gchq · n1474335 · Apr 12, 2024 · Apr 12, 2024
diff --git a/src/core/config/Categories.json b/src/core/config/Categories.json
@@ -238,6 +238,7 @@
             "JA3 Fingerprint",
             "JA3S Fingerprint",
             "JA4 Fingerprint",
+            "JA4Server Fingerprint",
             "HASSH Client Fingerprint",
             "HASSH Server Fingerprint",
             "Format MAC addresses",

diff --git a/src/core/lib/JA4.mjs b/src/core/lib/JA4.mjs
@@ -25,6 +25,9 @@ export function toJA4(bytes) {
     let tlsr = {};
     try {
         tlsr = parseTLSRecord(bytes);
+        if (tlsr.handshake.value.handshakeType.value !== 0x01) {
+            throw new Error();
+        }
     } catch (err) {
         throw new OperationError("Data is not a valid TLS Client Hello. QUIC is not yet supported.\n" + err);
     }
@@ -48,16 +51,7 @@ export function toJA4(bytes) {
             break;
         }
     }
-    switch (version) {
-        case 0x0304: version = "13"; break; // TLS 1.3
-        case 0x0303: version = "12"; break; // TLS 1.2
-        case 0x0302: version = "11"; break; // TLS 1.1
-        case 0x0301: version = "10"; break; // TLS 1.0
-        case 0x0300: version = "s3"; break; // SSL 3.0
-        case 0x0200: version = "s2"; break; // SSL 2.0
-        case 0x0100: version = "s1"; break; // SSL 1.0
-        default: version = "00"; // Unknown
-    }
+    version = tlsVersionMapper(version);
 
     /* SNI
         If the SNI extension (0x0000) exists, then the destination of the connection is a domain, or “d” in the fingerprint.
@@ -99,6 +93,7 @@ export function toJA4(bytes) {
         if (ext.type.value === "application_layer_protocol_negotiation") {
             alpn = parseFirstALPNValue(ext.value.data);
             alpn = alpn.charAt(0) + alpn.charAt(alpn.length - 1);
+            if (alpn.charCodeAt(0) > 127) alpn = "99";
             break;
         }
     }
@@ -164,3 +159,106 @@ export function toJA4(bytes) {
         "JA4_ro": `${ptype}${version}${sni}${cipherLen}${extLen}${alpn}_${originalCiphersRaw}_${originalExtensionsRaw}`,
     };
 }
+
+
+/**
+ * Calculate the JA4Server from a given TLS Server Hello Stream
+ * @param {Uint8Array} bytes
+ * @returns {string}
+ */
+export function toJA4S(bytes) {
+    let tlsr = {};
+    try {
+        tlsr = parseTLSRecord(bytes);
+        if (tlsr.handshake.value.handshakeType.value !== 0x02) {
+            throw new Error();
+        }
+    } catch (err) {
+        throw new OperationError("Data is not a valid TLS Server Hello. QUIC is not yet supported.\n" + err);
+    }
+
+    /* QUIC
+        “q” or “t”, which denotes whether the hello packet is for QUIC or TCP.
+        TODO: Implement QUIC
+    */
+    const ptype = "t";
+
+    /* TLS Version
+        TLS version is shown in 3 different places. If extension 0x002b exists (supported_versions), then the version
+        is the highest value in the extension. Remember to ignore GREASE values. If the extension doesn’t exist, then
+        the TLS version is the value of the Protocol Version. Handshake version (located at the top of the packet)
+        should be ignored.
+    */
+    let version = tlsr.version.value;
+    for (const ext of tlsr.handshake.value.extensions.value) {
+        if (ext.type.value === "supported_versions") {
+            version = parseHighestSupportedVersion(ext.value.data);
+            break;
+        }
+    }
+    version = tlsVersionMapper(version);
+
+    /* Number of Extensions
+        2 character number of cipher suites, so if there’s 6 cipher suites in the hello packet, then the value should be “06”.
+        If there’s > 99, which there should never be, then output “99”.
+    */
+    let extLen = tlsr.handshake.value.extensions.value.length;
+    extLen = extLen > 99 ? "99" : extLen.toString().padStart(2, "0");
+
+    /* ALPN Extension Chosen Value
+        The first and last characters of the ALPN (Application-Layer Protocol Negotiation) first value.
+        If there are no ALPN values or no ALPN extension then we print “00” as the value in the fingerprint.
+    */
+    let alpn = "00";
+    for (const ext of tlsr.handshake.value.extensions.value) {
+        if (ext.type.value === "application_layer_protocol_negotiation") {
+            alpn = parseFirstALPNValue(ext.value.data);
+            alpn = alpn.charAt(0) + alpn.charAt(alpn.length - 1);
+            if (alpn.charCodeAt(0) > 127) alpn = "99";
+            break;
+        }
+    }
+
+    /* Chosen Cipher
+        The hex value of the chosen cipher suite
+    */
+    const cipher = toHexFast(tlsr.handshake.value.cipherSuite.data);
+
+    /* Extension hash
+        A 12 character truncated sha256 hash of the list of extensions.
+        The extension list is created using the 4 character hex values of the extensions, lower case, comma delimited.
+    */
+    const extensionsList = [];
+    for (const ext of tlsr.handshake.value.extensions.value) {
+        extensionsList.push(toHexFast(ext.type.data));
+    }
+    const extensionsRaw = extensionsList.join(",");
+    const extensionsHash = runHash(
+        "sha256",
+        Utils.strToArrayBuffer(extensionsRaw)
+    ).substring(0, 12);
+
+    return {
+        "JA4S":    `${ptype}${version}${extLen}${alpn}_${cipher}_${extensionsHash}`,
+        "JA4S_r":  `${ptype}${version}${extLen}${alpn}_${cipher}_${extensionsRaw}`,
+    };
+}
+
+
+/**
+ * Takes a TLS version value and returns a JA4 TLS version string
+ * @param {Uint8Array} version - Two byte array of version number
+ * @returns {string}
+ */
+function tlsVersionMapper(version) {
+    switch (version) {
+        case 0x0304: return "13"; // TLS 1.3
+        case 0x0303: return "12"; // TLS 1.2
+        case 0x0302: return "11"; // TLS 1.1
+        case 0x0301: return "10"; // TLS 1.0
+        case 0x0300: return "s3"; // SSL 3.0
+        case 0x0200: return "s2"; // SSL 2.0
+        case 0x0100: return "s1"; // SSL 1.0
+        default: return "00"; // Unknown
+    }
+}
diff --git a/src/core/lib/TLS.mjs b/src/core/lib/TLS.mjs
@@ -70,13 +70,11 @@ function parseHandshake(bytes) {
 
     // Handshake type
     h.handshakeType = {
-        description: "Client Hello",
+        description: "Handshake Type",
         length: 1,
         data: b.getBytes(1),
         value: s.readInt(1)
     };
-    if (h.handshakeType.value !== 0x01)
-        throw new OperationError("Not a Client Hello.");
 
     // Handshake length
     h.handshakeLength = {
@@ -86,8 +84,33 @@ function parseHandshake(bytes) {
         value: s.readInt(3)
     };
     if (s.length !== h.handshakeLength.value + 4)
-        throw new OperationError("Not enough data in Client Hello.");
+        throw new OperationError("Not enough data in Handshake message.");
+
+
+    switch (h.handshakeType.value) {
+        case 0x01:
+            h.handshakeType.description = "Client Hello";
+            parseClientHello(s, b, h);
+            break;
+        case 0x02:
+            h.handshakeType.description = "Server Hello";
+            parseServerHello(s, b, h);
+            break;
+        default:
+            throw new OperationError("Not a known handshake message.");
+    }
 
+    return h;
+}
+
+/**
+ * Parse a TLS Client Hello
+ * @param {Stream} s
+ * @param {Stream} b
+ * @param {Object} h
+ * @returns {JSON}
+ */
+function parseClientHello(s, b, h) {
     // Hello version
     h.helloVersion = {
         description: "Client Hello Version",
@@ -171,6 +194,79 @@ function parseHandshake(bytes) {
     return h;
 }
 
+/**
+ * Parse a TLS Server Hello
+ * @param {Stream} s
+ * @param {Stream} b
+ * @param {Object} h
+ * @returns {JSON}
+ */
+function parseServerHello(s, b, h) {
+    // Hello version
+    h.helloVersion = {
+        description: "Server Hello Version",
+        length: 2,
+        data: b.getBytes(2),
+        value: s.readInt(2)
+    };
+
+    // Random
+    h.random = {
+        description: "Server Random",
+        length: 32,
+        data: b.getBytes(32),
+        value: s.getBytes(32)
+    };
+
+    // Session ID Length
+    h.sessionIDLength = {
+        description: "Session ID Length",
+        length: 1,
+        data: b.getBytes(1),
+        value: s.readInt(1)
+    };
+
+    // Session ID
+    h.sessionID = {
+        description: "Session ID",
+        length: h.sessionIDLength.value,
+        data: b.getBytes(h.sessionIDLength.value),
+        value: s.getBytes(h.sessionIDLength.value)
+    };
+
+    // Cipher Suite
+    h.cipherSuite = {
+        description: "Selected Cipher Suite",
+        length: 2,
+        data: b.getBytes(2),
+        value: CIPHER_SUITES_LOOKUP[s.readInt(2)] || "Unknown"
+    };
+
+    // Compression Method
+    h.compressionMethod = {
+        description: "Selected Compression Method",
+        length: 1,
+        data: b.getBytes(1),
+        value: s.readInt(1) // TODO: Compression method name here
+    };
+
+    // Extensions Length
+    h.extensionsLength = {
+        description: "Extensions Length",
+        length: 2,
+        data: b.getBytes(2),
+        value: s.readInt(2)
+    };
+
+    // Extensions
+    h.extensions = {
+        description: "Extensions",
+        length: h.extensionsLength.value,
+        data: b.getBytes(h.extensionsLength.value),
+        value: parseExtensions(s.getBytes(h.extensionsLength.value))
+    };
+}
+
 /**
  * Parse Cipher Suites
  * @param {Uint8Array} bytes
@@ -748,6 +844,11 @@ export const GREASE_VALUES = [
 export function parseHighestSupportedVersion(bytes) {
     const s = new Stream(bytes);
 
+    // The Server Hello supported_versions extension simply contains the chosen version
+    if (s.length === 2) {
+        return s.readInt(2);
+    }
+
     // Length
     let i = s.readInt(1);
 

diff --git a/src/core/operations/JA4ServerFingerprint.mjs b/src/core/operations/JA4ServerFingerprint.mjs
@@ -0,0 +1,66 @@
+/**
+ * @author n1474335 [n1474335@gmail.com]
+ * @copyright Crown Copyright 2024
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import Utils from "../Utils.mjs";
+import {toJA4S} from "../lib/JA4.mjs";
+
+/**
+ * JA4Server Fingerprint operation
+ */
+class JA4ServerFingerprint extends Operation {
+
+    /**
+     * JA4ServerFingerprint constructor
+     */
+    constructor() {
+        super();
+
+        this.name = "JA4Server Fingerprint";
+        this.module = "Crypto";
+        this.description = "Generates a JA4Server Fingerprint (JA4S) to help identify TLS servers or sessions based on hashing together values from the Server Hello.<br><br>Input: A hex stream of the TLS or QUIC Server Hello packet application layer.";
+        this.infoURL = "https://medium.com/foxio/ja4-network-fingerprinting-9376fe9ca637";
+        this.inputType = "string";
+        this.outputType = "string";
+        this.args = [
+            {
+                name: "Input format",
+                type: "option",
+                value: ["Hex", "Base64", "Raw"]
+            },
+            {
+                name: "Output format",
+                type: "option",
+                value: ["JA4S", "JA4S Raw", "Both"]
+            }
+        ];
+    }
+
+    /**
+     * @param {string} input
+     * @param {Object[]} args
+     * @returns {string}
+     */
+    run(input, args) {
+        const [inputFormat, outputFormat] = args;
+        input = Utils.convertToByteArray(input, inputFormat);
+        const ja4s = toJA4S(new Uint8Array(input));
+
+        // Output
+        switch (outputFormat) {
+            case "JA4S":
+                return ja4s.JA4S;
+            case "JA4S Raw":
+                return ja4s.JA4S_r;
+            case "Both":
+            default:
+                return `JA4S:   ${ja4s.JA4S}\nJA4S_r: ${ja4s.JA4S_r}`;
+        }
+    }
+
+}
+
+export default JA4ServerFingerprint;
diff --git a/tests/operations/index.mjs b/tests/operations/index.mjs
@@ -83,7 +83,7 @@ import "./tests/HKDF.mjs";
 import "./tests/Image.mjs";
 import "./tests/IndexOfCoincidence.mjs";
 import "./tests/JA3Fingerprint.mjs";
-import "./tests/JA4Fingerprint.mjs";
+import "./tests/JA4.mjs";
 import "./tests/JA3SFingerprint.mjs";
 import "./tests/JSONBeautify.mjs";
 import "./tests/JSONMinify.mjs";