ASN.1 Unmarshalling using go structs gives tags do

Question

I am trying to do ASN.1 marshal/unmarshal for the following definition:

ACEI    ::= SEQUENCE {
message         MessageFields,
neRegNumber     OCTET STRING OPTIONAL,
gpsInfo         GpsInfo OPTIONAL,
siteInfo        OCTET STRING OPTIONAL,
nlementID   INTEGER(0..16777216) OPTIONAL,
...
}

GpsInfo         ::= SEQUENCE {
gpsLat      INTEGER(-900000000..900000000) OPTIONAL,
gpsLong     INTEGER(-1800000000..1800000000) OPTIONAL,
gpsAlt      INTEGER OPTIONAL,
...
}
MessageFields       ::= SEQUENCE {
messageSequence     INTEGER (1..65535),
bsId    INTEGER (1..65535) OPTIONAL,
neID        INTEGER(0..16777216) OPTIONAL, -- unsigned int
nelementID  INTEGER(0..16777216) OPTIONAL, -- unsigned int
...
}

The corresponding go structs are:

type ACEI struct {
    Message          MessageFields
    NeRegNumber      []byte `asn1:"optional"`
    GPSInfo          GPSInfo `asn1:"optional"`
    SiteInfo         []byte `asn1:"optional"`
    NElementID       int `asn1:"optional"`
}

type GPSInfo struct {
    GpsLatitude  int `asn1:"optional"`
    GpsLongitude int `asn1:"optional"`
    GpsAltitude  int `asn1:"optional"`
}

type MessageFields struct {
    MessageSequence  int
    BsId             int `asn1:"optional"`
    NeID             int `asn1:"optional"`
    NElementID       int `asn1:"optional"`
}

I am filling in the structs, marshalling them and then converting them to hex. When I do that, the hex sequence (seqA) obtained is:

302e300f020101020204d2020215b302021a0a040430413042300b02019c020200be020200c80404304330440202309c

When I do the same on http://asn1-playground.oss.com/, I get the following hex sequence (seqB):

302AA00F 80010181 0204D282 0215B383 021A0A81 020A0BA2 0B80019C 810200BE 820200C8 83020C0D 8402309C

I fed in both these hex sequences to the unmarshal function; while the seqA is correctly unmarshalled, unmarshalling seqB gives me the following error:

Error while unmarshalling: asn1: structure error: tags don't match (16 vs {class:2 tag:0 length:15 isCompound:true}) {optional:false explicit:false application:false defaultValue: tag: stringType:0 timeType:0 set:false omitEmpty:false} MessageFields @2

This is the code that marshals/unmarshals:

func main() {
    //Marshalling
    messageSequence := structs.MessageFields{1, 1234, 5555, 6666}
    gpsInfo := structs.GPSInfo{-100, 190, 200}
    val := structs.ACEI{messageSequence, []byte("0A0B"), gpsInfo, []byte("0C0D"), 12444}
    hexmdata := asn1Marshal(val)

    //Unmarshalling hex sequence (seqA) generated by go code
    res1, _ := asn1Unmarshal(hexmdata)
    fmt.Println(res1)

    //Unmarshalling hex sequence (seqB) generated by http://asn1-playground.oss.com/ 
    res2, _ := asn1Unmarshal(strings.ToLower("302AA00F800101810204D2820215B383021A0A81020A0BA20B80019C810200BE820200C883020C0D8402309C"))
    fmt.Println(res2)
}

func asn1Unmarshal(hexmdata string) (structs.ACEI, error){
    fmt.Println(hexmdata)
    s, _ := hex.DecodeString(hexmdata)
    res := structs.ACEI{}
    _, err := asn1.Unmarshal(s, &res)
    if err != nil {
        fmt.Println("Error while unmarshalling: ", err)
    }
    return res, err
}

func asn1Marshal(data structs.ACEI) string {
    mdata, _ := asn1.Marshal(data)
    hexmdata := hex.EncodeToString(mdata)
    return hexmdata
}

1. Why are the hex sequences different for the same definition?
2. How can I correct the error that I get during unmarshalling of seqB?

EDIT: Conversely, the go code sequence (seqA) when fed to http://asn1-playground.oss.com/ gives me this error:

ACEI SEQUENCE: tag = [UNIVERSAL 16] constructed; length = 46
D0033E: Tag mismatch or tag not expected: [UNIVERSAL 16] (expected tag [0]); check field 'message' (type: MessageFields) of PDU #1 'ACEI'.
  *SKIPPED*: tag = [UNIVERSAL 16] constructed; length = 15
    <skipped>
D0033E: Tag mismatch or tag not expected: [UNIVERSAL 4] (expected tag [0]); check field 'message' (type: MessageFields) of PDU #1 'ACEI'.
  *SKIPPED*: tag = [UNIVERSAL 4] primitive; length = 4
    <skipped>
D0033E: Tag mismatch or tag not expected: [UNIVERSAL 16] (expected tag [0]); check field 'message' (type: MessageFields) of PDU #1 'ACEI'.
  *SKIPPED*: tag = [UNIVERSAL 16] constructed; length = 11
    <skipped>
D0033E: Tag mismatch or tag not expected: [UNIVERSAL 4] (expected tag [0]); check field 'message' (type: MessageFields) of PDU #1 'ACEI'.
  *SKIPPED*: tag = [UNIVERSAL 4] primitive; length = 4
    <skipped>
D0033E: Tag mismatch or tag not expected: [UNIVERSAL 2] (expected tag [0]); check field 'message' (type: MessageFields) of PDU #1 'ACEI'.
  *SKIPPED*: tag = [UNIVERSAL 2] primitive; length = 2
    <skipped>
D0049E: Field omitted: "message"; check PDU #1 'ACEI'.
S0012E: Decoding of PDU #1 failed with the return code '5'.

EDIT 2: After editing the structs according to @YaFred's suggestion, my structs look like this now:

type ACEI struct {
        Message          MessageFields `asn1:"application,tag:0,implicit"`
        NeRegNumber      []byte `asn1:"application,tag:1,implicit,optional"`
        GPSInfo          GPSInfo `asn1:"application,tag:2,implicit,optional"`
        SiteInfo         []byte `asn1:"application,tag:3,implicit,optional"`
        NElementID int `asn1:"application,tag:4,implicit,optional"`
    }

type GPSInfo struct {
    GpsLatitude  int `asn1:"application,tag:0,implicit,optional"`
    GpsLongitude int `asn1:"application,tag:1,implicit,optional"`
    GpsAltitude int `asn1:"application,tag:2,implicit,optional"`
}

type MessageFields struct {
    MessageSequence  int `asn1:"application,tag:0,implicit"`
    BsId int `asn1:"application,tag:1,implicit,optional"`
    NeID        int `asn1:"application,tag:2,implicit,optional"`
    NElementID int `asn1:"application,tag:3,implicit,optional"`
}

Marshalling using these structs gives me the same hex code as that obtained from asn playground. But, the unmarshalling fails with the following error:

Hex code: 302aa00f800101810204d2820215b383021a0a81020a0ba20b80019c810200be820200c883020c0d8402309c

Error (the same error I was getting before when I tried to unmarshall hex code (from asn playground) using go code):

Error while unmarshalling: asn1: structure error: tags don't match (0 vs {class:2 tag:0 length:15 isCompound:true}) {optional:false explicit:false application:true defaultValue: tag:0xc042008348 stringType:0 timeType:0 set:false omitEmpty:false} MessageFields @2

EDIT 3: Removing the "application" tag from the structs helps me unmarshal the hex code as expected.

Answer 1

Where you input your types (with your tool as well as with http://asn1-playground.oss.com/), you must use a module. If not, it is not a valid asn1 specification (and your tool as well as oss.com should reject it).

My-module DEFINITIONS ::= 
BEGIN 

ACEI    ::= SEQUENCE {
etc ...

END

The fact that tools accept to encode types without knowing the module tagging context is the reason for many questions on https://stackoverflow.com/questions/tagged/asn.1

To answer the second question (about the difference of length), you should show what values you gave to oss.com .. it's likely:

neRegNumber  '0A0B'H
siteInfo     '0C0D'H    

They are both 2 bytes long

But when you write []byte("0A0B") you are certainly asking Go to give you the bytes of the string "0A0B" (indeed 30 41 30 42 in ASCII)

I think []byte{10,11} should give you the bytes for '0A0B'H

EDITED

If your tool does not take an asn.1 module, you can still do the automatic tagging yourself

ACEI    ::= SEQUENCE {
message         [0] IMPLICIT MessageFields,
neRegNumber     [1] IMPLICIT OCTET STRING OPTIONAL,
gpsInfo         [2] IMPLICIT GpsInfo OPTIONAL,
siteInfo        [3] IMPLICIT OCTET STRING OPTIONAL,
nlementID       [4] IMPLICIT INTEGER(0..16777216) OPTIONAL,
...
}

GpsInfo         ::= SEQUENCE {
gpsLat      [0] IMPLICIT INTEGER(-900000000..900000000) OPTIONAL,
gpsLong     [1] IMPLICIT INTEGER(-1800000000..1800000000) OPTIONAL,
gpsAlt      [2] IMPLICIT INTEGER OPTIONAL,
...
}
MessageFields       ::= SEQUENCE {
messageSequence     [0] IMPLICIT INTEGER (1..65535),
bsId                [1] IMPLICIT INTEGER (1..65535) OPTIONAL,
neID                [2] IMPLICIT INTEGER(0..16777216) OPTIONAL, -- unsigned int
nelementID          [3] IMPLICIT INTEGER(0..16777216) OPTIONAL, -- unsigned int
...
}

EDITED 2 (based on chat with @Aarvi)

It just occurred to me that https://golang.org/pkg/encoding/asn1/ does not involve an ASN.1 compiler.

Instead a Go struct is manually written and annotated.

So following ASN.1 type (in a module with AUTOMATICS TAGS selected)

GpsInfo         ::= SEQUENCE {
   gpsLat      INTEGER(-900000000..900000000) OPTIONAL,
   gpsLong     INTEGER(-1800000000..1800000000) OPTIONAL,
   gpsAlt      INTEGER OPTIONAL,
   ...
}

has to be written manually like this

type GPSInfo struct {
    GpsLatitude  int `asn1:"tag:0,implicit,optional"`
    GpsLongitude int `asn1:"tag:1,implicit,optional"`
    GpsAltitude int `asn1:"tag:2,implicit,optional"`
}