GPG Signature Verification

Peter Lebbing peter at
Wed Apr 26 12:23:49 CEST 2017

On 24/04/17 19:23, Peter Lebbing wrote:
> The part of "cmp" that would correspond to the constant part of the DER
> encoding I do not recognise.

It is still proper ASN.1, but it encodes a slightly different structure.

I wondered whether it was DER encoded or BER encoded, because I read 
that BER was valid for old PKCS#1 v1.5 structures. DER is a subset of 
BER. If the Python ASN.1 module pyasn1 rejects malformed DER encoding 
then it is proper DER; or would pyasn1 be liberal in what it accepts?

Anyway, the two ASN.1 encoded objects are slightly different:

$ python
>>> import pyasn1.codec.der.decoder
>>> sha256der = b'\x30\x31\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x01\x05\x00\x04\x20' + chr(0)*32
>>> unknown_enc=b'\x30\x2f\x30\x0b\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x01\x04\x20'+chr(0)*32
>>> pyasn1.codec.der.decoder.decode(sha256der)
(Sequence().setComponentByPosition(0, Sequence().setComponentByPosition(0, ObjectIdentifier(2.16.840., Null(''))).setComponentByPosition(1, OctetString(hexValue='0000000000000000000000000000000000000000000000000000000000000000')), '')
>>> pyasn1.codec.der.decoder.decode(unknown_enc)
(Sequence().setComponentByPosition(0, Sequence().setComponentByPosition(0, ObjectIdentifier(2.16.840., OctetString(hexValue='0000000000000000000000000000000000000000000000000000000000000000')), '')

There's an extra setComponentByPosition(1, Null('')) in the properly 
encoded ASN.1. It would appear that the library you're using *is* 
trying to generate a PKCS#1 v1.5 message, but that it ends up with a 
slightly different DER encoding than what is defined for OpenPGP. You 
will have to find a way to generate a EMSA-PKCS1-v1_5 structure that is 
compatible to RFC 4880 (and RFC 3447 PKCS #1 Version 2.1).

I don't know much about ASN.1, so I can't really say anything useful 
about the results of the experiment above.



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