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Cryptography is a set of tools with different jobs: encryption protects secrecy, hashes help detect changes, digital signatures support integrity and signatory authentication, certificates connect public keys to identities, and key exchange establishes shared keying material. Secure systems combine these tools; none is a substitute for all the others.
The cryptography toolkit at a glance
The names AES, RSA, ECC, hashing, PKI, digital signatures and key exchange describe different mechanisms or systems. The first question is what a mechanism is meant to do—not which name sounds most secure.
| Mechanism | Main job | Key model | Important distinction |
|---|---|---|---|
| AES | Confidentiality | A shared secret key | It is a block cipher, not a complete communication protocol; mode, authentication, nonce or IV handling, and key management also matter. |
| RSA | Signatures or key establishment, depending on the scheme | A related public and private key pair | Signing and key establishment are distinct uses, covered by separate NIST standards. |
| ECC | Key agreement or signatures, depending on the technique | A public and private key pair based on elliptic-curve mathematics | ECDH is for key agreement; ECDSA and EdDSA are signature techniques. |
| Plain hash | Produce a digest useful for detecting changes | No secret key | A digest alone does not establish who created a message. |
| Digital signature | Integrity checking and signatory authentication | Private key to sign; corresponding public key to verify | A signature does not conceal the signed data. |
| PKI certificate | Bind a public key to an identity or owner | Public data signed by an issuing authority | A certificate is not the corresponding private key. |
| Key exchange or establishment | Produce shared keying material | Depends on the protocol and scheme | It sets up material for later protection; it does not itself encrypt an entire session. |
What is the difference between symmetric and public-key cryptography?
Symmetric cryptography uses the same secret key on both sides of an operation such as encryption and decryption. Public-key cryptography uses a related pair: a private key kept under its owner’s control and a public key that can be shared. Public-key systems can support operations such as signatures or key establishment, but the exact operation depends on the algorithm and scheme.
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The practical trade-off is key handling. With a symmetric cipher such as AES, both communicating parties need the right secret key and must keep it protected. Public-key methods avoid handing the private key to the other party, but they do not remove the need to establish trust in public keys or manage keys safely.
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How does AES encryption work?
AES, the Advanced Encryption Standard, is a symmetric block cipher: communicating parties use a shared secret key. It is commonly used as a building block for confidentiality, but AES alone does not specify every step needed to protect a real conversation or stored data.
A system built around AES must select an appropriate mode of operation and handle its key and any required nonce or initialization vector correctly. Where protection against tampering is needed, the system also needs authentication; encryption by itself should not be assumed to prove that data is genuine or unchanged.
NIST’s FIPS 197 specifies AES. The standard was originally published in 2001 and updated on May 9, 2023; NIST said that update modernized editorial material and made no technical changes to the algorithm.
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RSA and ECC are public-key families, not synonyms for one particular task. Each can appear in different kinds of cryptographic operation, and using the family name without naming the operation can be misleading.
RSA: distinguish signatures from key establishment
RSA uses mathematically related public and private keys. NIST FIPS 186-5 addresses RSA digital signatures, while NIST SP 800-56B Rev. 2 covers RSA-based key establishment, including key agreement and key transport. These are separate uses, with separate standards and requirements; a reference to “RSA encryption” does not by itself explain which operation or protocol is involved.
NIST reaffirmed SP 800-56B Rev. 2 as current on January 6, 2026. FIPS 186-5, published February 3, 2023, specifies RSA, ECDSA and EdDSA signature techniques; DSA is retained only for verifying existing signatures. NIST’s FIPS 186-5 abstract says: “This standard specifies a suite of algorithms that can be used to generate a digital signature.”
ECC: name the specific technique
Elliptic-curve cryptography (ECC) is a public-key family based on elliptic-curve mathematics. Elliptic-curve Diffie–Hellman (ECDH) is used for key agreement. Elliptic Curve Digital Signature Algorithm (ECDSA) is used for signatures, as is EdDSA, a different signature technique using Edwards curves. An ECC key-agreement method and an ECC signature method are not interchangeable simply because both use curves.
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NIST SP 800-56A Rev. 3, published in April 2018, covers finite-field and elliptic-curve discrete-logarithm key-establishment schemes. On January 6, 2026, NIST announced plans to update the publication, including changes addressing widely adopted x-coordinate-only ECC key-agreement implementations. That announcement is a plan to revise the publication, not a replacement standard or a new requirement already in force.
What does hashing do—and what can it not do?
A cryptographic hash function maps an input message to a fixed-length digest. A changed input should produce a different digest with very high probability, so a digest can help a recipient detect whether data changed when they have a trustworthy reference digest.
Hashing is not encryption: a hash is not designed to be reversed to recover the original message. And a plain hash does not prove who generated the message. If an attacker can replace both a message and its unprotected digest, the digest alone cannot establish authenticity. A digital signature or a keyed construction is needed when the sender’s authenticity matters.
NIST FIPS 180-4, the Secure Hash Standard, specifies SHA-1, SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224 and SHA-512/256. The standard was published in August 2015, and NIST’s landing page notes that a revision was planned. Which hash is suitable depends on the use case and current transition guidance; the algorithm name alone is not enough to make that decision.
How do digital signatures work?
A signer uses a private key to generate a signature for data; a verifier uses the corresponding public key to check it. A successful verification can help detect unauthorized modification and authenticate the signatory, provided the verifier has a trustworthy basis for associating that public key with the claimed signer.
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Signatures do not encrypt the signed content. Anyone who can read the data may still be able to read it; a system that needs secrecy must use encryption as well. Hashes often form part of signature mechanisms, but hashing and signing are not the same operation: a digest summarizes data, while the signature operation uses a private key and can be verified with the public key.
NIST FIPS 186-5 specifies RSA, ECDSA and EdDSA for signature generation and verification. NIST’s 2023 publication announcement says DSA is retained only for verification of existing signatures, not as one of the permitted signature-generation techniques in that standard.
What are certificates and PKI?
A public-key certificate is data that identifies a public key and its authorized owner, with a trusted certification authority’s digital signature binding the key to the owner. The certificate carries public information and identity claims; the corresponding private key is separate and should remain under its owner’s control.
PKI, or public key infrastructure, is the wider trust arrangement around certificates. It includes certification authorities, policies, software, and the processes used to validate certificates and handle revocation. Possessing a certificate is not, by itself, proof that it should be trusted: a verifier needs to validate the certificate and its chain against the applicable trust rules. NIST’s glossary defines “Public key certificate” in these terms.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What does key exchange do?
Key exchange, also called key establishment, lets parties produce shared keying material. A protocol can then use that material in subsequent steps to protect data. This setup function is different from bulk encryption: an exchange does not itself encrypt every message in a session.
Key establishment and authentication are also separate concerns. A protocol may produce shared material without, by that fact alone, proving that the other participant is the intended person or service. The protocol must address peer authentication as well as key establishment.
- NIST SP 800-56A Rev. 3 covers discrete-logarithm key-establishment schemes over finite fields and elliptic curves, including Diffie–Hellman variants.
- NIST SP 800-56B Rev. 2 covers key-establishment schemes based on integer-factorization cryptography, particularly RSA.
How do the pieces fit together in a secure system?
A useful way to understand a cryptographic system is to follow its jobs in sequence rather than treating one algorithm as the whole solution:
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- Authenticate the peer when needed: signatures and validated certificates can help connect a public key to the intended identity.
- Protect data: a symmetric cipher such as AES can provide confidentiality when used with an appropriate mode and correct key and nonce or IV handling.
- Check integrity and authenticity: the design uses an appropriate authenticated mechanism, such as a signature or keyed construction, rather than relying on a plain hash to identify the sender.
The exact construction depends on the protocol and threat model. NIST SP 800-131A Rev. 2, published March 21, 2019, provides transition guidance on cryptographic algorithms and key lengths and includes post-quantum algorithms in its scope. Treat it as a reason to plan and review algorithm transitions, not evidence that every current system is immediately vulnerable. Standards also change: NIST has announced an update to SP 800-56A Rev. 3, reaffirmed SP 800-56B Rev. 2 as current on January 6, 2026, and noted planned revisions or future corrections for FIPS 180-4 and FIPS 186-5. Consult the relevant current NIST publication and errata when making implementation or compliance decisions.
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