// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package crypto collects common cryptographic constants.
package crypto

import (
	"hash"
	"io"
	"strconv"
)

// Hash identifies a cryptographic hash function that is implemented in another
// package.
type Hash uint

// HashFunc simply returns the value of h so that Hash implements SignerOpts.
func (h Hash) HashFunc() Hash {
	return h
}

const (
	MD4       Hash = 1 + iota // import code.google.com/p/go.crypto/md4
	MD5                       // import crypto/md5
	SHA1                      // import crypto/sha1
	SHA224                    // import crypto/sha256
	SHA256                    // import crypto/sha256
	SHA384                    // import crypto/sha512
	SHA512                    // import crypto/sha512
	MD5SHA1                   // no implementation; MD5+SHA1 used for TLS RSA
	RIPEMD160                 // import code.google.com/p/go.crypto/ripemd160
	SHA3_224                  // import code.google.com/p/go.crypto/sha3
	SHA3_256                  // import code.google.com/p/go.crypto/sha3
	SHA3_384                  // import code.google.com/p/go.crypto/sha3
	SHA3_512                  // import code.google.com/p/go.crypto/sha3
	maxHash
)

var digestSizes = []uint8{
	MD4:       16,
	MD5:       16,
	SHA1:      20,
	SHA224:    28,
	SHA256:    32,
	SHA384:    48,
	SHA512:    64,
	SHA3_224:  28,
	SHA3_256:  32,
	SHA3_384:  48,
	SHA3_512:  64,
	MD5SHA1:   36,
	RIPEMD160: 20,
}

// Size returns the length, in bytes, of a digest resulting from the given hash
// function. It doesn't require that the hash function in question be linked
// into the program.
func (h Hash) Size() int {
	if h > 0 && h < maxHash {
		return int(digestSizes[h])
	}
	panic("crypto: Size of unknown hash function")
}

var hashes = make([]func() hash.Hash, maxHash)

// New returns a new hash.Hash calculating the given hash function. New panics
// if the hash function is not linked into the binary.
func (h Hash) New() hash.Hash {
	if h > 0 && h < maxHash {
		f := hashes[h]
		if f != nil {
			return f()
		}
	}
	panic("crypto: requested hash function #" + strconv.Itoa(int(h)) + " is unavailable")
}

// Available reports whether the given hash function is linked into the binary.
func (h Hash) Available() bool {
	return h < maxHash && hashes[h] != nil
}

// RegisterHash registers a function that returns a new instance of the given
// hash function. This is intended to be called from the init function in
// packages that implement hash functions.
func RegisterHash(h Hash, f func() hash.Hash) {
	if h >= maxHash {
		panic("crypto: RegisterHash of unknown hash function")
	}
	hashes[h] = f
}

// PublicKey represents a public key using an unspecified algorithm.
type PublicKey interface{}

// PrivateKey represents a private key using an unspecified algorithm.
type PrivateKey interface{}

// Signer is an interface for an opaque private key that can be used for
// signing operations. For example, an RSA key kept in a hardware module.
type Signer interface {
	// Public returns the public key corresponding to the opaque,
	// private key.
	Public() PublicKey

	// Sign signs msg with the private key, possibly using entropy from
	// rand. For an RSA key, the resulting signature should be either a
	// PKCS#1 v1.5 or PSS signature (as indicated by opts). For an (EC)DSA
	// key, it should be a DER-serialised, ASN.1 signature structure.
	//
	// Hash implements the SignerOpts interface and, in most cases, one can
	// simply pass in the hash function used as opts. Sign may also attempt
	// to type assert opts to other types in order to obtain algorithm
	// specific values. See the documentation in each package for details.
	Sign(rand io.Reader, msg []byte, opts SignerOpts) (signature []byte, err error)
}

// SignerOpts contains options for signing with a Signer.
type SignerOpts interface {
	// HashFunc returns an identifier for the hash function used to produce
	// the message passed to Signer.Sign, or else zero to indicate that no
	// hashing was done.
	HashFunc() Hash
}