feat: ber to der convertor

cms/encoding/ber/ber.go

@@ -0,0 +1,301 @@
+// Copyright The Notary Project Authors.
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package ber decodes BER-encoded ASN.1 data structures and encodes in DER.
+// Note:
+//   - DER is a subset of BER.
+//   - Indefinite length is not supported.
+//   - The length of the encoded data must fit the memory space of the int type
+//     (4 bytes).
+//
+// Reference:
+// - http://luca.ntop.org/Teaching/Appunti/asn1.html
+// - ISO/IEC 8825-1:2021
+package ber
+
+import (
+	"bytes"
+	"encoding/asn1"
+	"fmt"
+)
+
+// value is the interface for an ASN.1 value node.
+type value interface {
+	// EncodeMetadata encodes the identifier and length in DER to the buffer.
+	EncodeMetadata(Writer) error
+
+	// EncodedLen returns the length in bytes of the data when encoding in DER.
+	EncodedLen() int
+
+	// Content returns the content of the value.
+	// For primitive values, it returns the content octets.
+	// For constructed values, it returns nil because the content is
+	// the data of all members.
+	Content() []byte
+}
+
+// ConvertToDER converts BER-encoded ASN.1 data structures to DER-encoded.
+func ConvertToDER(ber []byte) ([]byte, error) {
+	if ber == nil || len(ber) == 0 {
+		return nil, asn1.SyntaxError{Msg: "BER-encoded ASN.1 data structures is empty"}
+	}
+
+	flatValues, err := decode(ber)
+	if err != nil {
+		return nil, err
+	}
+
+	// get the total length from the root value and allocate a buffer
+	buf := bytes.NewBuffer(make([]byte, 0, flatValues[0].EncodedLen()))
+	for _, v := range flatValues {
+		if err = v.EncodeMetadata(buf); err != nil {
+			return nil, err
+		}
+
+		if content := v.Content(); content != nil {
+			// primitive value
+			_, err = buf.Write(content)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	return buf.Bytes(), nil
+}
+
+// decode decodes BER-encoded ASN.1 data structures.
+// To get the DER of `r`, encode the values
+// in the returned slice in order.
+//
+// Parameters:
+// r - The input byte slice.
+//
+// Return:
+// []value - The returned value, which is the flat slice of ASN.1 values,
+// contains the nodes from a depth-first traversal.
+// error - An error that can occur during the decoding process.
+//
+// Reference: ISO/IEC 8825-1: 8.1.1.3
+func decode(r []byte) ([]value, error) {
+	// prepare the first value
+	identifier, contentLen, r, err := decodeMetadata(r)
+	if err != nil {
+		return nil, err
+	}
+	if len(r) != contentLen {
+		return nil, asn1.SyntaxError{Msg: fmt.Sprintf("decoding BER: length octets value %d does not match with content length %d", contentLen, len(r))}
+	}
+
+	// primitive value
+	if isPrimitive(identifier) {
+		return []value{&primitive{
+			identifier: identifier,
+			content:    r,
+		}}, nil
+	}
+
+	// constructed value
+	rootConstructed := &constructed{
+		identifier: identifier,
+		rawContent: r,
+	}
+	flatValues := []value{rootConstructed}
+
+	// start depth-first decoding with stack
+	contructedStack := []*constructed{rootConstructed}
+	for len(contructedStack) > 0 {
+		stackLen := len(contructedStack)
+		// top
+		node := contructedStack[stackLen-1]
+
+		// check that the constructed value is fully decoded
+		if len(node.rawContent) == 0 {
+			// calculate the length of the members
+			for _, m := range node.members {
+				node.length += m.EncodedLen()
+			}
+			// pop
+			contructedStack = contructedStack[:stackLen-1]
+			continue
+		}
+
+		// decode the next member of the constructed value
+		nextNodeIdentifier, nextNodeContentLen, remainingContent, err := decodeMetadata(node.rawContent)
+		if err != nil {
+			return nil, err
+		}
+		nextNodeContent := remainingContent[:nextNodeContentLen]
+		node.rawContent = remainingContent[nextNodeContentLen:]
+
+		if isPrimitive(nextNodeIdentifier) {
+			// primitive value
+			primitiveNode := &primitive{
+				identifier: nextNodeIdentifier,
+				content:    nextNodeContent,
+			}
+			node.members = append(node.members, primitiveNode)
+			flatValues = append(flatValues, primitiveNode)
+		} else {
+			// constructed value
+			constructedNode := &constructed{
+				identifier: nextNodeIdentifier,
+				rawContent: nextNodeContent,
+			}
+			node.members = append(node.members, constructedNode)
+
+			// add a new constructed node to the stack
+			contructedStack = append(contructedStack, constructedNode)
+			flatValues = append(flatValues, constructedNode)
+		}
+	}
+	return flatValues, nil
+}
+
+// decodeMetadata decodes the metadata of a BER-encoded ASN.1 value.
+//
+// Parameters:
+// r - The input byte slice.
+//
+// Return:
+// []byte - The identifier octets.
+// int - The length octets value.
+// []byte - The subsequent octets after the length octets.
+// error - An error that can occur during the decoding process.
+//
+// Reference: ISO/IEC 8825-1: 8.1.1.3
+func decodeMetadata(r []byte) ([]byte, int, []byte, error) {
+	// structure of an encoding (primitive or constructed)
+	// +----------------+----------------+----------------+
+	// | identifier     | length         | content        |
+	// +----------------+----------------+----------------+
+	identifier, r, err := decodeIdentifier(r)
+	if err != nil {
+		return nil, 0, nil, err
+	}
+
+	contentLen, r, err := decodeLength(r)
+	if err != nil {
+		return nil, 0, nil, err
+	}
+
+	return identifier, contentLen, r, nil
+}
+
+// decodeIdentifier decodes decodeIdentifier octets.
+//
+// Parameters:
+// r - The input byte slice from which the identifier octets are to be decoded.
+//
+// Returns:
+// []byte - The identifier octets decoded from the input byte slice.
+// []byte - The remaining part of the input byte slice after the identifier octets.
+// error - An error that can occur during the decoding process.
+//
+// Reference: ISO/IEC 8825-1: 8.1.2
+func decodeIdentifier(r []byte) ([]byte, []byte, error) {
+	if len(r) < 1 {
+		return nil, nil, asn1.SyntaxError{Msg: "decoding BER identifier octets: identifier octets is empty"}
+	}
+	offset := 0
+	b := r[offset]
+	offset++
+
+	// high-tag-number form
+	// Reference: ISO/IEC 8825-1: 8.1.2.4
+	if b&0x1f == 0x1f {
+		for offset < len(r) && r[offset]&0x80 == 0x80 {
+			offset++
+		}
+		if offset >= len(r) {
+			return nil, nil, asn1.SyntaxError{Msg: "decoding BER identifier octets: high-tag-number form with early EOF"}
+		}
+		offset++
+	}
+
+	if offset >= len(r) {
+		return nil, nil, asn1.SyntaxError{Msg: "decoding BER identifier octets: early EOF due to missing length and content octets"}
+	}
+	return r[:offset], r[offset:], nil
+}
+
+// decodeLength decodes length octets.
+// Indefinite length is not supported
+//
+// Parameters:
+// r - The input byte slice from which the length octets are to be decoded.
+//
+// Returns:
+// int - The length decoded from the input byte slice.
+// []byte - The remaining part of the input byte slice after the length octets.
+// error - An error that can occur during the decoding process.
+//
+// Reference: ISO/IEC 8825-1: 8.1.3
+func decodeLength(r []byte) (int, []byte, error) {
+	if len(r) < 1 {
+		return 0, nil, asn1.SyntaxError{Msg: "decoding BER length octets: length octets is empty"}
+	}
+	offset := 0
+	b := r[offset]
+	offset++
+
+	if b < 0x80 {
+		// short form
+		// Reference: ISO/IEC 8825-1: 8.1.3.4
+		contentLen := int(b)
+		subsequentOctets := r[offset:]
+		if contentLen > len(subsequentOctets) {
+			return 0, nil, asn1.SyntaxError{Msg: "decoding BER length octets: short form length octets value should be less or equal to the subsequent octets length"}
+		}
+		return contentLen, subsequentOctets, nil
+	} else if b == 0x80 {
+		// Indefinite-length method is not supported.
+		// Reference: ISO/IEC 8825-1: 8.1.3.6.1
+		return 0, nil, asn1.StructuralError{Msg: "decoding BER length octets: indefinite length not supported"}
+	}
+
+	// long form
+	// Reference: ISO/IEC 8825-1: 8.1.3.5
+	n := int(b & 0x7f)
+	if n > 4 {
+		// length must fit the memory space of the int type (4 bytes).
+		return 0, nil, asn1.StructuralError{Msg: fmt.Sprintf("decoding BER length octets: length of encoded data (%d bytes) cannot exceed 4 bytes", n)}
+	}
+	if offset+n >= len(r) {
+		return 0, nil, asn1.SyntaxError{Msg: "decoding BER length octets: long form length octets with early EOF"}
+	}
+	var length uint64
+	for i := 0; i < n; i++ {
+		length = (length << 8) | uint64(r[offset])
+		offset++
+	}
+
+	// length must fit the memory space of the int32.
+	if (length >> 31) > 0 {
+		return 0, nil, asn1.StructuralError{Msg: fmt.Sprintf("decoding BER length octets: length %d does not fit the memory space of int32", length)}
+	}
+
+	contentLen := int(length)
+	subsequentOctets := r[offset:]
+	if contentLen > len(subsequentOctets) {
+		return 0, nil, asn1.SyntaxError{Msg: "decoding BER length octets: long form length octets value should be less or equal to the subsequent octets length"}
+	}
+	return contentLen, subsequentOctets, nil
+}
+
+// isPrimitive returns true if the first identifier octet is marked
+// as primitive.
+// Reference: ISO/IEC 8825-1: 8.1.2.5
+func isPrimitive(identifier []byte) bool {
+	return identifier[0]&0x20 == 0
+}