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Cryptography

WebAuthn Passkey Assertion Decoder & Challenge Debugger

Decode WebAuthn/FIDO2 assertion fields, inspect authenticatorData flags and signCount, check challenge/origin/rpIdHash binding, and optionally verify ES256, RS256, or Ed25519 signatures with a supplied COSE public key.

json· JSON resultOpen tool

Quick start

Call this tool from your code in three languages.

cURL
curl -X POST 'https://api.elysiatools.com/en/api/tools/webauthn-passkey-assertion-decoder-challenge-debugger' \
  -H 'Content-Type: application/json' \
  -d '{"clientDataJson":"{\"type\":\"webauthn.get\",\"challenge\":\"Y2hhbGxlbmdl\",\"origin\":\"https://example.com\"}","clientDataEncoding":"json","authenticatorData":"a379a6f6eeafb9a55e378c118034e2751e682fab9f2d30ab13d2125586ce19478500000002","signature":"00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000","binaryEncoding":"hex","expectedChallenge":"Y2hhbGxlbmdl","expectedOrigin":"","rpId":"example.com","previousSignCount":1,"cosePublicKey":"","coseEncoding":"base64url"}'

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API reference

Send a POST request with your inputs as JSON. File parameters require a separate upload first.

Endpoint

HTTP
POST https://api.elysiatools.com/en/api/tools/webauthn-passkey-assertion-decoder-challenge-debugger

Parameters

NameTypeRequiredDescription
clientDataJsontextareaYes—
clientDataEncodingselectNo—
authenticatorDatatextareaYes—
signaturetextareaYes—
binaryEncodingselectNo—
expectedChallengetextNo—
expectedOrigintextNo—
rpIdtextNo—
previousSignCountnumberNo—
cosePublicKeytextareaNo—
coseEncodingselectNo—

Response format

JSON result

JSON
{
  "key": {...},
  "metadata": {
    "key": "value"
  },
  "error": "Error message (optional)",
  "message": "Notification message (optional)"
}

MCP integration

Add this tool to your Model Context Protocol server so AI agents can list and call it.

Server configuration

Add this block to your MCP client configuration:

mcp.json
{
  "mcpServers": {
    "elysiatools-webauthn-passkey-assertion-decoder-challenge-debugger": {
      "name": "webauthn-passkey-assertion-decoder-challenge-debugger",
      "description": "Decode WebAuthn/FIDO2 assertion fields, inspect authenticatorData flags and signCount, check challenge/origin/rpIdHash binding, and optionally verify ES256, RS256, or Ed25519 signatures with a supplied COSE public key.",
      "baseUrl": "https://api.elysiatools.com/mcp/sse?toolId=webauthn-passkey-assertion-decoder-challenge-debugger",
      "command": "",
      "args": [],
      "env": {},
      "isActive": true,
      "type": "sse"
    }
  }
}

List available tools

After connecting to the SSE endpoint, list the exposed tools:

tools/list
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/list"
}

Call this tool

Invoke the tool by its id, passing arguments built from its parameters:

tools/call
{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "tools/call",
  "params": {
    "name": "webauthn-passkey-assertion-decoder-challenge-debugger",
    "arguments": {
  "clientDataJson": "{\"type\":\"webauthn.get\",\"challenge\":\"Y2hhbGxlbmdl\",\"origin\":\"https://example.com\"}",
  "clientDataEncoding": "json",
  "authenticatorData": "a379a6f6eeafb9a55e378c118034e2751e682fab9f2d30ab13d2125586ce19478500000002",
  "signature": "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
  "binaryEncoding": "hex",
  "expectedChallenge": "Y2hhbGxlbmdl",
  "expectedOrigin": "",
  "rpId": "example.com",
  "previousSignCount": 1,
  "cosePublicKey": "",
  "coseEncoding": "base64url"
}
  }
}
Chain multiple tools in one session with a comma-separated toolId list, e.g. /mcp/sse?toolId=png-to-webp,jpg-to-webp,gif-to-webp (max 20).

Questions or issues? Contact [email protected]

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