Digital Signatures: Algorithms, Standards, and Legal Standing in the US

Digital signatures occupy a critical intersection of cryptographic engineering and legal enforceability in the United States, governing how identity, consent, and document integrity are verified across commercial, governmental, and healthcare transactions. This page covers the algorithmic foundations of digital signatures, the standards bodies and regulatory frameworks that govern their use, the principal deployment scenarios across US sectors, and the decision criteria that distinguish one signature scheme from another. The subject is relevant to procurement officers, compliance teams, legal professionals, and system architects operating in regulated industries.

Definition and scope

A digital signature is a cryptographic mechanism that binds a signer's identity to a specific piece of data, producing a verifiable proof that the data has not been altered since signing and that the signing key was applied by a specific party. This is distinct from an electronic signature — a broader legal category that includes typed names, checkbox acknowledgments, and image-based signatures — though the two terms are frequently conflated in contract workflows.

The scope of digital signature technology is defined primarily by NIST FIPS 186-5, the Digital Signature Standard (DSS), which specifies the approved algorithms for federal use. Three algorithm families are approved under FIPS 186-5: DSA (Digital Signature Algorithm), RSA-based signatures, and ECDSA (Elliptic Curve Digital Signature Algorithm). NIST deprecated DSA in FIPS 186-5 for new signature generation, leaving RSA and ECDSA as the primary federally sanctioned options, with EdDSA (Edwards-curve Digital Signature Algorithm) added as an approved scheme.

Legal standing at the federal level derives from two statutes: the Electronic Signatures in Global and National Commerce Act (E-SIGN Act, 15 U.S.C. § 7001 et seq.) and the Uniform Electronic Transactions Act (UETA), adopted by 49 states and the District of Columbia as of the Uniform Law Commission's published adoption table. Together, these frameworks establish that digital signatures cannot be denied legal effect solely because they are electronic in form. Sector-specific requirements — including those under HIPAA, PCI DSS, and federal acquisition regulations — layer additional technical constraints on top of this baseline.

How it works

Digital signatures operate through asymmetric encryption, using a mathematically linked key pair: a private key held exclusively by the signer and a public key distributed freely. The signing process follows a discrete sequence:

1. Hash generation — A cryptographic hash function (SHA-256 or SHA-384 under NIST guidelines) is applied to the message or document, producing a fixed-length digest that uniquely represents the content.
2. Private key encryption of the digest — The signer's private key encrypts the hash value, producing the signature. This step is the core of non-repudiation: only the holder of the private key could have produced that specific signature.
3. Transmission — The signed document and the signature value (and typically the signer's digital certificate) are transmitted to the relying party.
4. Verification — The verifier decrypts the signature using the signer's public key, re-hashes the received document independently, and compares the two hash values. A match confirms integrity and authenticity.

The security of this process depends directly on the hash algorithm and key length. NIST SP 800-131A Rev. 2 disallows SHA-1 for digital signature generation as of 2014 and sets minimum RSA key sizes at 2048 bits for current use, transitioning toward 3072 bits for post-2030 security margins.

Certificates that bind public keys to verified identities are issued by Certificate Authorities (CAs), operating within the Public Key Infrastructure (PKI). Federal agencies use the Federal PKI (FPKI), governed by the Federal CIO Council, which maintains cross-certified trust anchors across civilian and defense systems.

Common scenarios

Digital signatures appear across a wide range of regulated and commercial contexts in the US:

• Federal contracting — The General Services Administration and Department of Defense require digitally signed documents for many procurement workflows, with PIV (Personal Identity Verification) cards carrying ECDSA or RSA credentials used by federal employees.
• Healthcare records — HIPAA-regulated entities use digital signatures to authenticate ePHI (electronic Protected Health Information) transactions and to meet audit-trail requirements. Many implementations rely on encryption compliance frameworks to satisfy 45 CFR Part 164 technical safeguard standards.
• Financial services — SEC Rule 17a-4 and FINRA guidance reference electronic recordkeeping in ways that make digitally signed audit logs a de facto standard for broker-dealers and investment advisers.
• Code signing — Software publishers sign executable code and packages using code signing certificates, allowing operating systems and app distribution platforms to verify publisher identity and code integrity before execution.
• Legal and real estate transactions — E-SIGN-compliant digital signatures are routinely used in mortgage origination, title transfer, and notarized document workflows, with state-level Remote Online Notarization (RON) laws introducing additional requirements in jurisdictions including Virginia, Texas, and Florida.

Decision boundaries

Selecting a digital signature scheme requires evaluating four intersecting criteria: regulatory mandate, algorithm longevity, infrastructure compatibility, and threat horizon.

RSA vs. ECDSA — RSA at 2048 bits is broadly compatible with legacy systems and is specified in older standards, but it produces larger signature values and requires greater computational overhead than ECDSA at equivalent security levels. ECDSA with the P-256 curve provides approximately 128-bit security with a 64-byte signature, compared to RSA-2048's 256-byte output. Systems constrained by bandwidth or processing power — including IoT devices — favor ECDSA for this reason.

Algorithm longevity and post-quantum risk — Neither RSA nor ECDSA is considered resistant to attacks from cryptographically relevant quantum computers. NIST's Post-Quantum Cryptography standardization process finalized FIPS 204 (ML-DSA, based on CRYSTALS-Dilithium) and FIPS 205 (SLH-DSA, based on SPHINCS+) in 2024 as quantum-resistant digital signature standards. Federal agencies subject to CNSS Policy 15 and NSA's CNSA 2.0 guidance face explicit migration timelines toward these schemes.

Legally qualified electronic signatures (QESCD) — The EU eIDAS Regulation, relevant to US entities operating across borders, defines a hierarchy of signature types. While eIDAS does not govern domestic US transactions, multinational contracts may require alignment with its Qualified Electronic Signature (QES) requirements, which mandate use of a Qualified Signature Creation Device (QSCD) — typically a hardware token or hardware security module.

Trust anchor requirements — Applications operating within federal networks must use CA certificates that chain to the Federal Common Policy CA (FCPCA), maintained by the GSA FPKI program. Commercial applications may rely on browser-trusted public CAs, but the trust root selected determines which signature schemes and key types are accepted by verifying parties.

References

• NIST FIPS 186-5 — Digital Signature Standard (DSS)
• NIST SP 800-131A Rev. 2 — Transitioning the Use of Cryptographic Algorithms and Key Lengths
• NIST Post-Quantum Cryptography Standardization
• NIST FIPS 180-4 — Secure Hash Standard
• Electronic Signatures in Global and National Commerce Act (E-SIGN), 15 U.S.C. § 7001
• Uniform Electronic Transactions Act — Uniform Law Commission
• GSA Federal PKI Program (FPKI)
• HHS HIPAA Security Rule — 45 CFR Part 164
• NIST SP 800-57 Part 1 Rev. 5 — Recommendation for Key Management