Understanding about AS4 (Applicability Statement 4)

What is AS4 in EDI?

AS4 (Applicability Statement 4) is a modern secure web services-based communication protocol used for B2B data exchange, especially in EDI (Electronic Data Interchange). It is a successor to AS2, offering enhanced security, reliability, and flexibility using SOAP and web services standards.

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Key Components of AS4 Architecture

Component	Description
SOAP Envelope	AS4 uses SOAP 1.2 for message structure.
ebMS Header	Metadata such as sender, receiver, message ID, and action.
Payload	Can be any type (XML, EDIFACT, X12, etc.).
WS-Security	Ensures message encryption, signature, and integrity.
Reliability	Built-in retry, duplicate detection, and delivery tracking.
P-Mode	Configuration setting that governs how messages are exchanged between partners (like service agreements).

Where is AS4 Used?

• Peppol Network (EU e-Invoicing and procurement)
• Government projects 
• Logistics and healthcare sectors
• Large enterprises and ERP integrations

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Advantages over AS2

AS2	AS4
S/MIME-based	SOAP/WS-based
No message pulling	Supports pull & push
Less standard metadata	Richer metadata and payload formats
Limited error handling	Enhanced error, retry, and security mechanisms

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AS4 Message Exchange Patterns

1. Push Model
• Sender initiates the message transfer.
• Most common model (similar to AS2).
2. Pull Model
• Receiver retrieves messages from the sender’s AS4 endpoint.
• Useful when recipient has limited inbound connectivity.
3. One-Way / Two-Way Messaging
• AS4 supports both one-way messaging (send only or receive only) and two-way messaging (request-response type).

Message Flow (Push Model)

Step-by-step:

1. Sender ERP sends EDI 810 (Invoice) to AS4 Gateway.
2. AS4 wraps the message in a SOAP envelope.
3. Message is signed and encrypted.
4. Sent over HTTPS to receiver.
5. Receiver processes and sends back acknowledgment (SOAP Receipt).

Security in AS4

AS4 uses WS-Security standards for:

• Authentication (via certificates)
• Confidentiality (via encryption)
• Integrity (via digital signatures)
• Non-repudiation (auditable message logs)

Certificates (X.509) are used to sign and encrypt the message or its parts.