~~NOTOC~~
====== 7. Encryption ======

<WRAP round info>
**Scenarios:** 3 \\
**FFI Functions:** ~25 \\
**Status:** Planned
</WRAP>

This category covers all scenarios for Post-Quantum secure encryption. Hybrid encryption with ML-KEM, key encapsulation, and symmetric encryption.

----

===== Scenarios =====

^  ID  ^  Scenario  ^  Description  ^  Complexity  ^  Status  ^
| [[.:hybrid_encryption|7.1]] | Hybrid Encryption | ECDH + ML-KEM combined | **** | Planned |
| [[.:key_encapsulation|7.2]] | Key Encapsulation | ML-KEM for key exchange | *** | Planned |
| [[.:file_encryption|7.3]] | File Encryption | Encrypt large files | *** | Planned |

----

===== Hybrid Encryption Workflow =====

<mermaid>
sequenceDiagram
    participant A as Alice (Sender)
    participant B as Bob (Recipient)

    Note over B: Generate key pair
    B->>A: Public Key (ECDH + ML-KEM)

    Note over A: Key Encapsulation
    A->>A: ECDH SharedSecret + ML-KEM Ciphertext
    A->>A: HKDF(ECDH || ML-KEM) -> KEK
    A->>A: AES-GCM Encrypt(KEK, Plaintext)

    A->>B: ECDH Ephemeral PK + ML-KEM Ciphertext + AES Ciphertext

    Note over B: Key Decapsulation
    B->>B: ECDH SharedSecret + ML-KEM Decapsulate
    B->>B: HKDF(ECDH || ML-KEM) -> KEK
    B->>B: AES-GCM Decrypt(KEK, Ciphertext)
</mermaid>

----

===== Algorithms =====

^  Component  ^  Algorithm  ^  Security Level  ^  Standard  ^
| Key Encapsulation (PQ) | ML-KEM-768 | 192 bit | NIST FIPS 203 |
| Key Exchange (classical) | ECDH P-384 | 192 bit | NIST SP 800-56A |
| Key Derivation | HKDF-SHA384 | - | RFC 5869 |
| Symmetric Encryption | AES-256-GCM | 256 bit | NIST SP 800-38D |

<WRAP round important>
**Hybrid Principle:** Both algorithms (ECDH + ML-KEM) must be broken to compromise the encryption.
</WRAP>

----

===== Usage Scenarios =====

^  Application  ^  Recommended Mode  ^  Rationale  ^
| Long-term archiving | ML-KEM (PQ-only) | Data must remain secure 20+ years |
| Current communication | Hybrid | Compatibility + future-proofing |
| Legacy systems | ECDH (Classic) | No PQ support |

----

===== Quick Start Code =====

==== Hybrid Encryption ====

<code csharp>
using WvdS.Security.Cryptography.Extensions.PQ;

// Recipient: Generate key pair
using var recipientKey = ctx.GenerateHybridKeyPair(
    classicAlgorithm: EcdhCurve.P384,
    pqAlgorithm: PqAlgorithm.MlKem768
);
var publicKeyPem = recipientKey.ExportPublicKeyPem();

// Sender: Encrypt
var (ciphertext, encapsulatedKey) = ctx.HybridEncrypt(
    plaintext: Encoding.UTF8.GetBytes("Secret message"),
    recipientPublicKey: publicKeyPem
);

// Recipient: Decrypt
var plaintext = ctx.HybridDecrypt(
    ciphertext: ciphertext,
    encapsulatedKey: encapsulatedKey,
    privateKey: recipientKey
);
</code>

==== File Encryption ====

<code csharp>
// Encrypt large file (streaming)
await ctx.EncryptFileAsync(
    inputPath: "document.pdf",
    outputPath: "document.pdf.enc",
    recipientPublicKey: publicKeyPem,
    mode: CryptoMode.Hybrid
);

// Decrypt
await ctx.DecryptFileAsync(
    inputPath: "document.pdf.enc",
    outputPath: "document-decrypted.pdf",
    privateKey: recipientKey
);
</code>

----

===== Related Categories =====

^  Category  ^  Relationship  ^
| [[.:schluessel:start|11. Key Management]] | Manage key pairs |
| [[.:tls:start|10. TLS/mTLS]] | Key exchange in TLS handshake |
| [[.:interop:start|12. Import/Export]] | Export encrypted keys |

----

<< [[en:int:pqcrypt:szenarien:widerruf:start|<- 6. Revocation]] | [[en:int:pqcrypt:szenarien:start|^ Scenarios]] | [[.:signaturen:start|8. Digital Signatures ->]] >>

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//Wolfgang van der Stille @ EMSR DATA d.o.o. - Post-Quantum Cryptography Professional//

{{tag>category encryption hybrid mlkem}}