this link doesnt seem to add up, any suggestions?
closed as unclear what you're asking by Andrew Chow♦, Pieter Wuille, MCCCS, cdecker, Murch♦ Sep 5 '17 at 20:47
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The structure of a DER encoded ECDSA signature is as follows:
30 identifies a
SEQUENCE in ASN1 encoding, which is followed by the length of
z (the sequence).
scan be either 32 or 33 bytes long, depending on how big the DER encoded values are.
s are always leaded by
02, which identify an integer value in ASN1. Finally, the tailing (
ht) byte represents the hashtype
In order to parse a signature to extract both values you should check their length, which is always 1 byte long, and extract the value depending on the result. Here's the code to do so:
def parse_element(hex_str, offset, element_size): """ :param hex_str: string to parse the element from. :type hex_str: hex str :param offset: initial position of the object inside the hex_str. :type offset: int :param element_size: size of the element to extract. :type element_size: int :return: The extracted element from the provided string, and the updated offset after extracting it. :rtype tuple(str, int) """ return hex_str[offset:offset+element_size], offset+element_size def dissect_signature(hex_sig): """ Extracts the r, s and ht components from a Bitcoin ECDSA signature. :param hex_sig: Signature in hex format. :type hex_sig: hex str :return: r, s, t as a tuple. :rtype: tuple(str, str, str) """ offset = 0 # Check the sig contains at least the size and sequence marker assert len(hex_sig) > 4, "Wrong signature format." sequence, offset = parse_element(hex_sig, offset, 2) # Check sequence marker is correct assert sequence == '30', "Wrong sequence marker." signature_length, offset = parse_element(hex_sig, offset, 2) # Check the length of the remaining part matches the length of the signature + the length of the hashflag (1 byte) assert len(hex_sig[offset:])/2 == int(signature_length, 16) + 1, "Wrong length." # Get r marker, offset = parse_element(hex_sig, offset, 2) assert marker == '02', "Wrong r marker." len_r, offset = parse_element(hex_sig, offset, 2) len_r_int = int(len_r, 16) * 2 # Each byte represents 2 characters r, offset = parse_element(hex_sig, offset, len_r_int) # Get s marker, offset = parse_element(hex_sig, offset, 2) assert marker == '02', "Wrong s marker." len_s, offset = parse_element(hex_sig, offset, 2) len_s_int = int(len_s, 16) * 2 # Each byte represents 2 characters s, offset = parse_element(hex_sig, offset, len_s_int) # Get ht ht, offset = parse_element(hex_sig, offset, 2) assert offset == len(hex_sig), "Wrong parsing." return r, s, ht example_sig = None # insert_sig_here r, s, ht = dissect_signature(example_sig) print "r: %s\ns: %s\nht: %s\n" % (r, s, ht)
First "Signature" (sans the public key) which contains the R and S values is:
This breaks down into:
30, I think, is standard.
44 is the number of bytes that will be taken up by, R, S, and their preceding numbers. 44 in hex is 68 in decimal form. Thus referencing that after this point until (but not including) the
01 there will be 68 bytes of data or 136 alpha numeric characters.
02 is a number which identifies the following value (in this case, R) as an integer (I think)
20 is the number which identifies the number of bytes (or number of hex characters/numbers times 2) of the following value (in this case, R). This value is in hex (ie: 20 in hex is 32 in decimal, signifying 32 bytes or 64 alpha/numeric characters)
6878b5690514437a2342405029426cc2b25b4a03fc396fef845d656cf62bad2c is the actual "R" value referenced by the preceding
02 identifies the following value as an integer (in this case, S).
20 is the number which identifies the number of bytes (or number of hex characters/numbers times 2) of the following value (in this case, S). This value is in hex (ie: 20 in hex is 32 in decimal, signifying 32 bytes or 64 alpha/numeric characters)
18610a8d37e3384245176ab49ddbdbe8da4133f661bf5ea7ad4e3d2b912d856f is the "S" value referenced by the preceding
I think the
01 is merely a reference as to the end of this part of the transaction data.
Some R and S values may be different lengths, the preceding numbers will tell you how long they are.
Information for additional reference:
R = 47253809947851177065887724633329625063088643784040492056218945870752194997548
S = 11026965355983493404719379810734327200902731292741433431270495068542334764399
The Second Input in this Transaction
Breaks down to the following
To reference your comment in your question
In the first part of the example transaction I showed, to rip the R value from the data, you would have to remove the first 3 bytes (or the
304402, aka the first 6 individual numbers). Then you would have to convert the subsequent first byte (or 2 digit number) being
20. You would then have to convert this into decimal (which would be 32). Then erase the
20 and then read ONLY the next 32 bytes (aka the "R" value), which could also be described as reading the next 64 alphanumeric characters. You could then start the whole thing over again and shift the values over to identify the S value instead of the R value.