encoding/asn1: string type tags are not documented to work in Unmarshal

[mjg59]

What version of Go are you using (go version)?

1.14

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

linux amd64

What did you do?

I have the following struct:

type ComponentIdentifierV2 struct {
        ComponentClass           ComponentClass
        ComponentManufacturer    string
        ComponentModel           string
        ComponentSerial          string                `asn1:"optional,tag:0"`
        ComponentRevision        string                `asn1:"optional,tag:1"`
        ComponentManufacturerID  int                   `asn1:"optional,tag:2"`
        FieldReplaceable         bool                  `asn1:"optional,tag:3"`
        ComponentAddresses       []ComponentAddress    `asn1:"optional,tag:4"`
        ComponentPlatformCert    CertificateIdentifier `asn1:"optional,tag:5"`
        ComponentPlatformCertURI string                `asn1:"optional,tag:6"`
        Status                   asn1.Enumerated       `asn1:"optional,tag:7"`
}

Note that ComponentRevision is tag:1, so context specific. The fact that it's a string means that getUniversalType() will return PrintableString, and this will not be overridden because there's no explicit tag type in the encoded stream.

The spec definition of this struct is:

ComponentIdentifier ::= SEQUENCE {
componentClass ComponentClass,
componentManufacturer UTF8String (SIZE (1..STRMAX)),
componentModel UTF8String (SIZE (1..STRMAX)),
componentSerial[0] IMPLICIT UTF8String (SIZE (1..STRMAX)) OPTIONAL,
componentRevision [1] IMPLICIT UTF8String (SIZE (1..STRMAX)) OPTIONAL,
componentManufacturerId [2] IMPLICIT PrivateEnterpriseNumber OPTIONAL,
fieldReplaceable [3] IMPLICIT BOOLEAN OPTIONAL,
componentAddresses [4] IMPLICIT SEQUENCE(SIZE(1.. MAX)) OF ComponentAddress OPTIONAL
componentPlatformCert [5] IMPLICIT CertificateIdentifier OPTIONAL,
componentPlatformCertUri [6] IMPLICIT URIReference 
}

ie, componentRevision is a UTF8String, not a PrintableString. The certificate I'm parsing has an @ sign in, which is legitimate in a UTF8String but not a PrintableString. Since we have no explicit information in the stream to treat this as a UTF8String we continue treating it as a PrintableString, and then fail because of the "invalid" character.

What did you expect to see?

A string containing an @ sign.

What did you see instead?

asn1: syntax error: PrintableString contains invalid character

[mjg59]

This actually works if I do:

asn1:"utf8,optional,tag:1"

which is documented as doing something in Marshal() but not in Unmarshal().

[ericchiang]

I can take the documentation fix

[gopherbot]

Change https://golang.org/cl/269777 mentions this issue: encoding/asn1: add utf8 tag doc for Unmarshal method.

[gopherbot]

Change https://golang.org/cl/269798 mentions this issue: encoding/asn1: document unmarshaling behavior for IMPLICIT string fields