SLH-DSA (Stateless Hash-Based Digital Signature Algorithm), standardised as NIST FIPS 205 in August 2024, is the post-quantum signature standard based on hash functions rather than lattice problems. Where ML-DSA and ML-KEM both rest their security on the hardness of Module Learning With Errors — a relatively young mathematical assumption first formulated in 2005 — SLH-DSA’s security rests exclusively on the collision resistance and preimage resistance of an underlying hash function (SHA-256, SHA-512, or SHAKE, depending on parameter set). Hash function security against quantum computers is well-understood: Grover’s algorithm provides at most a quadratic speedup, which is fully mitigated by doubling output size (SHA-256 remains adequate against classical attacks; SHA-512 provides AES-256-equivalent quantum resistance). The decades-long cryptanalytic confidence in SHA-2 and SHA-3 makes SLH-DSA’s security argument the most conservative available: it requires no new mathematical assumption beyond the hash functions already trusted throughout the entire cryptographic stack.
SHA (Secure Hash Algorithm) is the name given to a series of cryptographic hash function families standardised by NIST under FIPS 180 and FIPS 202. Three generations exist with fundamentally different design lineages. SHA-1 (1995, FIPS 180-1) produces a 160-bit digest and is fully broken for collision resistance: the SHAttered attack (Google and CWI Amsterdam, 2017) produced a chosen-prefix collision — two different PDF files with identical SHA-1 hashes — using approximately 9.2 × 10^18 SHA-1 operations, within practical reach of well-resourced attackers. SHA-1 must not be used for any security purpose; it persists only in legacy Git object identifiers (SHA-1 is being phased out in Git’s object store in favour of SHA-256 under the sha256 object format) and in TOTP’s HMAC-SHA-1 inner construction (where collision resistance is not the relevant security property, but migration to SHA-256 variants is still recommended). SHA-2 (2001, FIPS 180-2 and subsequent revisions) is the Merkle-Damgård family that includes SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, and SHA-512/256. SHA-256 and SHA-512 are the two variants in universal production use; the others serve niche roles. SHA-3 (2015, FIPS 202) is the Keccak sponge construction — structurally independent of SHA-2 — providing algorithm diversity and including fixed-output variants (SHA3-256, SHA3-512) and extendable output functions (SHAKE128, SHAKE256).
Post-Quantum Cryptography (PQC) is the set of cryptographic algorithms designed to resist attacks from a Cryptographically Relevant Quantum Computer (CRQC) — a quantum computer large and stable enough to run Shor’s algorithm at scale. Shor’s algorithm can solve the integer factorisation and discrete logarithm problems that underpin RSA, ECDSA, and ECDH in polynomial time, meaning that every asymmetric algorithm in wide use today — TLS key exchange, X.509 certificate signatures, SSH host keys, code signing, and encrypted email — becomes trivially breakable by a CRQC. Symmetric algorithms (AES, SHA-256) are substantially less affected: Grover’s algorithm provides only a quadratic speedup against them, which is mitigated by doubling key lengths (AES-256 remains appropriate). PQC replaces the asymmetric primitives only, on hard mathematical problems for which no efficient quantum algorithm is known: structured lattices (the Learning With Errors and Module-LWE problems), hash functions (the security of SHA-3 family variants), and error-correcting codes.
NIST Special Publication 800-53 is published by the National Institute of Standards and Technology (NIST), a US federal agency within the Department of Commerce. The current version is Revision 5 (September 2020, updated December 2020), which defines over 1,000 security and privacy controls organized in 20 control families (Access Control, Audit and Accountability, Configuration Management, Incident Response, System and Communications Protection, Supply Chain Risk Management, etc.). NIST 800-53 is mandatory for US federal agencies and their contractors under FISMA (Federal Information Security Modernization Act) and serves as the control baseline for FedRAMP (cloud), CMMC (defense contractors), and many state/local government programs. Beyond the US, it is widely adopted internationally as a comprehensive reference catalog — organizations in finance, healthcare, and critical infrastructure worldwide use NIST 800-53 as their control framework. The standard defines three baselines (Low, Moderate, High) corresponding to the potential impact of a security breach. NIST 800-53 is not a certification itself but the control catalog against which systems are assessed; formal authorization (ATO — Authority to Operate) is granted by an authorizing official after an assessor verifies control implementation using NIST SP 800-53A assessment procedures. The companion OSCAL (Open Security Controls Assessment Language) standard, also from NIST, provides machine-readable formats for expressing 800-53 controls and assessment results.
ML-KEM (Module-Lattice-Based Key Encapsulation Mechanism), standardised as NIST FIPS 203 in August 2024, is the primary post-quantum replacement for key encapsulation and key exchange. It replaces the role of ECDH (X25519, P-256) and RSA key transport in TLS handshakes, IPsec IKEv2 negotiations, and any other protocol that needs two parties to establish a shared secret without prior key material. ML-KEM is derived from CRYSTALS-Kyber, the submission that won NIST’s lattice-based KEM selection, and its security rests on the Module Learning With Errors (MLWE) problem: distinguishing a structured noisy linear system from a random one is computationally hard, and no efficient quantum algorithm for this problem is known. The “module” qualifier means the construction uses polynomial rings structured in a way that allows a good balance between security and efficiency, contrasting with pure LWE (larger keys, simpler structure) and NTRU (smaller keys, different structure).
ML-DSA (Module-Lattice-Based Digital Signature Algorithm), standardised as NIST FIPS 204 in August 2024, is the primary post-quantum replacement for digital signatures. It replaces ECDSA, EdDSA, and RSA PSS/PKCS#1 signatures in X.509 certificates, code signing, TLS client and server authentication, SSH, JWT signing, and any other context where a party proves possession of a private key by producing a signature that others verify with the public key. ML-DSA is derived from CRYSTALS-Dilithium, the submission that won NIST’s lattice-based signature selection, and its security rests on the Module Learning With Errors (MLWE) and Module Short Integer Solution (MSIS) problems — the same mathematical family as ML-KEM, which is significant because both algorithms can share implementation code and hardware acceleration for the underlying polynomial arithmetic (NTT, number-theoretic transform).
FIPS 140 (Federal Information Processing Standard, Publication 140) is the US and Canadian government standard that defines security requirements for cryptographic modules — the hardware, software, or firmware components that perform cryptographic operations (encryption, decryption, hashing, signing, key management). It is published by NIST (National Institute of Standards and Technology) and jointly administered with CCCS (Canadian Centre for Cyber Security) through the Cryptographic Module Validation Program (CMVP). The standard has two active versions: FIPS 140-2 (published 2001, no longer accepting new submissions since April 2022) and FIPS 140-3 (effective September 2020, the current standard for all new validations). FIPS 140-2 certificates remain valid until 21 September 2026, after which they move to the Historical list — meaning only FIPS 140-3 validated modules will be accepted for new federal procurements. FIPS 140 defines four security levels (Level 1 through Level 4), with Level 1 being the baseline for software modules and Level 4 requiring physical tamper-active hardware. Compliance is mandatory for all US federal agencies and their contractors under FISMA, for Canadian federal systems, and is widely adopted by regulated industries (finance, healthcare, critical infrastructure) globally. Non-validated cryptography is treated as providing no protection — effectively plaintext — regardless of the algorithm strength. Validation is a formal, lab-based process: vendors submit modules to accredited Cryptographic and Security Testing (CST) laboratories, which test against the standard and submit results to CMVP for certificate issuance.
The Federal Risk and Authorization Management Program (FedRAMP) is a US government-wide program, codified into law by the FedRAMP Authorization Act of 2022, that provides a standardized approach to security assessment, authorization, and continuous monitoring for cloud products and services used by federal agencies. FedRAMP is administered by the General Services Administration (GSA) and is mandatory — any cloud service (SaaS, PaaS, IaaS) that stores, processes, or transmits federal data or metadata must achieve FedRAMP authorization before it can be used by US government agencies or their contractors. The program defines three impact levels: Low (limited adverse effect), Moderate (serious adverse effect), and High (severe or catastrophic effect — applies to law enforcement, emergency, financial, and health systems). Each level maps to NIST SP 800-53 Rev 5 control baselines: FedRAMP High requires implementation of approximately 421 controls. Authorization is achieved through either an Agency ATO (a specific agency sponsors the assessment) or the newer FedRAMP 20-X experimental accelerated path. Once authorized, cloud service providers (CSPs) must maintain continuous monitoring — monthly vulnerability scans, annual penetration testing, and Plan of Action & Milestones (POA&M) reporting — or risk revocation. Authorized services are listed on the FedRAMP Marketplace.
AES (Advanced Encryption Standard), standardised as NIST FIPS 197 in 2001, is the symmetric block cipher that underlies virtually all data encryption in modern infrastructure. It was selected through a five-year open competition that evaluated 15 candidate algorithms; the winner, Rijndael (designed by Joan Daemen and Vincent Rijmen), became AES. A block cipher takes a fixed-size block of plaintext and a key and produces a fixed-size block of ciphertext — AES always operates on 128-bit (16-byte) blocks, regardless of key size. Three key lengths are standardised: AES-128 (128-bit key, 10 rounds), AES-192 (192-bit key, 12 rounds), and AES-256 (256-bit key, 14 rounds), providing 128, 192, and 256 bits of security respectively against classical attacks. AES-256 is the conservative choice for data with long confidentiality requirements and is mandated by CNSA 2.0 for national security systems; AES-128 is widely deployed in TLS and provides adequate security for most workloads. The internal structure — SubBytes, ShiftRows, MixColumns, AddRoundKey — is fully public and has withstood over two decades of cryptanalysis; the best known attacks against full-round AES are theoretical and computationally infeasible, requiring work far beyond brute force but not threatening practical security.
