// Copyright 2013 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 rsa

// This file implements the RSASSA-PSS signature scheme according to RFC 8017.

import (
	
	
	
	
	
	
)

// Per RFC 8017, Section 9.1
//
//     EM = MGF1 xor DB || H( 8*0x00 || mHash || salt ) || 0xbc
//
// where
//
//     DB = PS || 0x01 || salt
//
// and PS can be empty so
//
//     emLen = dbLen + hLen + 1 = psLen + sLen + hLen + 2
//

func emsaPSSEncode( []byte,  int,  []byte,  hash.Hash) ([]byte, error) {
	// See RFC 8017, Section 9.1.1.

	 := .Size()
	 := len()
	 := ( + 7) / 8

	// 1.  If the length of M is greater than the input limitation for the
	//     hash function (2^61 - 1 octets for SHA-1), output "message too
	//     long" and stop.
	//
	// 2.  Let mHash = Hash(M), an octet string of length hLen.

	if len() !=  {
		return nil, errors.New("crypto/rsa: input must be hashed with given hash")
	}

	// 3.  If emLen < hLen + sLen + 2, output "encoding error" and stop.

	if  < ++2 {
		return nil, errors.New("crypto/rsa: key size too small for PSS signature")
	}

	 := make([]byte, )
	 :=  -  -  - 2
	 := [:+1+]
	 := [+1+ : -1]

	// 4.  Generate a random octet string salt of length sLen; if sLen = 0,
	//     then salt is the empty string.
	//
	// 5.  Let
	//       M' = (0x)00 00 00 00 00 00 00 00 || mHash || salt;
	//
	//     M' is an octet string of length 8 + hLen + sLen with eight
	//     initial zero octets.
	//
	// 6.  Let H = Hash(M'), an octet string of length hLen.

	var  [8]byte

	.Write([:])
	.Write()
	.Write()

	 = .Sum([:0])
	.Reset()

	// 7.  Generate an octet string PS consisting of emLen - sLen - hLen - 2
	//     zero octets. The length of PS may be 0.
	//
	// 8.  Let DB = PS || 0x01 || salt; DB is an octet string of length
	//     emLen - hLen - 1.

	[] = 0x01
	copy([+1:], )

	// 9.  Let dbMask = MGF(H, emLen - hLen - 1).
	//
	// 10. Let maskedDB = DB \xor dbMask.

	mgf1XOR(, , )

	// 11. Set the leftmost 8 * emLen - emBits bits of the leftmost octet in
	//     maskedDB to zero.

	[0] &= 0xff >> (8* - )

	// 12. Let EM = maskedDB || H || 0xbc.
	[-1] = 0xbc

	// 13. Output EM.
	return , nil
}

func emsaPSSVerify(,  []byte, ,  int,  hash.Hash) error {
	// See RFC 8017, Section 9.1.2.

	 := .Size()
	if  == PSSSaltLengthEqualsHash {
		 = 
	}
	 := ( + 7) / 8
	if  != len() {
		return errors.New("rsa: internal error: inconsistent length")
	}

	// 1.  If the length of M is greater than the input limitation for the
	//     hash function (2^61 - 1 octets for SHA-1), output "inconsistent"
	//     and stop.
	//
	// 2.  Let mHash = Hash(M), an octet string of length hLen.
	if  != len() {
		return ErrVerification
	}

	// 3.  If emLen < hLen + sLen + 2, output "inconsistent" and stop.
	if  < ++2 {
		return ErrVerification
	}

	// 4.  If the rightmost octet of EM does not have hexadecimal value
	//     0xbc, output "inconsistent" and stop.
	if [-1] != 0xbc {
		return ErrVerification
	}

	// 5.  Let maskedDB be the leftmost emLen - hLen - 1 octets of EM, and
	//     let H be the next hLen octets.
	 := [:--1]
	 := [--1 : -1]

	// 6.  If the leftmost 8 * emLen - emBits bits of the leftmost octet in
	//     maskedDB are not all equal to zero, output "inconsistent" and
	//     stop.
	var  byte = 0xff >> (8* - )
	if [0] & ^ != 0 {
		return ErrVerification
	}

	// 7.  Let dbMask = MGF(H, emLen - hLen - 1).
	//
	// 8.  Let DB = maskedDB \xor dbMask.
	mgf1XOR(, , )

	// 9.  Set the leftmost 8 * emLen - emBits bits of the leftmost octet in DB
	//     to zero.
	[0] &= 

	// If we don't know the salt length, look for the 0x01 delimiter.
	if  == PSSSaltLengthAuto {
		 := bytes.IndexByte(, 0x01)
		if  < 0 {
			return ErrVerification
		}
		 = len() -  - 1
	}

	// 10. If the emLen - hLen - sLen - 2 leftmost octets of DB are not zero
	//     or if the octet at position emLen - hLen - sLen - 1 (the leftmost
	//     position is "position 1") does not have hexadecimal value 0x01,
	//     output "inconsistent" and stop.
	 :=  -  -  - 2
	for ,  := range [:] {
		if  != 0x00 {
			return ErrVerification
		}
	}
	if [] != 0x01 {
		return ErrVerification
	}

	// 11.  Let salt be the last sLen octets of DB.
	 := [len()-:]

	// 12.  Let
	//          M' = (0x)00 00 00 00 00 00 00 00 || mHash || salt ;
	//     M' is an octet string of length 8 + hLen + sLen with eight
	//     initial zero octets.
	//
	// 13. Let H' = Hash(M'), an octet string of length hLen.
	var  [8]byte
	.Write([:])
	.Write()
	.Write()

	 := .Sum(nil)

	// 14. If H = H', output "consistent." Otherwise, output "inconsistent."
	if !bytes.Equal(, ) { // TODO: constant time?
		return ErrVerification
	}
	return nil
}

// signPSSWithSalt calculates the signature of hashed using PSS with specified salt.
// Note that hashed must be the result of hashing the input message using the
// given hash function. salt is a random sequence of bytes whose length will be
// later used to verify the signature.
func signPSSWithSalt( io.Reader,  *PrivateKey,  crypto.Hash, ,  []byte) ([]byte, error) {
	 := .N.BitLen() - 1
	,  := emsaPSSEncode(, , , .New())
	if  != nil {
		return nil, 
	}
	 := new(big.Int).SetBytes()
	,  := decryptAndCheck(, , )
	if  != nil {
		return nil, 
	}
	 := make([]byte, .Size())
	return .FillBytes(), nil
}

const (
	// PSSSaltLengthAuto causes the salt in a PSS signature to be as large
	// as possible when signing, and to be auto-detected when verifying.
	PSSSaltLengthAuto = 0
	// PSSSaltLengthEqualsHash causes the salt length to equal the length
	// of the hash used in the signature.
	PSSSaltLengthEqualsHash = -1
)

// PSSOptions contains options for creating and verifying PSS signatures.
type PSSOptions struct {
	// SaltLength controls the length of the salt used in the PSS
	// signature. It can either be a number of bytes, or one of the special
	// PSSSaltLength constants.
	SaltLength int

	// Hash is the hash function used to generate the message digest. If not
	// zero, it overrides the hash function passed to SignPSS. It's required
	// when using PrivateKey.Sign.
	Hash crypto.Hash
}

// HashFunc returns opts.Hash so that PSSOptions implements crypto.SignerOpts.
func ( *PSSOptions) () crypto.Hash {
	return .Hash
}

func ( *PSSOptions) () int {
	if  == nil {
		return PSSSaltLengthAuto
	}
	return .SaltLength
}

// SignPSS calculates the signature of digest using PSS.
//
// digest must be the result of hashing the input message using the given hash
// function. The opts argument may be nil, in which case sensible defaults are
// used. If opts.Hash is set, it overrides hash.
func ( io.Reader,  *PrivateKey,  crypto.Hash,  []byte,  *PSSOptions) ([]byte, error) {
	if  != nil && .Hash != 0 {
		 = .Hash
	}

	 := .saltLength()
	switch  {
	case PSSSaltLengthAuto:
		 = (.N.BitLen()-1+7)/8 - 2 - .Size()
	case PSSSaltLengthEqualsHash:
		 = .Size()
	}

	 := make([]byte, )
	if ,  := io.ReadFull(, );  != nil {
		return nil, 
	}
	return signPSSWithSalt(, , , , )
}

// VerifyPSS verifies a PSS signature.
//
// A valid signature is indicated by returning a nil error. digest must be the
// result of hashing the input message using the given hash function. The opts
// argument may be nil, in which case sensible defaults are used. opts.Hash is
// ignored.
func ( *PublicKey,  crypto.Hash,  []byte,  []byte,  *PSSOptions) error {
	if len() != .Size() {
		return ErrVerification
	}
	 := new(big.Int).SetBytes()
	 := encrypt(new(big.Int), , )
	 := .N.BitLen() - 1
	 := ( + 7) / 8
	if .BitLen() > *8 {
		return ErrVerification
	}
	 := .FillBytes(make([]byte, ))
	return emsaPSSVerify(, , , .saltLength(), .New())
}