# Digital signatures

## Purpose

A digital signature verifies the authenticity of a message and provides non-repudiation. This means any change to the message causes signature verification to fail, you know who signed the message, and someone cannot deny having signed a message.

Signing is done using a private key. The associated public key can then be publicly shared to allow others to verify signatures.

Private keys **MUST** **NOT** be shared. They **MUST** remain secret.

Generally, **avoid** using signatures with encryption and instead rely on **authenticated** key exchange. You can find out more here.

## Usage

### GenerateKeyPair

Fills a span with a randomly generated private key and another span with the associated public key.

#### Exceptions

`publicKey`

has a length not equal to `PublicKeySize`

.

`privateKey`

has a length not equal to `PrivateKeySize`

.

Unable to generate key pair.

### GenerateKeyPair

Fills a span with a private key generated using a random seed and another span with the associated public key.

#### Exceptions

`publicKey`

has a length not equal to `PublicKeySize`

.

`privateKey`

has a length not equal to `PrivateKeySize`

.

`seed`

has a length not equal to `SeedSize`

.

Unable to generate key pair from seed.

### ComputePublicKey

Fills a span with the public key computed from a private key.

#### Exceptions

`publicKey`

has a length not equal to `PublicKeySize`

.

`privateKey`

has a length not equal to `PrivateKeySize`

.

Unable to compute public key from private key.

### Sign

Fills a span with the signature for a message signed using a private key.

#### Exceptions

`signature`

has a length not equal to `SignatureSize`

.

`privateKey`

has a length not equal to `PrivateKeySize`

.

Unable to compute signature.

### Verify

Determines if a signature is valid for a message and public key. It returns `true`

if the signature is valid and `false`

otherwise.

#### Exceptions

`signature`

has a length not equal to `SignatureSize`

.

`publicKey`

has a length not equal to `PublicKeySize`

.

### IncrementalEd25519ph

Provides support for computing/verifying a signature from a sequence of messages using Ed25519ph.

`IncrementalEd25519ph.Finalize()`

fills a span with the signature for a chunked message signed using a private key.

`IncrementalEd25519ph.FinalizeAndVerify()`

determines if a signature is valid for a chunked message and public key. It returns `true`

if the signature is valid and `false`

otherwise.

This should **only** be used when the message is too large to fit into memory because prehashing is theoretically weaker than regular signing.

#### Exceptions

`signature`

has a length not equal to `SignatureSize`

.

`privateKey`

has a length not equal to `PrivateKeySize`

.

`publicKey`

has a length not equal to `PublicKeySize`

.

The signature could not be computed.

Cannot update after finalizing or finalize twice (without reinitializing).

## Constants

These are used for validation and/or save you defining your own constants.

## Notes

If you want to use BLAKE2b for prehashing instead of Ed25519ph, which uses SHA-512 internally, you can hash a domain separation constant (e.g. the protocol name) concatenated with the message and sign the 512-bit hash.

Ed25519 is vulnerable to fault attacks. Techniques like causing voltage glitches on a chip (e.g. on an Arduino) can be used to recover the secret key and create valid signatures.

This should generally not concern you as it's mostly relevant for embedded devices and requires physical or remote access to the device. Furthermore, most countermeasures are ineffective. Prehashing or hedged signatures can help but will not prevent all attacks.

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