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The Qustody API is a non-custodial orchestration layer. It manages transaction lifecycle, policy enforcement, and event delivery, but never holds private keys.

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Non-custodial signing model

Unlike traditional custodial solutions, the Custody API delegates all signing to your infrastructure. This means:
  • You control the keys. Post-quantum private keys live in your HSM, secure enclave, or key management system
  • The API orchestrates. It creates transactions, enforces policies, and broadcasts signed transactions to the Quantum Chain network
  • Signing is a callback. When a transaction needs signing, you retrieve the payload, sign it externally, and submit the signature back

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Transaction lifecycle

Every transaction moves through a deterministic state machine of 14 states. The diagram below shows the most common transitions; see Transaction state machine for the full table.

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Request flow

A typical transfer follows this sequence:
1

Create transaction

Your application calls POST /v1/transactions with source, destination, and amount. The API validates the request and runs it through the policy engine.
2

Policy evaluation

Configured policies are evaluated: spending limits, whitelist/blacklist checks, approval requirements, and time windows. If a policy requires approval, the transaction moves to PENDING_AUTHORIZATION.
3

Retrieve signing payload

Call GET /v1/transactions/{id}/signing_payload to get the unsigned transaction digest as a hex-encoded byte array.
4

Sign externally

Sign the digest with your post-quantum private key using your key management solution.
5

Submit signature

Call POST /v1/transactions/{id}/signature with the base64-encoded signature and the signer’s public key. The API verifies the signature before accepting it.
6

Broadcast and confirm

The API broadcasts the signed transaction to the Quantum Chain network via the node pool and tracks confirmation depth.
7

Webhook notification

Status change events are delivered to your registered webhook endpoints with HMAC-SHA256 signatures.

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System components

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Multi-tenancy

The API supports multiple tenants with full isolation:
  • Each tenant has its own API credentials, vaults, wallets, policies, and webhook endpoints
  • Cross-tenant access is prevented at the middleware layer
  • Tenant context is extracted from the API key and propagated through all service calls

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Post-quantum signatures

Quantum Chain uses a post-quantum digital signature scheme offering security equivalent to ~192-bit classical cryptography: The API verifies signatures server-side before broadcasting to ensure only valid transactions reach the network.