Encryption Types and Algorithms: A Reference Guide

Encryption is the foundational mechanism by which digital systems protect data confidentiality, authenticate identity, and enforce data integrity across virtually every regulated industry in the United States. This reference covers the primary encryption families — symmetric, asymmetric, and hash-based — along with the algorithms that implement them, the regulatory frameworks that mandate their use, and the structural tradeoffs that practitioners and procurement officers must navigate. The scope spans both at-rest and in-transit contexts, from enterprise storage systems to public-key infrastructure and post-quantum transition planning.

• Definition and Scope
• Core Mechanics or Structure
• Causal Relationships or Drivers
• Classification Boundaries
• Tradeoffs and Tensions
• Common Misconceptions
• Checklist or Steps
• Reference Table or Matrix
• References

Definition and Scope

Encryption is the process of transforming plaintext data into ciphertext using a defined mathematical algorithm and one or more cryptographic keys, such that only authorized parties holding the correct key material can reverse the transformation. The scope of encryption spans two primary operational contexts: data at rest, where stored data on disk, database, or backup media is protected, and data in transit, where information moving across networks is shielded from interception.

The National Institute of Standards and Technology (NIST) defines cryptographic algorithms and approves them for federal use through its Cryptographic Module Validation Program (CMVP) and the Federal Information Processing Standards (FIPS). FIPS 140-3, the current validation standard for cryptographic modules, governs which implementations are acceptable in federal procurement — a requirement that flows downstream into regulated commercial sectors through frameworks such as HIPAA, PCI DSS, and FedRAMP.

The encryption landscape is not monolithic. It encompasses algorithm families, key management architectures, protocol layers, and hardware implementations — each governed by distinct standards bodies and compliance obligations. The glossary of encryption terms provides definitions for technical vocabulary referenced throughout this sector.

Core Mechanics or Structure

Symmetric Encryption

Symmetric encryption uses a single key for both encryption and decryption. The sender and receiver must share this key through a secure channel before communication begins. The dominant symmetric algorithm is the Advanced Encryption Standard (AES), standardized by NIST in FIPS 197 (2001). AES operates on 128-bit block sizes with key lengths of 128, 192, or 256 bits. AES-256 is the variant mandated for classified U.S. government information at the SECRET level, as specified by the National Security Agency's Commercial National Security Algorithm Suite (CNSA 1.0, published 2015).

Symmetric algorithms are further divided by their mode of operation — ECB (Electronic Codebook), CBC (Cipher Block Chaining), GCM (Galois/Counter Mode), and CTR (Counter Mode) among others. GCM provides both encryption and authentication (AEAD — Authenticated Encryption with Associated Data), making it the preferred mode in TLS 1.3, as documented in RFC 8446.

Asymmetric Encryption

Asymmetric encryption uses a mathematically linked key pair: a public key for encryption and a private key for decryption. RSA (Rivest–Shamir–Adleman) is the most widely deployed asymmetric algorithm, relying on the computational hardness of integer factorization. NIST SP 800-131A Rev 2 disallows RSA key sizes below 2048 bits for new implementations after 2015, with 3072-bit keys recommended for security beyond 2030.

Elliptic Curve Cryptography (ECC) achieves equivalent security to RSA with substantially shorter keys — a 256-bit ECC key provides roughly the same security level as a 3072-bit RSA key, according to NIST SP 800-57 Part 1 Rev 5. This size advantage makes ECC dominant in constrained environments including mobile devices and IoT.

Hash Functions

Hash functions are one-way transformations producing a fixed-length digest from arbitrary-length input. SHA-256 and SHA-3 (standardized in FIPS 180-4 and FIPS 202 respectively) are the current NIST-approved constructions. Hash functions underpin digital signatures, certificate fingerprinting, and password storage — though they are not reversible and thus technically outside the definition of encryption proper. The distinction is elaborated in hashing vs. encryption.

Causal Relationships or Drivers

Regulatory pressure is the primary driver of algorithm adoption timelines. The Health Insurance Portability and Accountability Act (HIPAA) Security Rule (45 CFR §164.312) does not mandate specific algorithms but requires encryption that renders data "unusable, unreadable, or indecipherable" — with HHS guidance pointing to NIST standards as the compliance benchmark. The Payment Card Industry Data Security Standard (PCI DSS v4.0, released 2022) requires "strong cryptography" for cardholder data, explicitly deprecating SSL and TLS 1.0 and requiring at minimum TLS 1.2, per the PCI Security Standards Council.

The National Security Agency's disclosure of potential weaknesses in older NIST elliptic curves (specifically the Dual EC DRBG random number generator, flagged publicly by 2013 Snowden disclosures) drove significant institutional migration toward Curve25519 and Ed25519, which are specified in RFC 8032.

Quantum computing threat models drive the post-quantum cryptography transition. NIST completed its first round of post-quantum algorithm standardization in 2024, publishing FIPS 203 (ML-KEM, based on CRYSTALS-Kyber), FIPS 204 (ML-DSA, based on CRYSTALS-Dilithium), and FIPS 205 (SLH-DSA, based on SPHINCS+). The post-quantum cryptography reference page covers migration frameworks.

Encryption key management failures — rather than algorithm weaknesses — account for the preponderance of cryptographic failures in practice, a pattern documented in NIST SP 800-57's guidance on key lifecycle management.

Classification Boundaries

Encryption schemes are classified along four primary axes:

By key relationship: Symmetric (shared key), asymmetric (public/private pair), or hybrid (asymmetric key exchange + symmetric bulk encryption, as used in TLS).

By operation type: Block ciphers (AES, Triple-DES) operate on fixed-size data blocks; stream ciphers (ChaCha20, RC4) operate on data one bit or byte at a time. RC4 is deprecated by NIST and prohibited under PCI DSS.

By security primitive: Confidentiality-only (AES-CBC without MAC), authenticated encryption (AES-GCM, ChaCha20-Poly1305), or signing/authentication only (HMAC-SHA256, ECDSA).

By deployment layer: Transport-layer protocols (TLS/SSL), application-layer encryption (end-to-end encryption), storage-layer encryption (full-disk encryption), and database-layer encryption (database encryption methods).

The boundary between encryption and tokenization is frequently mischaracterized. Tokenization replaces sensitive data with a non-reversible surrogate token and is not encryption — the distinction has direct PCI DSS compliance implications, covered in tokenization vs. encryption.

Tradeoffs and Tensions

Key length vs. performance: Longer keys increase computational overhead. AES-256 is approximately 40% slower than AES-128 in software implementations without hardware acceleration (AES-NI), though the margin narrows substantially on processors with native AES instruction sets. The security margin difference between AES-128 and AES-256 is debated among cryptographers — NIST considers both secure under current threat models.

Centralized vs. distributed key management: Centralized public key infrastructure (PKI) provides administrative control but creates single points of failure. Distributed approaches such as bring-your-own-key (BYOK) shift custody to the data owner but increase operational complexity for key rotation and recovery.

Forward secrecy vs. decryptability: Ephemeral Diffie-Hellman key exchange (used in TLS 1.3) provides perfect forward secrecy — past sessions cannot be decrypted even if the server's private key is later compromised. This conflicts with enterprise TLS inspection architectures and lawful intercept requirements.

Standardization vs. agility: Reliance on any single algorithm family creates systemic risk if that algorithm is broken. Cryptographic agility — the architectural capacity to swap algorithms without application redesign — is a design requirement in NIST SP 800-208 but adds implementation complexity.

Authenticated encryption overhead: AEAD modes (GCM, CCM) add authentication tag computation to every operation, increasing per-packet processing. At high-throughput network scales — above 40 Gbps — this requires hardware offload via dedicated cryptographic accelerators or hardware security modules (HSMs).

Common Misconceptions

Misconception: Encryption alone guarantees data security. Encryption protects confidentiality but not availability or integrity unless an authenticated mode is used. A ciphertext can be tampered with in transit if no MAC is present — this is the basis of padding oracle attacks documented against CBC-mode TLS in CVE-2016-2107 (DROWN attack family).

Misconception: HTTPS means the site is trustworthy. TLS encrypts the channel but does not authenticate the content or the legitimacy of the endpoint's operations. Certificate issuance by a public certificate authority only confirms domain control, not organizational legitimacy.

Misconception: AES-256 is unbreakable. No known attack breaks AES-256 faster than brute force, but implementation vulnerabilities — padding errors, weak IVs, key reuse — routinely compromise AES-protected systems. The algorithm's security does not transfer to insecure implementations. Side-channel attacks exploit timing and power consumption rather than the algorithm itself.

Misconception: Hashing is a form of encryption. Hash functions are irreversible by design and serve authentication and integrity functions, not confidentiality. Referring to password hashing as "password encryption" misrepresents both the mechanism and its security properties.

Misconception: Quantum computers will break all encryption immediately. Symmetric algorithms like AES-256 require only doubling the key length to maintain security against Grover's algorithm (a quantum speedup). Asymmetric algorithms based on factoring (RSA) and discrete logarithm (ECC) are more vulnerable, but large-scale cryptographically relevant quantum computers capable of breaking 2048-bit RSA did not exist as of NIST's 2024 post-quantum standardization documentation.

Checklist or Steps

The following sequence represents the evaluation phases organizations and procurement teams apply when assessing encryption implementations against regulatory and technical requirements. This is a structural description of standard practice — not prescriptive advice.

Phase 1 — Algorithm Inventory
- Identify all algorithms in use across applications, storage, and transport layers
- Map algorithms to NIST-approved lists in FIPS 140-3 Annex A and SP 800-131A Rev 2
- Flag deprecated constructions: MD5, SHA-1 (for signatures), RC4, DES, Triple-DES with 64-bit blocks, SSL, TLS 1.0, TLS 1.1

Phase 2 — Key Management Assessment
- Identify key generation sources and verify use of cryptographically secure pseudorandom number generators (CSPRNG)
- Document key storage locations: software keystore, HSM, cloud KMS
- Verify key rotation schedules align with SP 800-57 Part 1 recommendations (AES-256 keys: maximum 2^64 block encryptions before rotation under CBC mode)

Phase 3 — Deployment Context Review
- Classify data by sensitivity tier and map to required encryption strength
- Confirm authenticated encryption (AEAD) is applied to all network-transmitted ciphertext
- Verify TLS configurations meet or exceed TLS 1.2 minimum (TLS 1.3 preferred per RFC 8446)

Phase 4 — Compliance Mapping
- Cross-reference FIPS 140 requirements for any federal or FedRAMP-adjacent deployments
- Verify HIPAA, PCI DSS, or applicable state-level data protection mandates are addressed
- Review US export controls on encryption under EAR (15 CFR Part 742.15) for products distributed internationally

Phase 5 — Post-Quantum Readiness
- Inventory systems relying on RSA, ECDH, and ECDSA for key exchange or signatures
- Evaluate cryptographic agility of affected systems for algorithm substitution
- Track NIST FIPS 203/204/205 adoption timelines per NSA CNSA 2.0 transition guidance

Reference Table or Matrix

References

• NIST FIPS 197 — Advanced Encryption Standard (AES)
• NIST FIPS 140-3 — Security Requirements for Cryptographic Modules
• NIST SP 800-57 Part 1 Rev 5 — Recommendation for Key Management
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