Decoding ScriptSig

Question

I noticed that there can be what seems to be more than 1 signature in a ScriptSig.

The transaction in question is: https://blockchain.info/tx/32e8677c968981549acc25f46641c74a3055d949c90bd22c15a5236bf92a622d

Below is a ScriptSig sample in question:

ScriptSig: 0[] PUSHDATA(71)[304402201c45383cc6e43202ed069e36184a97bf5dd489c6bea1372629540dea154c424902200a950b3557bba9ae9531237d46e6eb1afffd01eba989c3e53adaac443e5e2a2e01] PUSHDATA(71)[3044022032a681fb77589ce1a29a84494a6f9cb19630fcd65480ff646f30d8b826980390022006dc71baba1142abb275bbf3613b2a13b347bac4d09d41ccd9b69b02c270d8c201] PUSHDATA1[522103745aaaf364030720b2d14de50a3310eef521c91e36353dca20813713535c005a2102db8911b3989b43c43d8dd6e50459bd85c38faf3b2862eb78ef297002775a10bd210351e3f71b7cf9a5f5f86c1908fee02ebf5a1ed77b6748f7486505d155833645f253ae]

The above text is taken from blockchain.info webpage for the first ScriptSig in that particular transaction.

It seems like there is 2 pieces of PUSHDATA(71)304402...01 and from the sequence and formatting, it is a DER encoded signature.

My question would be how would there appear to be 2 pieces of PUSHDATA(71)304402...01 which is 2 pieces of digital signatures ?

Shouldn't a private key only generate a single digital signature and not two ?

Also, how do I interpret the final chunk regarding the PUSHDATA1[5221...53ae]

Answer 1

This is a 5 input one output multisig transaction. The decoding of the first sigscript

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

can be decomposed into these sections:

       00
sig1   47304402201C45383CC6E43202ED069E36184A97BF5DD489C6BEA1372629540DEA154C424902200A950B3557BBA9AE9531237D46E6EB1AFFFD01EBA989C3E53ADAAC443E5E2A2E01
sig2   473044022032A681FB77589CE1A29A84494A6F9CB19630FCD65480FF646F30D8B826980390022006DC71BABA1142ABB275BBF3613B2A13B347BAC4D09D41CCD9B69B02C270D8C201
length 4C69
OP_2   52
pk1    21 03745AAAF364030720B2D14DE50A3310EEF521C91E36353DCA20813713535C005A
pk2    21 02DB8911B3989B43C43D8DD6E50459BD85C38FAF3B2862EB78EF297002775A10BD
pk3    21 0351E3F71B7CF9A5F5F86C1908FEE02EBF5A1ED77B6748F7486505D155833645F2
OP_3   53
check  AE

So this is a two of three multisig tx, which has provided two signatures, and three public keys. To explain each command, here are the details:

00: OP_0, OP_FALSE:      an empty array is pushed onto the stack.
47: OP_DATA_0x47:        push hex 47 (decimal 71) bytes on stack
30: OP_SEQUENCE_0x30:    type tag indicating SEQUENCE, begin sigscript
44: OP_LENGTH_0x44:      length of R + S
02: OP_INT_0x02:         type tag INTEGER indicating length
20: OP_LENGTH_0x20:      this is SIG R (32 Bytes)
    1C45383CC6E43202:ED069E36184A97BF:5DD489C6BEA13726:29540DEA154C4249
02: OP_INT_0x02:         type tag INTEGER indicating length
20: OP_LENGTH_0x20:      this is SIG S (32 Bytes)
    0A950B3557BBA9AE:9531237D46E6EB1A:FFFD01EBA989C3E5:3ADAAC443E5E2A2E
01: OP_SIGHASHALL:       this terminates the ECDSA signature (ASN1-DER structure)
#########################################################
47: OP_DATA_0x47:        push hex 47 (decimal 71) bytes on stack
30: OP_SEQUENCE_0x30:    type tag indicating SEQUENCE, begin sigscript
44: OP_LENGTH_0x44:      length of R + S
02: OP_INT_0x02:         type tag INTEGER indicating length
20: OP_LENGTH_0x20:      this is SIG R (32 Bytes)
    32A681FB77589CE1:A29A84494A6F9CB1
    9630FCD65480FF64:6F30D8B826980390
02: OP_INT_0x02:         type tag INTEGER indicating length
20: OP_LENGTH_0x20:      this is SIG S (32 Bytes)
    06DC71BABA1142AB:B275BBF3613B2A13
    B347BAC4D09D41CC:D9B69B02C270D8C2
01: OP_SIGHASHALL:       this terminates the ECDSA signature (ASN1-DER structure)
#########################################################
4C: OP_PUSHDATA1:        next byte is # of bytes that go onto stack
69: OP_Int(0x01-0xff):   105 bytes onto the stack
52: OP_2:                the number 2 is pushed onto stack
    ################### we go multisig ####################################
21: OP_DATA_0x21:        compressed pub key (33 Bytes)
    03745AAAF3640307:20B2D14DE50A3310:EEF521C91E36353D:CA20813713535C00:5A
    This is MultiSig's compressed Public Key (X9.63 form)
    corresponding bitcoin address is:    1GNXpcYzasmmXvM4rNgkvZ5SzXgL4L9Ch6
21: OP_DATA_0x21:        compressed pub key (33 Bytes)
    02DB8911B3989B43:C43D8DD6E50459BD:85C38FAF3B2862EB:78EF297002775A10:BD
    This is MultiSig's compressed Public Key (X9.63 form)
    corresponding bitcoin address is:    1KHCXt29T7cHfPA88SpPL128h9PyZMi2uR
21: OP_DATA_0x21:        compressed pub key (33 Bytes)
        21: OP_DATA_0x21:        compressed pub key (33 Bytes)
    0351E3F71B7CF9A5:F5F86C1908FEE02E:BF5A1ED77B6748F7:486505D155833645:F2
    This is MultiSig's compressed Public Key (X9.63 form)
    corresponding bitcoin address is:    1PjUtVWKkr2GxA5HrkAmvhpyHGUwNPyQzH
53: OP_3:                the number 3 is pushed onto stack
    ################### 2-of-3 Multisig ###################################
AE: OP_CHECKMULTISIG:    terminating multisig
    corresponding bitcoin address is:    33jhmQXe4Q9dP24Jm9EVa6XHxn85sZnQMR

Now if all this looks like being to much for the beginning, I recommend reading Andreas' book "Mastering Bitcoin", here especially chapter 7 "Advanced Transactions and Scripting".