ASN.1 accross revisions

Pete Cordell pete.cordell at BT-SYS.BT.CO.UK
Tue Jun 30 14:43:18 EDT 1998

Dear All,

I admit that I am only just hanging in there with this debate, but I
think I have a possible solution 5 to throw in as a contender.

Looking at the problem a bit laterally, we have RasMessage in UDP
packets that we want to sign, and H323-UserInformation in the UUIE that
we want to sign.  Currently these are the only chunks of ASN.1 in these

We could add a second piece of ASN.1 into these fields (UDP packet and
UUIE) that contains the signatures, such as:

H323Extra ::= CHOICE
        icv     ICV,

This would be a separate ASN.1 tree.  Therefore in a RAS UDP packet you
would get:

        RasMessage chunk of ASN.1
        H323Extra chunk of ASN.1 typically containing signature

Similarly in the UUIE, you would have

        H323-UserInformation chunk of ASN.1
        H323Extra chunk of ASN.1

Note that all the key ids and time stamps etc., would remain in the
RasMessage and H323-UserInformation parts (so they get signed).

I agree this is not beautiful, but it does not require multiple ASN.1
encodings, and doesn't radically change the format of the message
depending on whether you want to sign it or not (as solution 4 seems

I hope this sounds vaguely as though I know what I'm talking about!!!


Pete Cordell
BT Labs
E-Mail: pete.cordell at
Tel: +44 1473 646436
Fax: +44 1473 645499

>From:  Bancroft Scott[SMTP:baos at OSS.COM]
>Sent:  28 June 1998 17:35
>Subject:       Re: ASN.1 accross revisions
>On Wed, 24 Jun 1998, Pekka Pessi wrote:
>>    The current H.225.0 algorithm for calculating the ICV does not work well
>>    with the PER extension mechanism.
>>    The above algorithm (AFAIK I have understood it correctly) assumes that
>>    the sender and receiver can encode the PDU exactly in the same way.
>>It is
>>    a reasonable assumption, as PER produces only one possible (canonical)
>>    encoding for given PDU.
>PER is not inherently canonical.  To get canonical behavior you have to
>either do as this set of standards have been doing and stay away from
>types REAL and SET OF, or you have to use Canonical PER Aligned or
>Canonical PER Unaligned.
>> However, there is a serious problem: the
>>    canonical representation changes if the ASN.1 SEQUENCE type is extended
>>    later.
>>    When encoding this, the length of extension bitfield is 2.  However,
>>    recipient B is using extended version of Foo like this:
>>       Foo ::= SEQUENCE
>>       {
>>            bar                 INTEGER (0..127),
>>            ...,
>>            baz                 INTEGER (0..255) OPTIONAL,
>>            integrityCheckValue ICV OPTIONAL,
>>            importantExtension  SomeType OPTIONAL
>>       }
>A lot of research has gone into how to most effectively create ICV's
>by the designers of the Secure Electronic Payment protocol (SET),
>e-check protocol and others.  That's why in SET you see the likes of:
>    UnsignedCertificate ::= SEQUENCE {
>       version               [0] CertificateVersion,
>       serialNumber          CertificateSerialNumber,
>       signature             AlgorithmIdentifier
>       issuer                Name,
>       validity              Validity,
>       subject               Name,
>       subjectPublicKeyInfo
>       issuerUniqueID        [1] IMPLICIT UniqueIdentifier OPTIONAL,
>       subjectUniqueID       [2] IMPLICIT UniqueIdentifier OPTIONAL,
>       extensions            [3] Extensions     -- Required for SET
>     }
>    -- Compute the encrypted hash of this value if issuing a
>    -- or recompute the issuer's signature on this value if validating
>    -- certificate.
>    --
>    EncodedCertificate ::= TYPE-IDENTIFIER.&Type (UnsignedCertificate)
>    Certificate::=   SIGNED {
>       EncodedCertificate
>    } ( CONSTRAINED BY { -- Verify Or Sign Certificate -- } )
>    SIGNED { ToBeSigned } ::= SEQUENCE {
>       toBeSigned  ToBeSigned,
>       algorithm   AlgorithmIdentifier {{SignatureAlgorithms}},
>       signature   BIT STRING
>    }
>The key thing to notice is that they DO NOT attempt to sign the
>certificate (i.e., UnsignedCertficate).  The actual signing occurs on
>open type which has been constrained to carry the value of the
>type that is to be signed (TYPE-IDENTIFIER.&Type
>In so doing, a) they clearly identify that the type should be encoded
>independently of the ICV, b) they avoid having to do a re-encode of the
>data after the CRV has been calculated, c) the type to which the ICV is
>be applied can be extended with no problem, and d) they prepared
>themselves for the possible use of PER by utilizing an open type to
>the value to be signed - something that is *crucial* to signing
>PER-encoded data.
>> Problems with Non-OPTIONAL Extensions
>>    Another problem with some ASN.1 compilers is inclusion of non-OPTIONAL
>>    extensions. Let us assume that software C uses following ASN.1
>>    for Foo:
>>       Foo ::= SEQUENCE
>>       {
>>            bar                 INTEGER (0..127),
>>            ...,
>>            baz                 INTEGER (0..255) OPTIONAL,
>>            integrityCheckValue ICV OPTIONAL,
>>            importantExtension  SomeType OPTIONAL,
>>            criticalExtension   OtherType
>>       }
>>    There are two kinds of problems with an extension like
>>    First, the encoder may try to ensure that all encoded PDUs conform
>>to the
>>    specification and signal an error when a PDU without
>>criticalExtension is
>>    encoded.
>That's the correct behavior only if you are originating a message.
>Encoders should not complain about criticalExtension being absent
>if they are not originating the message.
>>    Another problem is that the intermediate representation produced
>>    by the ASN.1 compiler may not provide means for application to express
>>    that criticalExtension is not present. (In other words, the produced
>>    structure usually contains a flag telling whether an optional field is
>>    present or not. Such flags are not included when the field is not
>>    optional.)
>It is an error for encoders not to encode extension additions that are
>mandatory but missing if such extension additions do not occur within
>extension addition group and if no other extension addition values
>the missing but mandatory extension. (I don't believe H.225 employs
>extension addition groups (i.e., the "[[" and "]]" notation.))  This is
>indicated in X.680:1997 clause 7.1 (X.680:1994 Amd.1 clause 6.1) which
>unconditionally mandates that it be possible for decoded components
>are defined to both the sender and receiver be re-encodable by the
>>    The presense or absence of the OPTIONAL flag in an extension does not
>>    change the PER encoding of the SEQUENCE. In order to avoid previously
>>    mentioned problems, application may use a version of ASN.1 notation that
>>    has extra OPTIONAL keyword after each extension.
>Neat!  A nice and simple solution for use with such compilers.
>> Solution 1: Clarification to the PER Encoding Process
>>    The text in PER document (X.691, 1994) is somewhat ambiguous how many
>>    bits should be included in the extension present bitfield of
>>    quote verbatim: "Let the number of extension additions in the type being
>Note that it says the *type* being encoded, not the value.  That's
>distinct from the value.
>>    encoded be "n", then a bit-field with "n" bits shall be produced for
>>    addition to the field-list." (Is the "type being encoded" the abstract
>>    syntax or an actual value like { bar 1, baz 2 }?)  However, the 0
>>bits at
>>    the end of extension present bitfield can be left out without changing
>>    the resulting semantics: the corresponding extensions are not present.
>It is true that they can be left out without changing the semantics,
>this is not what PER mandates (It should have mandated what you
>but hindsight is 20-20.)  Unless everyone encodes according to PER its
>canonical nature will be lost.
>>    As a result, the PER encoding does not change after a new extension
>>    is added to the ASN.1 specification.
>>    This solution, while leaving H.225.0 v2 protocol as it is, requires
>>    however changes to some existing ASN.1 compilers, and in a pessimal
>>    to the X.691 standard text, too.
>Yes, this would work, but as you point out, it requires that PER be
>> Solution 2: Hack
>>    The receiving application does not decode and the re-encode the PDU, but
>>    rather removes the ICV from the encoded PDU. In practice, this
>>requires that
>>    application can identify PER-encoded fields within the PDU and it can
>>    regenerate them, i.e.  it has effectively the same functionality as
>>a PER
>>    encoder/decoder.
>> Solution 3: Calculating ICV Differently
>>    The following algorithm for generating and checking the ICV makes it
>>    possible to avoid all the previously mentioned problems. The
>>problems are
>>    avoided by breaking the protocol layering, the application changes
>>    the PER-encoded PDU:
>>    integrityCheckValue - provides improved message integrity/message
>>         authentication of the RAS messages. The cryptographically based
>>         integrity check value is computed by the sender applying a
>>         negotiated integrity algorithm and the secret key upon the entire
>>         message. Prior to integrityCheckValue computation an ICV with a
>>         previously agreed magic value (or key when using MDC) will be
>>         inserted to this field. The magic value will contain same
>>         algorithmOID and exactly as many bits in the icv BIT STRING as the
>>         computed value. After computation, the sender replaces the magic
>>         value with the computed integrity check value and transmits
>>         message. The receiver decodes message, replaces the received
>>         integrity value with the magic value, calculates the ICV and
>>         compares it with the received value.
>>    NOTE: The sender or receiver can encode the ICV separately and replace
>>         it directly within the encoded PDU, when the magic ICV and computed
>>         ICV have exactly the same length.  When replacing ICV value within
>>         an encoded PDU, re-encoding the whole PDU can be avoided.
>I don't like this too much because of the possibility, though low,
>of the data containing the magic ICV value.  Still I prefer it to
>solutions 1 and 2.
>> Solution 4:
>>  Fourth alternative is to treat encoded RasMessage as octet string. IVC
>>  can be calculated over that octet string and appended to the PDU on
>>  separate layer.  E.g., RAS PDU could be defined as follows:
>>      RasMessage ::= CHOICE {
>>         -- all previous RasMessage CHOICEs are here
>>         authenticatedRasMessage SEQUENCE {
>>            plainRasMessage OCTET STRING,
>>            ivc             IVC,
>>            ...
>>         }
>>      }
>This is effectively what is done in SET and other security-conscious
>protocols, though they typically use open type instead of octet string.
>Alternate Solution 1:
>To do exactly as is done in SET, you would define RasMessage without an
>ICV, and define a container, AuthenticatedRasMessage, say, that carries
>the encoded RasMessage as an open type value (i.e., octet-aligned, and
>padded with 0-bits at the end to ensure that it is an integral multiple
>8 bits) followed by the ICV value. In other words:
>    AuthenticatedRasMessage ::= SEQUENCE {
>        plainRasMessage    TYPE-IDENTIFIER.&Type (RasMessage),
>        ivc                IVC
>    }
>Using this approach, the RasMessage (which would be defined without an
>ICV) would be encoded, then the ICV would be calculated using the
>RasMessage, then AuthenticatedRasMessage would be encoded.  This
>is efficient because RasMessage does not have to be encoded without
>the ICV value then re-encoded with it, nor does it require that the
>encoded value be tinkered with in any way.
>This technique is good only if backward compatibility is not an issue
>(i.e., if you don't already have a deployed version of H.225), for the
>open type carries a length component than an ordinary encoded
>does not have.
>Alternate Solution 2:
>If backward compatibility is an issue then Solution 4 above is a better
>alternative, though I would change it to:
>    RasMessage ::= CHOICE {
>        -- all previous RasMessage CHOICEs are here
>        authenticatedRasMessage  AuthenticatedRasMessage
>    }
>Open type and octet string produce identical encodings, but I have a
>preference for the open type notation because it allows you to clearly
>identify what the value is - in this case an encoded RasMessage, and
>because it can simplify implementations that may not have a need to
>authenticate the RasMessage upon decoding (e.g., line monitors.)
>I am not intimate with H.225, so I don't know for sure of Solution 4
>or this minor variation is backward compatible.  It is if the ICV was
>just introduced, else I don't think it is.
>Bancroft Scott                                Toll Free
>Open Systems Solutions, Inc.
>baos at                                  Tech Support
>                            Fax

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