asn1go

Rationale

Built-in Go libraries for asn1 support are either reflection-based (crypto/asn1) or very low-level (golang.org/x/crypto/cryptobyte). Idea is to provide Protobuf-like experience for working with ASN1 in Golang.

Note: currently provided code generator implementation creates definitions to be used with crypto/asn1, so all its limitations (no real CHOICE support) apply for this project as well.

Architecture

1. Custom Lexer consumes from bufio.Reader and called by Parser
2. Parser is built using goyacc based on BNF provided in X.680 standard. As the result, Parser produces ASN1 module AST.
3. AST is used by Code Generator to produce declarations, serialization, and deserialization code.

Supported features

Feature categories

Feature	Parsing	Codegen
Exports	Syntax only	No
Imports	Yes	No
Type assignments	Yes	Yes
Value assignments	Yes	Partial 1
XML	No
Objects	No
Parameterization	No

Types

Type	Parsing	Codegen
BIT STRING	Yes	Yes; named bits not translated
BOOLEAN	Yes	Yes
CHARACTER STRING	Yes	Yes
CHOICE	Yes	Yes; common demoninator type is used
Embedded PDV	No
External	No
ENUMERATED	Yes 2	Yes; alternative values not translated
Instance Of	No
INTEGER	Yes	Yes
NULL	Yes
Object Class	No
Object Identifier	Yes
OCTET STRING	Yes	Yes
REAL	Yes	Yes
Relative OID	No
SEQUENCE	Yes 3	Yes
SEQUENCE OF	Yes	Yes
SET	Yes 3	Yes
SET OF	Yes	Yes
ANY	Yes 4	Yes
Tagged types	Yes	Yes 5
Constrained types	Partial	Partial; generates wrapped type

Values

Value	Parsing	Codegen
BOOLEAN	Yes	Yes
INTEGER	Yes	Yes
OID	Yes	No
Real	Yes	Yes
Referenced	No
Object class fields	No
BIT STRING	No
Other	No

Roadmap

1. Lexer

• identifiers
• numbers
• keywords
• symbols
• strings, bit strings, hex strings
• XML

2. Parser

• module definition BNF
• parse Kerberos (rfc4120)
• yield AST from parser
• parse SNMPv1 (rfc1157, rfc1155); no codegen, depends on CHOICE
• parse LDAP (rfc4511, partially - required minor modifications); no codegen, depends on CHOICE
• parse X.509 (rfc 5280) - depends on ANY
• SNMPv2 (rfc3411–3418)

3. Code Generator

• declaration generator
• crypto/asn1 compatible generation mode
• verify serialization on Kerberos
• DER serialization generator
• DER deserialization generator

4. Supported ASN features

• SET type
• ANY type (1988 standard) - mapped to interface{}
• CHOICE type - mapped to interface{}, or asn1.RawValue if selections are tagged
• Extensions in SEQUENCE, SET, CHOICE
• Add more as found

Adding features

Real-world ASN.1 descriptions from RFC documents are used to gauge completeness of the implementation. Standard itself is pretty huge, so features are added as needed.

Typically, this requires:

1. Modifying asn1.y to uncomment unsupported branches of syntax notation, and add missing declarations. Refer to goyacc documentation for .y syntax notation.
2. Extending ast.go with necessary fields and types.
3. Modifying codegen.go to produce corresponding Go declarations. Note that not all ASN declarations can be mapped to crypto/asn1.

For example, refer to CHOICE implementation.

Footnotes

1. Only literal values are supported, referenced values are not implemented. ↩

2. With ASN.1 syntax limitations: explicit extensibility and non-literal values are not supported. ↩

3. With ASN.1 syntax limitations: two component type lists, exceptions and extension addition groups are not supported, extensions are not exposed in generated Go code. ↩ ↩²

4. Not defined in the latest ASN.1 standard. ↩

5. Used by encoding/asn1 only in SEQUENCE and SET fields. CHOICE with tagged alternatives is represented as RawValue. ↩