304 lines
8.4 KiB
Go
304 lines
8.4 KiB
Go
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// Copyright 2013 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package rsa
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// This file implements the RSASSA-PSS signature scheme according to RFC 8017.
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import (
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"bytes"
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"crypto"
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"errors"
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"hash"
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"io"
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"math/big"
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)
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// Per RFC 8017, Section 9.1
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//
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// EM = MGF1 xor DB || H( 8*0x00 || mHash || salt ) || 0xbc
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//
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// where
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//
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// DB = PS || 0x01 || salt
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//
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// and PS can be empty so
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//
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// emLen = dbLen + hLen + 1 = psLen + sLen + hLen + 2
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//
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func emsaPSSEncode(mHash []byte, emBits int, salt []byte, hash hash.Hash) ([]byte, error) {
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// See RFC 8017, Section 9.1.1.
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hLen := hash.Size()
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sLen := len(salt)
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emLen := (emBits + 7) / 8
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// 1. If the length of M is greater than the input limitation for the
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// hash function (2^61 - 1 octets for SHA-1), output "message too
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// long" and stop.
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//
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// 2. Let mHash = Hash(M), an octet string of length hLen.
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if len(mHash) != hLen {
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return nil, errors.New("crypto/rsa: input must be hashed with given hash")
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}
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// 3. If emLen < hLen + sLen + 2, output "encoding error" and stop.
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if emLen < hLen+sLen+2 {
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return nil, errors.New("crypto/rsa: key size too small for PSS signature")
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}
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em := make([]byte, emLen)
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psLen := emLen - sLen - hLen - 2
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db := em[:psLen+1+sLen]
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h := em[psLen+1+sLen : emLen-1]
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// 4. Generate a random octet string salt of length sLen; if sLen = 0,
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// then salt is the empty string.
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//
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// 5. Let
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// M' = (0x)00 00 00 00 00 00 00 00 || mHash || salt;
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//
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// M' is an octet string of length 8 + hLen + sLen with eight
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// initial zero octets.
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//
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// 6. Let H = Hash(M'), an octet string of length hLen.
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var prefix [8]byte
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hash.Write(prefix[:])
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hash.Write(mHash)
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hash.Write(salt)
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h = hash.Sum(h[:0])
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hash.Reset()
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// 7. Generate an octet string PS consisting of emLen - sLen - hLen - 2
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// zero octets. The length of PS may be 0.
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//
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// 8. Let DB = PS || 0x01 || salt; DB is an octet string of length
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// emLen - hLen - 1.
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db[psLen] = 0x01
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copy(db[psLen+1:], salt)
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// 9. Let dbMask = MGF(H, emLen - hLen - 1).
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//
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// 10. Let maskedDB = DB \xor dbMask.
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mgf1XOR(db, hash, h)
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// 11. Set the leftmost 8 * emLen - emBits bits of the leftmost octet in
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// maskedDB to zero.
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db[0] &= 0xff >> (8*emLen - emBits)
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// 12. Let EM = maskedDB || H || 0xbc.
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em[emLen-1] = 0xbc
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// 13. Output EM.
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return em, nil
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}
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func emsaPSSVerify(mHash, em []byte, emBits, sLen int, hash hash.Hash) error {
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// See RFC 8017, Section 9.1.2.
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hLen := hash.Size()
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if sLen == PSSSaltLengthEqualsHash {
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sLen = hLen
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}
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emLen := (emBits + 7) / 8
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if emLen != len(em) {
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return errors.New("rsa: internal error: inconsistent length")
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}
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// 1. If the length of M is greater than the input limitation for the
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// hash function (2^61 - 1 octets for SHA-1), output "inconsistent"
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// and stop.
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//
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// 2. Let mHash = Hash(M), an octet string of length hLen.
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if hLen != len(mHash) {
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return ErrVerification
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}
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// 3. If emLen < hLen + sLen + 2, output "inconsistent" and stop.
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if emLen < hLen+sLen+2 {
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return ErrVerification
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}
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// 4. If the rightmost octet of EM does not have hexadecimal value
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// 0xbc, output "inconsistent" and stop.
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if em[emLen-1] != 0xbc {
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return ErrVerification
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}
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// 5. Let maskedDB be the leftmost emLen - hLen - 1 octets of EM, and
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// let H be the next hLen octets.
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db := em[:emLen-hLen-1]
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h := em[emLen-hLen-1 : emLen-1]
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// 6. If the leftmost 8 * emLen - emBits bits of the leftmost octet in
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// maskedDB are not all equal to zero, output "inconsistent" and
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// stop.
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var bitMask byte = 0xff >> (8*emLen - emBits)
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if em[0] & ^bitMask != 0 {
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return ErrVerification
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}
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// 7. Let dbMask = MGF(H, emLen - hLen - 1).
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//
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// 8. Let DB = maskedDB \xor dbMask.
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mgf1XOR(db, hash, h)
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// 9. Set the leftmost 8 * emLen - emBits bits of the leftmost octet in DB
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// to zero.
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db[0] &= bitMask
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// If we don't know the salt length, look for the 0x01 delimiter.
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if sLen == PSSSaltLengthAuto {
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psLen := bytes.IndexByte(db, 0x01)
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if psLen < 0 {
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return ErrVerification
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}
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sLen = len(db) - psLen - 1
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}
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// 10. If the emLen - hLen - sLen - 2 leftmost octets of DB are not zero
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// or if the octet at position emLen - hLen - sLen - 1 (the leftmost
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// position is "position 1") does not have hexadecimal value 0x01,
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// output "inconsistent" and stop.
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psLen := emLen - hLen - sLen - 2
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for _, e := range db[:psLen] {
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if e != 0x00 {
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return ErrVerification
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}
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}
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if db[psLen] != 0x01 {
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return ErrVerification
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}
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// 11. Let salt be the last sLen octets of DB.
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salt := db[len(db)-sLen:]
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// 12. Let
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// M' = (0x)00 00 00 00 00 00 00 00 || mHash || salt ;
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// M' is an octet string of length 8 + hLen + sLen with eight
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// initial zero octets.
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//
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// 13. Let H' = Hash(M'), an octet string of length hLen.
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var prefix [8]byte
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hash.Write(prefix[:])
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hash.Write(mHash)
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hash.Write(salt)
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h0 := hash.Sum(nil)
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// 14. If H = H', output "consistent." Otherwise, output "inconsistent."
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if !bytes.Equal(h0, h) { // TODO: constant time?
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return ErrVerification
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}
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return nil
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}
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// signPSSWithSalt calculates the signature of hashed using PSS with specified salt.
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// Note that hashed must be the result of hashing the input message using the
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// given hash function. salt is a random sequence of bytes whose length will be
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// later used to verify the signature.
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func signPSSWithSalt(rand io.Reader, priv *PrivateKey, hash crypto.Hash, hashed, salt []byte) ([]byte, error) {
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emBits := priv.N.BitLen() - 1
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em, err := emsaPSSEncode(hashed, emBits, salt, hash.New())
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if err != nil {
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return nil, err
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}
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m := new(big.Int).SetBytes(em)
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c, err := decryptAndCheck(rand, priv, m)
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if err != nil {
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return nil, err
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}
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s := make([]byte, priv.Size())
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return c.FillBytes(s), nil
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}
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const (
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// PSSSaltLengthAuto causes the salt in a PSS signature to be as large
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// as possible when signing, and to be auto-detected when verifying.
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PSSSaltLengthAuto = 0
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// PSSSaltLengthEqualsHash causes the salt length to equal the length
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// of the hash used in the signature.
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PSSSaltLengthEqualsHash = -1
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)
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// PSSOptions contains options for creating and verifying PSS signatures.
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type PSSOptions struct {
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// SaltLength controls the length of the salt used in the PSS
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// signature. It can either be a number of bytes, or one of the special
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// PSSSaltLength constants.
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SaltLength int
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// Hash is the hash function used to generate the message digest. If not
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// zero, it overrides the hash function passed to SignPSS. It's required
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// when using PrivateKey.Sign.
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Hash crypto.Hash
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}
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// HashFunc returns opts.Hash so that PSSOptions implements crypto.SignerOpts.
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func (opts *PSSOptions) HashFunc() crypto.Hash {
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return opts.Hash
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}
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func (opts *PSSOptions) saltLength() int {
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if opts == nil {
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return PSSSaltLengthAuto
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}
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return opts.SaltLength
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}
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// SignPSS calculates the signature of digest using PSS.
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//
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// digest must be the result of hashing the input message using the given hash
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// function. The opts argument may be nil, in which case sensible defaults are
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// used. If opts.Hash is set, it overrides hash.
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func SignPSS(rand io.Reader, priv *PrivateKey, hash crypto.Hash, digest []byte, opts *PSSOptions) ([]byte, error) {
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if opts != nil && opts.Hash != 0 {
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hash = opts.Hash
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}
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saltLength := opts.saltLength()
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switch saltLength {
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case PSSSaltLengthAuto:
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saltLength = (priv.N.BitLen()-1+7)/8 - 2 - hash.Size()
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case PSSSaltLengthEqualsHash:
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saltLength = hash.Size()
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}
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salt := make([]byte, saltLength)
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if _, err := io.ReadFull(rand, salt); err != nil {
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return nil, err
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}
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return signPSSWithSalt(rand, priv, hash, digest, salt)
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}
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// VerifyPSS verifies a PSS signature.
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//
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// A valid signature is indicated by returning a nil error. digest must be the
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// result of hashing the input message using the given hash function. The opts
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// argument may be nil, in which case sensible defaults are used. opts.Hash is
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// ignored.
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func VerifyPSS(pub *PublicKey, hash crypto.Hash, digest []byte, sig []byte, opts *PSSOptions) error {
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if len(sig) != pub.Size() {
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return ErrVerification
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}
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s := new(big.Int).SetBytes(sig)
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m := encrypt(new(big.Int), pub, s)
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emBits := pub.N.BitLen() - 1
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emLen := (emBits + 7) / 8
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if m.BitLen() > emLen*8 {
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return ErrVerification
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}
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em := m.FillBytes(make([]byte, emLen))
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return emsaPSSVerify(digest, em, emBits, opts.saltLength(), hash.New())
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}
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