 | | From: | Robin de Krijger | | Subject: | From hex-notation to ASN.1 notation: how?!? | | Date: | Wed, 19 Jan 2005 16:50:03 +0100 |
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 | I have the following hex string:
0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d
Which is equal to the ASN.1 object identifier:
1.2.840.113549
I read this in the documentation. But how does this mapping work?!?
How can I calculatie the OID from the hex-string (and vica versa)
Regards,
Robin de Krijger
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| | From: | Bryan Olson | | Subject: | Re: From hex-notation to ASN.1 notation: how?!? | | Date: | Wed, 19 Jan 2005 17:12:57 GMT |
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| Robin de Krijger wrote:
> I have the following hex string:
> 0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d
>
> Which is equal to the ASN.1 object identifier:
> 1.2.840.113549
>
> I read this in the documentation. But how does this mapping work?!?
> How can I calculatie the OID from the hex-string (and vica versa)
Below is arc_list -> octets in the Python language. The actual
BER encoding would also have a tag and length field. If you want
to know more, you can download /ASN.1 Complete/ by John Larmouth
from:
http://www.oss.com/asn1/larmouth.html
I like Larmouth's book better than the /Layman's Guide/.
--Bryan
def to_base128(n):
""" Return a list of bytes holding the big-endian base-128
encoding used for tags > 30 and OID elements after the
first two. High bit is set except on last octet.
"""
# Build it least-significant byte first, then reverse the bytes.
bytes = [n & 0x7F]
while n > 127:
n >>= 7
bytes.append(n & 0x7F | 0x80)
bytes.reverse()
return bytes
def encode_arc_list(arc_list):
""" Encode an OID arc list into the value portion of a BER field.
"""
# First-octet special encoding of first two arcs
bytes = [40 * arc_list[0] + arc_list[1]]
# After that, they're base-128 big endian; top bit is continue flag.
for arc in arc_list[2:]:
bytes.extend(to_base128(arc))
return bytes
print [hex(byte) for byte in encode_arc_list([1, 2, 840, 113549])]
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| | From: | Robin de Krijger | | Subject: | Re: From hex-notation to ASN.1 notation: how?!? | | Date: | Thu, 20 Jan 2005 08:39:50 +0100 |
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| "Bryan Olson" wrote:
> Below is arc_list -> octets in the Python language. The actual
> BER encoding would also have a tag and length field.
Hm, it is not as straightfoward as I thought it would be... :-(
Thanks, I'll study it!
Regards,
Robin...
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| | From: | Bryan Olson | | Subject: | Re: From hex-notation to ASN.1 notation: how?!? | | Date: | Thu, 20 Jan 2005 08:56:44 GMT |
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| Robin de Krijger wrote:
> "Bryan Olson"
>>Below is arc_list -> octets [...]
>
> Hm, it is not as straightfoward as I thought it would be... :-(
The only really tricky part is that they encode the first two
arcs into the first octet. They assume that there are only a
few OIDs at that level (then enforce the assumption by not
granting more).
--
--Bryan
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| | From: | Paul Rubin | | Subject: | Re: From hex-notation to ASN.1 notation: how?!? | | Date: | 19 Jan 2005 08:21:45 -0800 |
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| "Robin de Krijger" writes:
> I read this in the documentation. But how does this mapping work?!?
> How can I calculatie the OID from the hex-string (and vica versa)
Google for "Layman's guide to BER" by Burt Kaliski.
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| | From: | Robin de Krijger | | Subject: | Re: From hex-notation to ASN.1 notation: how?!? | | Date: | Thu, 20 Jan 2005 08:38:33 +0100 |
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| "Paul Rubin" wrote:
> Google for "Layman's guide to BER" by Burt Kaliski.
Thanks, I'll get into it!
Regards,
Robin...
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| | From: | Bruce Stephens | | Subject: | Re: From hex-notation to ASN.1 notation: how?!? | | Date: | Wed, 19 Jan 2005 16:14:29 +0000 |
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| "Robin de Krijger" writes:
[...]
> I read this in the documentation. But how does this mapping work?!?
> How can I calculatie the OID from the hex-string (and vica versa)
If you really want to know, "A Layman's Guide to a Subset of ASN.1,
BER, and DER" will tell you. It's available all over the place, for
example, .
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| | From: | Robin de Krijger | | Subject: | Re: From hex-notation to ASN.1 notation: how?!? | | Date: | Thu, 20 Jan 2005 08:38:06 +0100 |
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| "Bruce Stephens" wrote:
[...]
> If you really want to know, "A Layman's Guide to a Subset of ASN.1,
> BER, and DER" will tell you. It's available all over the place, for
> example, .
Thanks, I'll get into it!
Regards,
Robin...
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