HKDF derive

POST /api/hkdf-derive1 quota unit per call · cache hits free

Run HKDF — the HMAC-based key derivation function from RFC 5869. Extract-then-expand from already-random input material plus an optional info string for domain separation.

HKDF (RFC 5869) is the standard extract-then-expand KDF for **already-high-entropy** input material — session keys, raw shared secrets, output of an ECDH exchange. Don't use HKDF on passwords (use PBKDF2 or scrypt for that). Inputs: - `ikm`: input keying material (the high-entropy seed). - `salt`: optional, RFC 5869 §3.1. - `info`: optional context/domain-separation string. - `hash`: underlying HMAC (SHA-256/384/512). - `bitsLength`: output length in bits (1 byte to 255×HashLen). Useful for splitting one shared secret into multiple per-purpose subkeys via different `info` strings.

Inputs

Name	Type	Default	Description
ikm*	string	—	Input keying material.
salt	string	""	Optional salt (recommended).
info	string	""	Optional context/info string for domain separation.
hash	enum (SHA-256 \| SHA-384 \| SHA-512)	"SHA-256"	Underlying HMAC hash.
bitsLength	number (8…8192)	256	Output key length in bits.
ikmFormat	enum (text \| hex \| base64)	"text"	Input encoding of `ikm`.
saltFormat	enum (text \| hex \| base64)	"text"	Input encoding of `salt`.
infoFormat	enum (text \| hex \| base64)	"text"	Input encoding of `info`.
outputFormat	enum (hex \| base64 \| base64url)	"hex"	Output encoding of the derived key.

Response

Modes: json. Cache: not cacheable.

Every response carries x-cache (HIT / MISS / BYPASS), x-cache-signature (stable across identical inputs), and the rate-limit headers listed below.

Quota & limits

Cost per call	1 unit against the 10,000-unit monthly quota. Cache hits are free — only cache misses decrement.
Burst limit	200 requests per rolling minute, shared across all tools.
Max request size	`64 KiB` (base64 inflates file payloads ~33% — the limit applies to the decoded bytes).
Max response size	`16 KiB`
Timeout	10s wall clock.
Rate-limit headers	`X-RateLimit-Limit`, `X-RateLimit-Remaining`, `X-RateLimit-Burst-Limit` on every response; `X-Quota-Warning` once 80% of the monthly quota is spent; `Retry-After` on 429s.

Errors

Errors are JSON with an error string; schema failures add a details object (Zod's flattened field errors).

Status	When	Body
400	Input fails the tool's schema (wrong type, missing required field, malformed base64) or the file bytes can't be parsed as the expected format.	{ "error": "invalid inputs", "details": { "fieldErrors": { … } } }
401	Missing or invalid X-Api-Key / X-Account-Id headers (or no session when called from the browser).	{ "error": "unauthorized" }
404	Unknown tool id, or calling /api/* on the website hostname instead of api.xtract.bot.	{ "error": "not found" }
413	Request body over the tool's max request size, or an image header declaring more megapixels than the tool's pixel budget.	{ "error": "…exceeds maxRequestBytes…" }
415	Content-Type isn't application/json on the json-body transport.	{ "error": "unsupported content-type; expected application/json" }
429	Monthly quota or the 200/min burst limit exhausted. Check Retry-After and the X-RateLimit-* headers.	{ "error": "monthly quota exceeded", "monthRemaining": 0, … }
5xx	Conversion engine failure. Safe to retry; if it persists, the input is hitting a bug — please report it.	{ "error": "…" }

Code samples

Built from the default example.


curl -X POST https://api.xtract.bot/api/hkdf-derive \
  -H "Content-Type: application/json" \
  -H "Accept: application/json" \
  -H "X-Account-Id: $XTRACT_ACCOUNT_ID" \
  -H "X-Api-Key: $XTRACT_API_KEY" \
  -d '{
  "ikm": "0011223344556677889900aabbccddeeff",
  "ikmFormat": "hex",
  "salt": "x-shared-salt",
  "info": "encryption-subkey",
  "hash": "SHA-256",
  "bitsLength": 256,
  "outputFormat": "hex"
}'