What Is SPHINCS+ and How Does It Sign Transactions?

SPHINCS+ (Stateless Practical Hash-based Incredibly Nice Cryptographic Signatures), standardized as SLH-DSA under NIST FIPS 205, is a hash-based digital signature algorithm designed for quantum resistance. It authorizes cryptocurrency transactions by producing signatures that cannot be forged, even by quantum computers.

Unlike ECDSA signatures used by Bitcoin and Ethereum, SPHINCS+ derives its security solely from the properties of cryptographic hash functions. This makes it the most conservative post-quantum signature scheme, as hash functions have decades of security analysis and no structural weaknesses exploitable by quantum algorithms.

SPHINCS+ constructs signatures using a hierarchy of hash-based components: WOTS+ (Winternitz One-Time Signatures), FORS (Forest of Random Subsets), and Merkle tree authentication paths. The stateless design means no counter or state management is required between signatures, simplifying wallet implementation and eliminating synchronization risks.

Transaction signing with SPHINCS+ follows a straightforward process. The wallet hashes the transaction data, then uses the private key to generate a SPHINCS+ signature. This signature, attached to the transaction, can be verified by any node using the public key. Valid signatures prove transaction authorization without revealing the private key.

SPHINCS+ signatures are larger than ECDSA (ranging from 7KB to 49KB depending on parameter selection), but this trade-off provides maximum confidence in quantum security. Signing takes far longer than ECDSA (tens to hundreds of milliseconds against well under one), while verification remains fast.

SynX uses SPHINCS+ as its transaction signature algorithm, ensuring that every fund transfer and staking operation is protected by hash-based quantum-resistant cryptography. Combined with Kyber-768 for key encapsulation, SynX signs with post-quantum algorithms, so there is no elliptic-curve key for a future quantum computer to attack.