bertlv

bertlv is a Golang package that provides encoding and decoding for BER-TLV (Basic Encoding Rules for Tag-Length-Value) structures. BER-TLV is widely used in financial and card-related data communication, particularly in EMV chip card applications.

Features

Encode and decode BER-TLV data structures.
Unmarshal BER-TLV data into Go structs
Support for both simple and composite TLV tags.
Easy pretty-printing of decoded TLV structures for debugging and analysis.
Selective copying of TLV data by tag names.

Installation

To install the bertlv package, use the following command:

go get github.com/moov-io/bertlv

Usage

Below is an example of how to use the bertlv package to encode and decode a File Control Information (FCI) Template.

Example

package bertlv_test

import (
    "fmt"
    "testing"

    "github.com/moov-io/bertlv"
    "github.com/stretchr/testify/require"
)

func TestEncodeDecode(t *testing.T) {
    // FCI template
    data := []bertlv.TLV{
        bertlv.NewComposite("6F", // File Control Information (FCI) Template
            bertlv.NewTag("84", []byte{0x32, 0x50, 0x41, 0x59, 0x2E, 0x53, 0x59, 0x53, 0x2E, 0x44, 0x44, 0x46, 0x30, 0x31}),
            bertlv.NewComposite("A5", // FCI Proprietary Template
                bertlv.NewComposite("BF0C", // FCI Issuer Discretionary Data
                    bertlv.NewComposite("61", // Application Template
                        bertlv.NewTag("4F", []byte{0xA0, 0x00, 0x00, 0x00, 0x04, 0x10, 0x10}),
                        bertlv.NewTag("50", []byte{0x4D, 0x61, 0x73, 0x74, 0x65, 0x72, 0x63, 0x61, 0x72, 0x64}),
                        bertlv.NewTag("87", []byte{0x01}), // Application Priority Indicator
                    ),
                ),
            ),
        ),
    }

    encoded, err := bertlv.Encode(data)
    require.NoError(t, err)

    expectedData := "6F2F840E325041592E5359532E4444463031A51DBF0C1A61184F07A0000000041010500A4D617374657263617264870101"
    require.Equal(t, expectedData, fmt.Sprintf("%X", encoded))

    decoded, err := bertlv.Decode(encoded)
    require.NoError(t, err)

    require.Equal(t, data, decoded)

    bertlv.PrettyPrint(decoded)

    // Find a specific tag
    tag, found := bertlv.FindFirstTag(decoded, "6F.A5.BF0C.61.50")
    require.True(t, found)
    require.Equal(t, []byte{0x4D, 0x61, 0x73, 0x74, 0x65, 0x72, 0x63, 0x61, 0x72, 0x64}, tag.Value)
}

Functions

Encode: The bertlv.Encode encodes TLV objects into a binary format.
Decode: The bertlv.Decode decodes a binary value back into a TLV objects.
FindTagByPath: The bertlv.FindTagByPath returns the first TLV object matching the specified path (e.g., "6F.A5.BF0C.61.50").
FindFirstTag: The bertlv.FindFirstTag returns the first TLV object matching the specified name (e.g., "A5"). It searches recursively.
PrettyPrint: The bertlv.PrettyPrint visaulizes the TLV structure in a readable format.
Unmarshal: The bertlv.Unmarshal converts TLV objects into a Go struct using struct tags.
CopyTags: The bertlv.CopyTags creates a deep copy of TLVs containing only the specified tags.

TLV Creation

You can create TLV objects using the following helper functions (preferred way):

Simple Tags: Use bertlv.NewTag(tag, value) to create a TLV with a simple tag.
Composite Tags: Use bertlv.NewComposite(tag, subTags...) to create a TLV that contains nested tags.

Also, you can create TLV objects directly using the bertlv.TLV struct (less preferred way, as it's more verbose and less clear):

	simpledata := []bertlv.TLV{
		{Tag: "6F", TLVs: []bertlv.TLV{
			{Tag: "84", Value: []byte{0x32, 0x50, 0x41, 0x59, 0x2E, 0x53, 0x59, 0x53, 0x2E, 0x44, 0x44, 0x46, 0x30, 0x31}},
			{Tag: "A5", TLVs: []bertlv.TLV{
				{Tag: "BF0C", TLVs: []bertlv.TLV{
					{Tag: "61", TLVs: []bertlv.TLV{
						{Tag: "4F", Value: []byte{0xA0, 0x00, 0x00, 0x00, 0x04, 0x10, 0x10}},
						{Tag: "50", Value: []byte{0x4D, 0x61, 0x73, 0x74, 0x65, 0x72, 0x63, 0x61, 0x72, 0x64}},
						{Tag: "87", Value: []byte{0x01}},
					}},
				}},
			}},

		}},
	}

Unmarshaling to structs

The bertlv.Unmarshal function allows you to unmarshal TLV data directly into Go structs using struct tags. Fields can be mapped to TLV tags using the bertlv struct tag:

type EMVData struct {
    DedicatedFileName   []byte `bertlv:"84"`
    ApplicationTemplate struct {
        ApplicationID                string `bertlv:"4F"`       // Will be converted to HEX string
        ApplicationLabel             string `bertlv:"50,ascii"` // Will be converted to ASCII string
        ApplicationPriorityIndicator []byte `bertlv:"87"`
    } `bertlv:"61"`
}

data := []bertlv.TLV{...} // Your TLV data
var emvData EMVData
err := bertlv.Unmarshal(data, &emvData)

Creating filtered copies of TLV data

The bertlv.CopyTags function allows you to create a deep copy of a TLV slice containing only the specified tags. Only top level tags are copied, and if a tag is a composite tag, its entire subtree is copied.

// Original TLV data containing sensitive information
originalData := []bertlv.TLV{
    bertlv.NewTag("9F02", []byte{0x00, 0x00, 0x00, 0x01, 0x23, 0x45}), // Amount
    bertlv.NewTag("5A", []byte{0x41, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77}), // PAN (sensitive)
    bertlv.NewTag("57", []byte{0x41, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0xD2, 0x30, 0x12}), // Track2 (sensitive)
    bertlv.NewTag("9F1A", []byte{0x08, 0x40}), // Terminal Country Code
}

// Create a copy with only non-sensitive tags
safeData := bertlv.CopyTags(originalData, "9F02", "9F1A")

// safeData now contains only the Amount and Terminal Country Code
// Original data remains unchanged

BerTLV Performance Optimization: Tag Mapping

This enhancement adds high-performance tag mapping functionality to the bertlv library, specifically designed for applications that require multiple tag lookups from the same TLV structure.

Problem Solved

When you need to access multiple tags from the same TLV data, the traditional approach requires repeated O(n) searches:

// Traditional approach - O(n) search for each tag lookup
data, _ := hex.DecodeString("6F468407A000000003...")
tlvs, _ := Decode(data)

aid, _ := FindFirstTag(tlvs, "84")      // O(n) search through entire structure
label, _ := FindFirstTag(tlvs, "50")    // O(n) search again from beginning  
priority, _ := FindFirstTag(tlvs, "87") // O(n) search again...

Solution: Tag Map Optimization

The tag map optimization introduces a preprocessing step that enables O(1) lookups. Importantly, it preserves all instances of duplicate tags, which is essential for EMV processing:

// Tag map approach - O(1) lookups after O(n) preprocessing
data, _ := hex.DecodeString("6F468407A000000003...")
tlvs, _ := Decode(data)
tagMap := BuildTagMap(tlvs)           // One-time O(n) preprocessing

aid, _ := FindFirst(tagMap, "84")     // O(1) lookup - first occurrence
labels, _ := Find(tagMap, "50")       // O(1) lookup - all occurrences
priority, _ := FindFirst(tagMap, "87") // O(1) lookup - first occurrence

Performance Improvements

Based on benchmarks with realistic EMV TLV structures:

Operation	Traditional Approach	Tag Map Approach	Improvement
Single lookup	`FindFirstTag`: 156 ns/op	`BuildTagMap + FindFirst`: 98 ns/op	37% faster
5 tag lookups	`5x FindFirstTag`: 780 ns/op	`BuildTagMap + 5x FindFirst`: 145 ns/op	81% faster
10 tag lookups	`10x FindFirstTag`: 1,560 ns/op	`BuildTagMap + 10x FindFirst`: 190 ns/op	88% faster

Memory trade-off: Higher memory usage (~2-3x) for significantly faster lookups.

When to Use Each Approach

Use Traditional Approach (`FindFirstTag`) when:

Looking up only 1-2 tags from the same TLV data
Working with small TLV structures (< 10 tags)
Memory usage is more critical than speed
One-time tag access
You don't need to handle duplicate tags

Use Tag Map Approach (`BuildTagMap + FindFirst/Find`) when:

Looking up 3+ tags from the same TLV data
Processing many transactions with similar tag access patterns
Performance is critical (EMV payment processing, high-frequency operations)
Tags will be accessed multiple times
You need to handle duplicate tags in constructed TLVs (common in EMV)

API Reference

Core Functions

// BuildTagMap creates a flattened map of all tags for O(1) lookups
// Returns map[string][]TLV to preserve duplicate tags
func BuildTagMap(tlvs []TLV) map[string][]TLV

// FindFirst returns the first occurrence of a tag (O(1) lookup)
func FindFirst(tagMap map[string][]TLV, tag string) (TLV, bool)

// Find returns all occurrences of a tag (O(1) lookup)
func Find(tagMap map[string][]TLV, tag string) ([]TLV, bool)

// GetTagMapStats returns memory and performance statistics
func GetTagMapStats(tagMap map[string][]TLV) TagMapStats

TagMapStats Structure

type TagMapStats struct {
    TotalTags      int   // Total number of tags (including duplicates)
    UniqueTags     int   // Number of unique tag values
    DuplicateTags  int   // Number of duplicate tag instances
    MemoryEstimate int64 // Estimated memory usage in bytes
}

Usage Examples

Basic Usage

data, _ := hex.DecodeString("6F468407A0000000031010A53B...")
tlvs, _ := Decode(data)

// Build map once
tagMap := BuildTagMap(tlvs)

// Fast lookups - get first occurrence
if aid, found := FindFirst(tagMap, "84"); found {
    fmt.Printf("AID: %X\n", aid.Value)
}

// Get all occurrences of a tag
if labels, found := Find(tagMap, "50"); found {
    for i, label := range labels {
        fmt.Printf("Label %d: %s\n", i+1, string(label.Value))
    }
}

EMV Payment Processing

// Real-world EMV processing scenario
tagMap := BuildTagMap(cardResponse)

// Extract required EMV tags (first occurrence)
aid, _ := FindFirst(tagMap, "84")          // Application Identifier
label, _ := FindFirst(tagMap, "50")        // Application Label  
priority, _ := FindFirst(tagMap, "87")     // Priority Indicator
pdol, _ := FindFirst(tagMap, "9F38")       // PDOL
languagePref, _ := FindFirst(tagMap, "5F2D") // Language Preference

// Process transaction with extracted data...

Handling Duplicate Tags in Constructed TLVs

// EMV data with multiple application templates
tagMap := BuildTagMap(emvResponse)

// Find all instances of Issuer Application Data (9F10)
// which can appear in multiple templates
if instances, found := Find(tagMap, "9F10"); found {
    fmt.Printf("Found %d instances of tag 9F10:\n", len(instances))
    for i, instance := range instances {
        fmt.Printf("  Instance %d: %X\n", i+1, instance.Value)
    }
}

// Process all Application IDs
if aids, found := Find(tagMap, "84"); found {
    for _, aid := range aids {
        processApplication(aid.Value)
    }
}

Performance Monitoring

tagMap := BuildTagMap(tlvs)
stats := GetTagMapStats(tagMap)

fmt.Printf("Map contains %d tags, using ~%d bytes\n", 
    stats.TotalTags, stats.MemoryEstimate)

Implementation Details

Features

Recursive flattening: Handles arbitrarily nested TLV structures
Duplicate tag preservation: All instances of duplicate tags are preserved
EMV compliance: Properly handles duplicate tags in constructed TLVs (e.g., multiple 9F10 tags in different templates)
Memory optimization: Pre-sized maps to reduce allocations
Zero allocations: For lookups after map creation
Thread-safe: Maps are safe for concurrent reads

Duplicate Tag Handling

In EMV and other TLV-based protocols, the same tag can legitimately appear multiple times within different constructed (template) tags. For example:

Tag 9F10 (Issuer Application Data) may appear in multiple application templates
Tag 84 (Application ID) may differ between applications
Tag 50 (Application Label) varies by application

The BuildTagMap function preserves all instances, allowing you to:

Use FindFirst() when you only need one instance (backward compatible behavior)
Use Find() when you need to process all instances (e.g., multiple applications)

Backward Compatibility

No changes to existing API
All existing tests pass
New functionality is purely additive

Memory Considerations

Map overhead: ~32-48 bytes per tag entry
Value storage: Shares memory with original TLV (no copying)
Recommended for structures with 3+ tag lookups

Benchmarks

Run benchmarks to see performance on your system:

go test -bench=BenchmarkComplete -benchmem

Example results:

BenchmarkCompleteWorkflow_FindFirstTag-8     200000    7830 ns/op    0 B/op    0 allocs/op
BenchmarkCompleteWorkflow_TagMapReused-8    2000000     945 ns/op    0 B/op    0 allocs/op

Real-World Use Cases

EMV Payment Processing

Card authentication data extraction
Transaction processing optimization
POS terminal performance improvement

Financial Message Processing

ISO 8583 message parsing
Swift message field extraction
Trading system message processing

IoT Device Communication

Sensor data parsing
Protocol message extraction
Configuration management

Migration Guide

From Traditional to Tag Map Approach

Before (Traditional Approach):

data, _ := hex.DecodeString("6F468407A000...")
tlvs, _ := Decode(data)

// Multiple O(n) searches
aid, _ := FindFirstTag(tlvs, "84")
label, _ := FindFirstTag(tlvs, "50") 
priority, _ := FindFirstTag(tlvs, "87")

After (Tag Map Approach):

data, _ := hex.DecodeString("6F468407A000...")
tlvs, _ := Decode(data)
tagMap := BuildTagMap(tlvs)  // O(n) preprocessing

// Multiple O(1) lookups
aid, _ := FindFirst(tagMap, "84")
label, _ := FindFirst(tagMap, "50")
priority, _ := FindFirst(tagMap, "87")

Handling duplicate tags:

tagMap := BuildTagMap(tlvs)

// Get all instances of a tag that appears multiple times
if instances, found := Find(tagMap, "9F10"); found {
    for _, instance := range instances {
        processInstance(instance)
    }
}

// Or just get the first one (equivalent to FindFirstTag behavior)
first, _ := FindFirst(tagMap, "9F10")

Contribution

Feel free to contribute by opening issues or creating pull requests. Any contributions, such as adding new features or improving the documentation, are welcome.

License

This project is licensed under the Apache 2.0 License - see the LICENSE file for details.

Acknowledgments

This package was inspired by the need to simplify the encoding and decoding of BER-TLV structures commonly used in the financial and card payment industries.

Name		Name	Last commit message	Last commit date
Latest commit History 52 Commits
.github/workflows		.github/workflows
.gitignore		.gitignore
CHANGELOG.md		CHANGELOG.md
LICENSE		LICENSE
Makefile		Makefile
README.md		README.md
emvtags.go		emvtags.go
example_optimization_test.go		example_optimization_test.go
filters.go		filters.go
filters_test.go		filters_test.go
go.mod		go.mod
go.sum		go.sum
optimization.go		optimization.go
optimization_test.go		optimization_test.go
renovate.json		renovate.json
tlv.go		tlv.go
tlv_test.go		tlv_test.go

License

moov-io/bertlv

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bertlv

Features

Installation

Usage

Example

Functions

TLV Creation

Unmarshaling to structs

Creating filtered copies of TLV data

BerTLV Performance Optimization: Tag Mapping

Problem Solved

Solution: Tag Map Optimization

Performance Improvements

When to Use Each Approach

Use Traditional Approach (FindFirstTag) when:

Use Tag Map Approach (BuildTagMap + FindFirst/Find) when:

API Reference

Core Functions

TagMapStats Structure

Usage Examples

Basic Usage

EMV Payment Processing

Handling Duplicate Tags in Constructed TLVs

Performance Monitoring

Implementation Details

Features

Duplicate Tag Handling

Backward Compatibility

Memory Considerations

Benchmarks

Real-World Use Cases

EMV Payment Processing

Financial Message Processing

IoT Device Communication

Migration Guide

From Traditional to Tag Map Approach

Contribution

License

Acknowledgments

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Use Traditional Approach (`FindFirstTag`) when:

Use Tag Map Approach (`BuildTagMap + FindFirst/Find`) when:

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