Class ResponseKeyDerivation

Namespace

Tessio.Verifier.AspNetCore

Assembly

Tessio.Verifier.AspNetCore.dll

Derives a P-256 response-encryption key from a master secret and a salt, so any instance holding the master can recreate the key for a request from its salt alone, and no private key is stored anywhere.

public static class ResponseKeyDerivation

Inheritance

object

ResponseKeyDerivation

Inherited Members

object.Equals(object)

object.Equals(object, object)

object.GetHashCode()

object.GetType()

object.MemberwiseClone()

object.ReferenceEquals(object, object)

object.ToString()

Remarks

HKDF-SHA256 (RFC 5869) expands the master and salt into 48 bytes, which are reduced to a private scalar d in [1, n - 1] by the step NIST SP 800-56A Rev. 3 section 5.6.1.2.1 uses for key-establishment key pairs: d = (c mod (n - 1)) + 1. That section draws c from a random bit generator; here it comes from HKDF, so this follows its reduction and is not its approved key-pair generation. The derivation is fixed by its info string: changing anything here changes every key, so a new derivation takes a new info string rather than editing this one.

Fields

MinimumMasterLength

The shortest master accepted: as long as the private scalar it derives.

public const int MinimumMasterLength = 32

Field Value

int

MinimumSaltLength

The shortest salt accepted, enough that two random salts colliding is negligible.

public const int MinimumSaltLength = 16

Field Value

int

Methods

Derive(ReadOnlySpan<byte>, ReadOnlySpan<byte>)

The P-256 key that master and salt derive. The caller owns and disposes it.

public static ECDsa Derive(ReadOnlySpan<byte> master, ReadOnlySpan<byte> salt)

Parameters

master ReadOnlySpan<byte>

salt ReadOnlySpan<byte>

Returns

ECDsa

Exceptions

ArgumentException

The master is shorter than MinimumMasterLength bytes or all zeros, or the salt is shorter than MinimumSaltLength.