Selective Encrypted Transmission Security Pattern

Entities actively encrypt specific sensitive data elements before transmitting them over uncontrolled channels. Entities directly interact with cryptographic libraries to encrypt only necessary parts.

Problem Addressed

Leak action request or data in transit: Sensitive data exposed while being transmitted over a channel that may be observed by unauthorized parties.

Core Components

Role	Type	Responsibility
Sender	Entity	Encrypts and transmits data
Receiver	Entity	Receives and decrypts data
CryptographerS	Cryptographic Primitive	Encryption library for Sender
CryptographerR	Cryptographic Primitive	Decryption library for Receiver

Data Elements

• d: Plaintext data element
• {d}_k: Ciphertext (d encrypted with key k)
• keyInfoS: Key information for encryption
• keyInfoR: Key information for decryption
• configS/configR: Cipher configuration

Pattern Flow

Sender → [encrypt(d, keyInfoS, configS)] → CryptographerS
CryptographerS → [{d}_k] → Sender
Sender → [{d}_k] → Receiver (over channel)
Receiver → [decrypt({d}_k, keyInfoR, configR)] → CryptographerR
CryptographerR → [d] → Receiver

1. Sender decides which data to encrypt
2. Sender requests encryption from CryptographerS
3. CryptographerS returns ciphertext
4. Sender transmits ciphertext
5. Receiver receives ciphertext
6. Receiver requests decryption from CryptographerR
7. CryptographerR returns plaintext

Key Characteristics

Selective Encryption

• Entities choose WHAT to encrypt
• Not all data needs encryption
• Application-level decision making

Active Entity Participation

• Entities directly invoke cryptographic operations
• Entities manage when encryption occurs
• Different from channel-level encryption

Cipher Negotiation

Sender and Receiver must agree on:

• Encryption algorithm
• Configuration (mode, block size)
• Cryptographic key(s)

Negotiation Timing

Stage	Approach
Design time	Hardcoded compatible algorithms
Deployment	Configured shared keys/certificates
Runtime	Dynamic negotiation protocol

Critical: Use only standardized negotiation protocols from cryptographic libraries. Never implement custom negotiation.

Security Considerations

Use Existing Libraries

• Never implement custom cryptography
• Use well-known libraries (OpenSSL, libsodium, etc.)
• Follow library best practices

Key Management

• Protect key confidentiality
• Protect key integrity
• Secure key exchange/distribution
• See: Cryptographic key management pattern

Configuration Integrity

• Protect cipher configuration from tampering
• Attacker changing config could weaken encryption
• Protect during transmission and storage

Public Key Infrastructure

For asymmetric encryption:

• Sender encrypts with Receiver's public key
• Protect public key integrity
• Receiver's private key must remain secret

Symmetric Key Sharing

For symmetric encryption:

• Both parties need same secret key
• Secure distribution challenge
• Consider key exchange protocols

Encryption Algorithm Selection

Follow Encryption pattern recommendations:

• Symmetric: AES-256, AES-128 minimum
• Asymmetric: RSA-3072+ or ECDH-256+
• Authenticated encryption preferred

Comparison with Encrypted Tunnel

Aspect	Selective Encryption	Encrypted Tunnel
Scope	Specific data elements	All communication
Control	Application decides	Infrastructure manages
Overhead	Lower (selective)	Higher (everything)
Complexity	Application manages	Delegated to endpoints

Use selective encryption when:

• Only some data is sensitive
• Different data needs different keys
• Application needs encryption control

Implementation Checklist

• Using established cryptographic library
• Strong algorithm selected (AES-256, RSA-3072+)
• Secure key management
• Configuration integrity protected
• Key exchange secured
• No custom cryptography implemented
• Sender/Receiver use compatible ciphers
• Public keys verified for authenticity

Related Patterns

• Encryption (underlying cryptographic operations)
• Encrypted tunnel (alternative: encrypt all traffic)
• Cryptographic key management (key handling)
• Selective encrypted storage (encryption at rest)

References

• Source: https://securitypatterns.distrinet-research.be/patterns/06_01_001__selective_encrypted_transmission/
• BSI TR-02102 Cryptographic Mechanisms
• NIST SP 800-175B Cryptographic Standards