1. [JA3 Fingerprint](https://scrapfly.io/web-scraping-tools/ja3-fingerprint)
2. [ TLS Extensions ](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extensions "Browse all TLS Extensions")
3. pre\_shared\_key
 
   [  Browse All TLS Extensions ](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extensions) 

  # pre\_shared\_key

TLS Extension Code: `41`

 

  ### Extension Details

 

Extension Name

`pre_shared_key`

 

Extension Code (Decimal)

`41`

 

Extension Code (Hex)

`0x0029`

 

RFC Reference

 [ RFC 8446  ](https://datatracker.ietf.org/doc/html/rfc8446#section-4.2.11) 

 

 

 

  ### What is pre\_shared\_key?

 

The Pre-Shared Key (PSK) extension in TLS 1.3 enables fast session resumption and 0-RTT (Zero Round-Trip Time) data. After an initial full handshake, the client and server derive a PSK which can be used to resume future sessions with reduced latency. 0-RTT allows the client to send application data in the first flight, eliminating a round trip. However, 0-RTT data lacks forward secrecy and is vulnerable to replay attacks, so it should only be used for idempotent operations. PSK resumption is more secure than TLS 1.2 session tickets.

 

 

  ### Role in JA3 Fingerprinting

 

  How This Extension Affects Fingerprinting 

 The **pre\_shared\_key** extension is part of the TLS Client Hello packet that JA3 analyzes to create a unique fingerprint of your browser or HTTP client.

 **JA3 Construction:** JA3 concatenates TLS parameters including extensions in a specific format:
 `TLS_VERSION,CIPHERS,EXTENSIONS,CURVES,POINT_FORMATS`

 Different browsers and HTTP clients support different sets of TLS extensions, making this a key differentiator in fingerprinting. The presence, absence, or order of extensions like `pre_shared_key` can reveal:

- Browser type and version (Chrome, Firefox, Safari, Edge)
- Operating system (Windows, macOS, Linux, Android, iOS)
- HTTP library (curl, Python requests, Go http.Client, Node.js)
- Bot detection - automated tools often have distinctive extension sets
 
 

 

 

  ### Test Your TLS Configuration

 

 See if your browser or HTTP client includes the `pre_shared_key` extension in its TLS handshake:

 [  Test Your JA3 Fingerprint ](https://scrapfly.io/web-scraping-tools/ja3-fingerprint) 

 The JA3 tool will show all TLS extensions your client advertises, including pre\_shared\_key.

 

  ### Browser & Client Support

 

**Modern Browser Support:**

- **Chrome/Edge:** Full support for pre\_shared\_key
- **Firefox:** Full support for pre\_shared\_key
- **Safari:** Full support for pre\_shared\_key
 
**HTTP Client Libraries:**

- **curl:** Support varies by OpenSSL/LibreSSL version
- **Python requests:** Limited TLS extension control
- **curl\_cffi:** Can mimic browser TLS configurations
- **Scrapfly API:** Automatically matches real browser TLS fingerprints
 
 

 

 

  ### Related TLS Resources

 

##### TLS Extensions

- [server\_name (SNI)](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extension/server-name)
- [supported\_groups](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extension/supported-groups)
- [ALPN](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extension/application-layer-protocol-negotiation)
- [supported\_versions](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extension/supported-versions)
 
 

##### Testing Tools

- [JA3 Fingerprint Tool](https://scrapfly.io/web-scraping-tools/ja3-fingerprint)
- [Browser Fingerprint Test](https://scrapfly.io/web-scraping-tools/browser-fingerprint)
 
 

 

 

  ### Technical References

 

- **RFC Specification:** [ RFC 8446 - TLS Extensions  ](https://datatracker.ietf.org/doc/html/rfc8446#section-4.2.11)
- **IANA Registry:** [ TLS ExtensionType Values  ](https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml)
- **JA3 Project:** [ JA3 - TLS Client Fingerprinting  ](https://github.com/salesforce/ja3)