SEED-128 for .NET (C#)

A lightweight implementation of the Korean SEED 128-bit block cipher for .NET. The library targets .NET Standard 2.0 and the console example targets .NET 9.0.

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Highlights

• SEED-128 block cipher (16-byte block size)
• CBC/ECB operation modes
  • Padding=true: CBC mode (uses IV)
  • Padding=false: ECB mode (no IV)
• PKCS#7-style padding (enabled by default)
• Helpers for byte/string/Base64 conversions

Supported runtimes

• Library: .NET Standard 2.0/2.1, .NET Core 8/9
• Example/Tests: .NET 9.0

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Background and Compliance (KR)

SEED is a 128-bit block cipher developed under the direction of the Government of the Republic of Korea and has been widely used as a domestic standard across e-government, public, and financial sectors. In certain Korean contexts, use of a KCMVP-approved crypto module and specific national algorithms may be required. Actual certification/review requirements vary by organization and project; verify the latest guidelines and requirements before deployment.

This repository follows the official specifications; the docs/seed-specs folder includes specifications, test vectors, and evaluation reports. Use those documents to validate this implementation with the provided test vectors where needed.

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Installation

Use the source in this repository either via a Project Reference or by including the code directly.

1. Add a Project Reference to src/Seed128/seed128.csproj in your solution
2. Or include the source files under src/Seed128

Note: seed128.csproj is configured to pack a package icon. If local builds fail due to the icon path, see “Build Issues & Fixes” below.

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Quick start

Namespace: Seed.Net.Cryptography

using System;
using System.Text;
using System.Security.Cryptography;
using Seed.Net.Cryptography;

// Prepare 16-byte key/IV
var key = new byte[16];
var iv  = new byte[16];
RandomNumberGenerator.Fill(key);
RandomNumberGenerator.Fill(iv);

// Default Padding=true (CBC mode)
var seed = new Seed128(key, iv);

// Encrypt/Decrypt (byte[])
var plain = Encoding.UTF8.GetBytes("hello SEED-128");
var cipher = seed.Encrypt(plain);
var roundtrip = seed.Decrypt(cipher);
Console.WriteLine(Encoding.UTF8.GetString(roundtrip)); // hello SEED-128

// Base64 helpers
string cipherB64 = seed.PlainBytesToChiperBase64(plain);
byte[] plain2 = seed.ChiperBase64ToPlainBytes(cipherB64);

// String <-> Base64 helpers
string encB64 = seed.PlainStringToChiperBase64("password");
string decB64 = seed.ChiperBase64ToPlainString(encB64);

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API summary

Class: Seed128 : Seed

• Constructor: Seed128(byte[] seed_key, byte[] seed_iv)
  • Default padding: true (CBC mode)
• byte[] Encrypt(byte[] plain_data)
• byte[] Decrypt(byte[] encrypted_data)
• string PlainBase64ToChiperBase64(string plain_text)
• string ChiperBase64ToPlainBase64(string chiper_text)
• string PlainBytesToChiperBase64(byte[] plain_data)
• byte[] ChiperBase64ToPlainBytes(string chiper_text)
• string PlainStringToChiperBase64(string plain_text)
• string ChiperBase64ToPlainString(string chiper_text)
• string PlainStringToChiperString(string plain_text)
• string ChiperStringToPlainString(string chiper_text)

Note: Some helpers use Encoding.Default for encoding and Encoding.UTF8 for decoding. For consistent cross-platform behavior, prefer using the byte-array APIs (Encrypt/Decrypt) and manage encoding explicitly.

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Modes and padding

• Block size: 16 bytes
• Padding: PKCS#7-like (applied when Padding=true)
• Modes:
  • Padding=true: CBC (requires IV, XOR chaining)
  • Padding=false: ECB (no IV/padding)

For security, CBC with padding is generally recommended. Avoid ECB for structured or repetitive data.

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Build & run

This repository includes the library (src/Seed128) and a console example (samples/seed128.sample).

• Build library: src/Seed128/seed128.csproj
• Run example: samples/Seed128.Sample/seed128.sample.csproj

Build issues & fixes:

• seed128.csproj is configured to pack an icon from ..\..\docs\logo-files\odinsoft-logo.png, while the repo contains docs/logo-files/odinsoft-logo.png. If local build fails, choose one:

  1. Disable packing: set GeneratePackageOnBuild to False
  2. Update icon path: point to the actual file (e.g., docs/logo-files/odinsoft-logo.png) and update the ItemGroup Include/PackagePath
  3. Add missing path/file: create docs/logo-files/odinsoft-logo.png

• The solution file (seed.net.sln) may reference a different test project path. Building projects individually is recommended.

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Example project

Located under samples/seed128.sample:

• Program.cs: Demonstrates encrypting a string with Seed128 and encrypting the generated symmetric key with RSA.
• RsaEncryption.cs: A simple example using .NET RSACryptoServiceProvider. For production, consider OAEP padding and robust key management.

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Specs and references

The docs/seed-specs folder contains official documents and test vectors related to SEED.

• [1] SEED Algorithm Specification (Korean): Algorithm overview, S-Boxes, key schedule, round function
  • File: [1]_SEED_Algorithm_Specification_korean_M.pdf
• [2] SEED+128 Specification (English): The SEED 128-bit specification and implementation notes
  • File: [2]_SEED+128_Specification_english_M.pdf
• [3] SEED+128 Self Evaluation (Korean): Self-evaluation report (security, performance, etc.)
  • File: [3]_SEED+128_Self_Evaluation-Korean-M.pdf
• [4] SEED+128 Self Evaluation (English): English version of the self-evaluation report
  • File: [4]_SEED+128_Self_Evaluation-English_M.pdf
• [5] SEED+128 Test Vector: Official test vectors (key/IV/plaintext/ciphertext)
  • File: [5]_SEED+128_Test_Vector_M.pdf
• [6] SEED+128 OID: Algorithm identifiers (OID)
  • File: [6]_SEED+128_OID+20091203_M.pdf
• SEED Evaluation Report by CRYPTREC: CRYPTREC evaluation report
  • File: SEED_Evaluation_Report_by_CRYPTREC.pdf
• SEED Test Vectors for Modified SEED: Test vectors for modified SEED
  • File: SEED_Test_Vectors_for_Modified_SEED.pdf

Validating with official test vectors

To quickly validate this implementation:

1. Choose a 16-byte key, 16-byte IV, and block-aligned plaintext from [5]_SEED+128_Test_Vector_M.pdf.
2. Encrypt with the same key/IV using CBC (Padding=true) or ECB (Padding=false) as specified by the vector.
3. Compare the resulting ciphertext byte-for-byte with the document.

Tip: If the test vector is block-aligned, comparing with Padding=false (ECB or CBC without padding) is convenient. For CBC, ensure chaining (XOR with previous block) matches the vector’s definition.

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👥 Team

Core Development Team

• SEONGAHN - Lead Developer & Project Architect (lisa@odinsoft.co.kr)
• YUJIN - Senior Developer & Exchange Integration Specialist (yoojin@odinsoft.co.kr)
• SEJIN - Software Developer & API Implementation (saejin@odinsoft.co.kr)

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