I try to understand why the signature length is variable in ECDSA.
Normally with 160bits EC, it must be 46 bytes long: 20 bytes for each of
the 2 components and 4 + 2 bytes for ASN.1.

I think it must be a bug in the ASN.1 creation of the signature.

Here is the details:

I'm testing this with openssl-0.9.8g.
I create a key:
openssl ecparam -out ec_key.pem -name secp160k1 -genkey
And I sign with it:
openssl dgst -ecdsa-with-SHA1 -sign ec_key.pem -out test_ec.sign test.txt

Now this signature (test_ec.sign) is between 46 and 48 bytes long if I
done it a few times.

I compared the asn1parse output with the hexadecimal content of the
signature, and the difference is an extra 0x00 before one or the two
members of the pair (r,s).

1. the most frequent case: 47 bytes:
asn.1 structure:
0:d=0 hl=2 l= 45 cons: SEQUENCE 2:d=1 hl=2 l= 21 prim:
INTEGER :BD8188D4FB9445C456FF257BC9A77E759CC63DA9
25:d=1 hl=2 l= 20 prim: INTEGER
the signature in hexadecimal:
302d0215_00bd8188d4fb9445c456ff257bc9a77e759cc63da 9_0214_2ac486bb6df4d81a44b38ce319935270b22cacc8

I put a _ to clearly separate the elements.

2. 48 bytes signature:
0:d=0 hl=2 l= 46 cons: SEQUENCE 2:d=1 hl=2 l= 21 prim:
INTEGER :95CB1F3A35F4358D158BE94BA41031CE1563CD0F
25:d=1 hl=2 l= 21 prim: INTEGER
302e0215_0095cb1f3a35f4358d158be94ba41031ce1563cd0 f_0215_00a07d76ef47cf74d385ff60da7ebf8e86652ad230

3. 46 bytes signature:
0:d=0 hl=2 l= 44 cons: SEQUENCE 2:d=1 hl=2 l= 20 prim:
INTEGER :22294F048F61B727DB3B0786D440717532601082
24:d=1 hl=2 l= 20 prim: INTEGER
302c0214_22294f048f61b727db3b0786d440717532601082_ 0214_09d21753a2dd8395cb965d583f27835b051e7c42

I reproduced this on a recent snapshot of the 0.9.9-dev branch.

Now, if I modified the signature to remove the extra 0x00 preceding one
of the members and modifying the length component in ASN.1, the
signature is still valid...

Thanks for your help,

Best Regards,


Dr Jean-Pierre Szikora e-mail: jean-pierre.szikora@uclouvain.be
tel: 32-2-764.75.00
74, av. Hippocrate - UCL 7459 fax: 32-2-764.65.65
1200 Brussels - Belgium

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