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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11A public/private key pair consists of two mathematically related cryptographic keys: the public key can be shared, while its matching private key must remain secret. A cryptographic hash is different: it turns input of any length into a fixed-length digest. Keys, hashes, encryption, and digital signatures serve distinct roles.
What are public and private keys?
In public-key cryptography, a public key is associated with an entity and may be distributed. Its corresponding private key is kept secret. The keys are mathematically related, but NIST says the private key cannot efficiently be determined from knowledge of the public key. The exact operations available depend on the algorithm; a key pair should not be assumed to support every cryptographic task. NIST glossary: public key
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Public key
A public key can be used to verify signatures, encrypt data or keys for the matching private key, or participate in key agreement, depending on the algorithm and protocol.
Private key
The private key is the secret counterpart. It can create signatures in a signature scheme, or decrypt data encrypted for its corresponding public key in a suitable encryption scheme. Keep it private; sharing the public key does not mean sharing the private one.
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What is hashing?
A cryptographic hash function maps a message of arbitrary length to a fixed-length message digest, often called a hash. The digest is a compact representation of the input, not encrypted content. Hashing does not create a public/private key pair, and it does not provide a decryption key. NIST: Hash Functions
Approved hash functions are designed to resist three kinds of attack: finding two inputs with the same digest (a collision), finding an input for a given digest (a preimage), and finding another input with the same digest as a known input (a second preimage). A digest is not proof by itself that a message is authentic; its value depends on how it is used and protected.
How do signatures, encryption, and key agreement differ?
| Operation | Key role | What it provides |
|---|---|---|
| Digital signature | The private key signs; the matching public key verifies. | Authenticity and integrity protection. It does not provide confidentiality or, by itself, replay-attack protection. |
| Public-key encryption | A public key can encrypt data or key material that the matching private key can recover. | Confidentiality for the intended private-key holder, when used in a suitable scheme. |
| Key agreement | Public-key material is used in a protocol to help compute a shared secret. | A shared secret for subsequent cryptographic use; it is not itself a signature. |
These roles are algorithm- and protocol-dependent. In particular, a signature is not “encrypting with the private key”: signing and verification are a distinct operation. NIST defines a digital signature as an asymmetric operation in which the private key signs data and the public key verifies the signature. NIST SP 800-63-4, Digital Identity Guidelines
How does a hash work with a digital signature?
For a large message, a signature workflow commonly hashes the message and uses the resulting digest as input to the signature algorithm. The signer uses the private key to create the signature; a verifier uses the corresponding public key to check it. The hash is not the signature, and hashing alone does not authenticate the message. NISTIR 8202: Blockchain Technology Overview
A verified signature supports confidence that the signed data has not changed and that it was signed using the corresponding private key. It does not hide the message. Nor does a valid signature alone stop someone from replaying a previously valid signed message; protocols need separate measures for that risk.
Are all hash functions equally suitable?
No. The appropriate choice depends on the application, algorithm, protocol, and any standards requirements. NIST lists SHA-2 and SHA-3 options. Its current hash-functions page gives SHA-256 and SHA3-256 a collision-resistance strength of 128 bits and a preimage-resistance strength of 256 bits; these are security-strength estimates, not a guarantee about every implementation. NIST reports that SHA-1 was deprecated in 2011 and disallowed for digital signatures at the end of 2013, so it is not an appropriate choice for new digital signatures. NIST: Hash Functions
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Is a blockchain address the same as a public key?
Not necessarily. Some blockchain implementations derive an address from a public key by hashing it and adding other data. The resulting address is not necessarily the public key itself or simply its raw hash; the derivation rules vary by implementation. NIST’s 2018 blockchain overview notes that each implementation may use a different address-derivation method. NISTIR 8202: Blockchain Technology Overview
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Key generation is a separate operation from hashing. It must follow the requirements of the relevant algorithm and protocol; a hash function does not turn an arbitrary value into a secure key pair. NIST SP 800-133 Rev. 2 provides U.S. federal recommendations for cryptographic key generation, but the existence of that guidance does not establish that any particular software or implementation follows it. NIST SP 800-133 Rev. 2: Recommendation for Cryptographic Key Generation
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