schastsp/internal/keyset/keyset.go

package keyset

import (
	"encoding/binary"
	"errors"
	"git.xdrm.io/schastsp/context"
	"git.xdrm.io/schastsp/util/scha"
	"io"
)

const SecretSize = scha.HSIZE * 4

/* Attributes */
type T struct {
	ctx   *context.T // current context
	depth uint16     // cur depth

	sec   []byte // secret
	mcode uint8  // migration code (secret renewal)
	// 0: none
	// 1: need to migrate
	// 2: waiting migration
}

/* (1) Creates a new KeySet
*
* @ctx<Context>						Context constants
*
---------------------------------------------------------*/
func Create(ctx *context.T) (*T, error) {

	/* (1) Fail if min+thre >= max */
	if ctx == nil {
		return nil, errors.New("Context must not be null")
	}

	/* (2) Instanciate */
	var instance = new(T)

	/* (3) Set attributes */
	instance.ctx = ctx

	/* (4) Generate values if <nil> secret */
	if instance.sec == nil {
		instance.generate()
	}

	return instance, nil
}

/* (2) Get current hash
*
* @return digest<[]byte]>			Current hash representing the set
*
---------------------------------------------------------*/
func (s T) CurrentHash() ([]byte, error) {

	/* (1) Get digest */
	digest, err := scha.Hash(s.sec, s.depth)

	/* (2) Dispatch error */
	if err != nil {
		return nil, err
	}

	/* (3) Else -> return digest */
	return digest, nil

}

/* (3) Decrement depth
*
* @return remaining<uint>			Remaining hashes before migration
*
---------------------------------------------------------*/
func (s *T) Decrement() uint16 {

	/* (1) Decrement the depth */
	s.depth--

	/* (2) If near minDepth (10 far): set consumption to 1 */
	if s.mcode == 0 && s.depth <= s.ctx.MinDepth()+s.ctx.DepthThreshold() {
		s.mcode = 1
	}

	/* (3) Return remaining attempts */
	return s.depth - s.ctx.MinDepth()

}

/* (4) Serialisation
*
* @return serial<string>			String representation
*
*
* === FORMAT ===
*
* |   hsize   |   secret   |  depth  |
* +-----------+------------+---------+
* |  16 bits  | hsize bits | 16 bits |
*
* == ENDIANNESS ==
*
* network endianness -> big-endian
*
---------------------------------------------------------*/
func (s *T) Store(writer io.Writer) error {

	var err error

	/* (1) Copy secret size */
	err = binary.Write(writer, binary.BigEndian, uint16(len(s.sec)))
	if err != nil {
		return err
	}

	/* (2) Copy secret */
	err = binary.Write(writer, binary.BigEndian, s.sec)
	if err != nil {
		return err
	}

	/* (3) Copy depth */
	err = binary.Write(writer, binary.BigEndian, s.depth)
	if err != nil {
		return err
	}

	/* (4) Copy migration code */
	err = binary.Write(writer, binary.BigEndian, s.mcode)
	if err != nil {
		return err
	}

	return nil

}

/* (5) Builds a KeySet from its serial representation
*
* @serial<string>			String representation
*
* @return instance<*Set>	The corresponding set
* @return err<error>		Optional error
*
---------------------------------------------------------*/
func (s *T) Fetch(reader io.Reader) error {

	var err error
	var secretLength uint16

	/* (1) Read the secret size */
	err = binary.Read(reader, binary.BigEndian, &secretLength)
	if err != nil {
		return err
	}

	/* (2) Fail if secretLength lower than digest size */
	if secretLength < scha.HSIZE {
		return errors.New("Invalid secret size (must be at least the same size as a digest)")
	}

	/* (3) Try to copy the secret */
	s.sec = make([]byte, secretLength)
	err = binary.Read(reader, binary.BigEndian, &s.sec)
	if err != nil {
		return err
	}

	/* (4) Manage invalid secret size (mismatch secretLength) */
	if uint16(len(s.sec)) != secretLength {
		return errors.New("Mismatching secret size")
	}

	/* (5) Try to copy the depth */
	err = binary.Read(reader, binary.BigEndian, &s.depth)
	if err != nil {
		return err
	}

	/* (6) Try to copy the migration code */
	err = binary.Read(reader, binary.BigEndian, &s.mcode)
	if err != nil {
		return err
	}

	return nil

}

/* (6) Getter/Setter for migration code 'mcode'
*
* @mcode<uint8>					[OPT] New value
*
* @return mcode<uint8>			Migration code
*
---------------------------------------------------------*/
func (s *T) MigrationCode(optional ...uint8) uint8 {

	/* (1) If no valid code given -> return current one */
	if len(optional) < 1 || optional[0] > 2 {
		return s.mcode
	}

	/* (2) Set new code */
	s.mcode = optional[0]

	/* (3) Return new code */
	return s.mcode

}

/* (7) Updates depth for rescuing from desynchroisation
*
* @lastHash<[]byte>					Last received hash
*
* @return error<err>				Error or NIL if not
*
* @description						We must update @depth so the next sent hash h_depth(k)
*										hashed MIN times equals the received hash @lastHash
*									We must find depth = i-MIN+1
*									with i so that h_MIN( h_i(k) ) = lastHash
*
---------------------------------------------------------*/
func (s *T) Rescue(lastHash []byte) error {

	/* (1) Browse possible values
	---------------------------------------------------------*/
	for i := s.depth; i <= s.depth+s.ctx.MinDepth(); i++ {

		/* (1) Process hash */
		currentHash, err := scha.Hash(s.sec, i)
		if err != nil { return err }

		/* (2) If not found -> try again */
		if string(currentHash) != string(lastHash) {
			continue
		}

		/* (3) Store new depth */
		s.depth = i - s.ctx.MinDepth() + 1

		/* (4) Update migration code */
		s.mcode = 2

		return nil

	}

	return errors.New("Cannot find an available rescue depth")

}