forked from gitea/gitea
1
0
Fork 0
gitea/vendor/github.com/ProtonMail/go-crypto/eax/eax.go

163 lines
4.6 KiB
Go
Raw Blame History

This file contains ambiguous Unicode characters

This file contains Unicode characters that might be confused with other characters. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.

// Copyright (C) 2019 ProtonTech AG
// Package eax provides an implementation of the EAX
// (encrypt-authenticate-translate) mode of operation, as described in
// Bellare, Rogaway, and Wagner "THE EAX MODE OF OPERATION: A TWO-PASS
// AUTHENTICATED-ENCRYPTION SCHEME OPTIMIZED FOR SIMPLICITY AND EFFICIENCY."
// In FSE'04, volume 3017 of LNCS, 2004
package eax
import (
"crypto/cipher"
"crypto/subtle"
"errors"
"github.com/ProtonMail/go-crypto/internal/byteutil"
)
const (
defaultTagSize = 16
defaultNonceSize = 16
)
type eax struct {
block cipher.Block // Only AES-{128, 192, 256} supported
tagSize int // At least 12 bytes recommended
nonceSize int
}
func (e *eax) NonceSize() int {
return e.nonceSize
}
func (e *eax) Overhead() int {
return e.tagSize
}
// NewEAX returns an EAX instance with AES-{KEYLENGTH} and default nonce and
// tag lengths. Supports {128, 192, 256}- bit key length.
func NewEAX(block cipher.Block) (cipher.AEAD, error) {
return NewEAXWithNonceAndTagSize(block, defaultNonceSize, defaultTagSize)
}
// NewEAXWithNonceAndTagSize returns an EAX instance with AES-{keyLength} and
// given nonce and tag lengths in bytes. Panics on zero nonceSize and
// exceedingly long tags.
//
// It is recommended to use at least 12 bytes as tag length (see, for instance,
// NIST SP 800-38D).
//
// Only to be used for compatibility with existing cryptosystems with
// non-standard parameters. For all other cases, prefer NewEAX.
func NewEAXWithNonceAndTagSize(
block cipher.Block, nonceSize, tagSize int) (cipher.AEAD, error) {
if nonceSize < 1 {
return nil, eaxError("Cannot initialize EAX with nonceSize = 0")
}
if tagSize > block.BlockSize() {
return nil, eaxError("Custom tag length exceeds blocksize")
}
return &eax{
block: block,
tagSize: tagSize,
nonceSize: nonceSize,
}, nil
}
func (e *eax) Seal(dst, nonce, plaintext, adata []byte) []byte {
if len(nonce) > e.nonceSize {
panic("crypto/eax: Nonce too long for this instance")
}
ret, out := byteutil.SliceForAppend(dst, len(plaintext) + e.tagSize)
omacNonce := e.omacT(0, nonce)
omacAdata := e.omacT(1, adata)
// Encrypt message using CTR mode and omacNonce as IV
ctr := cipher.NewCTR(e.block, omacNonce)
ciphertextData := out[:len(plaintext)]
ctr.XORKeyStream(ciphertextData, plaintext)
omacCiphertext := e.omacT(2, ciphertextData)
tag := out[len(plaintext):]
for i := 0; i < e.tagSize; i++ {
tag[i] = omacCiphertext[i] ^ omacNonce[i] ^ omacAdata[i]
}
return ret
}
func (e* eax) Open(dst, nonce, ciphertext, adata []byte) ([]byte, error) {
if len(nonce) > e.nonceSize {
panic("crypto/eax: Nonce too long for this instance")
}
if len(ciphertext) < e.tagSize {
return nil, eaxError("Ciphertext shorter than tag length")
}
sep := len(ciphertext) - e.tagSize
// Compute tag
omacNonce := e.omacT(0, nonce)
omacAdata := e.omacT(1, adata)
omacCiphertext := e.omacT(2, ciphertext[:sep])
tag := make([]byte, e.tagSize)
for i := 0; i < e.tagSize; i++ {
tag[i] = omacCiphertext[i] ^ omacNonce[i] ^ omacAdata[i]
}
// Compare tags
if subtle.ConstantTimeCompare(ciphertext[sep:], tag) != 1 {
return nil, eaxError("Tag authentication failed")
}
// Decrypt ciphertext
ret, out := byteutil.SliceForAppend(dst, len(ciphertext))
ctr := cipher.NewCTR(e.block, omacNonce)
ctr.XORKeyStream(out, ciphertext[:sep])
return ret[:sep], nil
}
// Tweakable OMAC - Calls OMAC_K([t]_n || plaintext)
func (e *eax) omacT(t byte, plaintext []byte) []byte {
blockSize := e.block.BlockSize()
byteT := make([]byte, blockSize)
byteT[blockSize-1] = t
concat := append(byteT, plaintext...)
return e.omac(concat)
}
func (e *eax) omac(plaintext []byte) []byte {
blockSize := e.block.BlockSize()
// L ← E_K(0^n); B ← 2L; P ← 4L
L := make([]byte, blockSize)
e.block.Encrypt(L, L)
B := byteutil.GfnDouble(L)
P := byteutil.GfnDouble(B)
// CBC with IV = 0
cbc := cipher.NewCBCEncrypter(e.block, make([]byte, blockSize))
padded := e.pad(plaintext, B, P)
cbcCiphertext := make([]byte, len(padded))
cbc.CryptBlocks(cbcCiphertext, padded)
return cbcCiphertext[len(cbcCiphertext)-blockSize:]
}
func (e *eax) pad(plaintext, B, P []byte) []byte {
// if |M| in {n, 2n, 3n, ...}
blockSize := e.block.BlockSize()
if len(plaintext) != 0 && len(plaintext)%blockSize == 0 {
return byteutil.RightXor(plaintext, B)
}
// else return (M || 1 || 0^(n1(|M| % n))) xor→ P
ending := make([]byte, blockSize-len(plaintext)%blockSize)
ending[0] = 0x80
padded := append(plaintext, ending...)
return byteutil.RightXor(padded, P)
}
func eaxError(err string) error {
return errors.New("crypto/eax: " + err)
}