 0x41, 0x02, 0x20, 0x18, 0x15, 0x22, 0xec, 0x8e, 0xca, 0x07, 0xde, 0x48, 0x60, 0xa4, 0xac, 0xdd, 0x12, 0x90, 0x9d, 0x83, 0x1c, 0xc5, 0x6c, 0xbb, 0xac, 0x46, 0x22, 0x08, 0x22, 0x21, 0xa8, 0x76, 0x8d, 0x1d, 0x09};
    BITCOIN_ASSERT(signature.size() == 70);

    elliptic_curve_key key;
    if (!key.set_public_key(pubkey))
    {
        log_fatal() << "unable to set EC public key";
        return -1;
    }

    log_info() << "checksig returns: " << (key.verify(tx_hash, signature) ? "true" : "false");
    return 0;
}
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