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// Package cryptocore wraps OpenSSL and Go GCM crypto and provides
// a nonce generator.
package cryptocore
import (
"crypto/aes"
"crypto/cipher"
"crypto/sha512"
"fmt"
"log"
"github.com/rfjakob/eme"
"github.com/rfjakob/gocryptfs/internal/siv_aead"
"github.com/rfjakob/gocryptfs/internal/stupidgcm"
)
// AEADTypeEnum indicates the type of AEAD backend in use.
type AEADTypeEnum int
const (
// KeyLen is the cipher key length in bytes. 32 for AES-256.
KeyLen = 32
// AuthTagLen is the length of a GCM auth tag in bytes.
AuthTagLen = 16
_ = iota // Skip zero
// BackendOpenSSL specifies the OpenSSL backend.
BackendOpenSSL AEADTypeEnum = iota
// BackendGoGCM specifies the Go based GCM backend.
BackendGoGCM AEADTypeEnum = iota
// BackendAESSIV specifies an AESSIV backend.
BackendAESSIV AEADTypeEnum = iota
)
// CryptoCore is the low level crypto implementation.
type CryptoCore struct {
// EME is used for filename encryption.
EMECipher *eme.EMECipher
// GCM or AES-SIV. This is used for content encryption.
AEADCipher cipher.AEAD
// Which backend is behind AEADCipher?
AEADBackend AEADTypeEnum
// GCM needs unique IVs (nonces)
IVGenerator *nonceGenerator
IVLen int
}
// New returns a new CryptoCore object or panics.
//
// Even though the "GCMIV128" feature flag is now mandatory, we must still
// support 96-bit IVs here because they were used for encrypting the master
// key in gocryptfs.conf up to gocryptfs v1.2. v1.3 switched to 128 bits.
func New(key []byte, aeadType AEADTypeEnum, IVBitLen int, useHKDF bool, forceDecode bool) *CryptoCore {
if len(key) != KeyLen {
log.Panic(fmt.Sprintf("Unsupported key length %d", len(key)))
}
// We want the IV size in bytes
IVLen := IVBitLen / 8
// Initialize EME for filename encryption.
var emeCipher *eme.EMECipher
{
emeKey := key
if useHKDF {
info := "EME filename encryption"
emeKey = hkdfDerive(key, info, KeyLen)
}
emeBlockCipher, err := aes.NewCipher(emeKey)
if err != nil {
log.Panic(err)
}
emeCipher = eme.New(emeBlockCipher)
}
// Initilize an AEAD cipher for file content encryption.
var aeadCipher cipher.AEAD
if aeadType == BackendOpenSSL || aeadType == BackendGoGCM {
gcmKey := key
if useHKDF {
info := "AES-GCM file content encryption"
gcmKey = hkdfDerive(key, info, KeyLen)
}
switch aeadType {
case BackendOpenSSL:
if IVLen != 16 {
log.Panic("stupidgcm only supports 128-bit IVs")
}
aeadCipher = stupidgcm.New(gcmKey, forceDecode)
case BackendGoGCM:
goGcmBlockCipher, err := aes.NewCipher(gcmKey)
if err != nil {
log.Panic(err)
}
aeadCipher, err = cipher.NewGCMWithNonceSize(goGcmBlockCipher, IVLen)
if err != nil {
log.Panic(err)
}
}
} else if aeadType == BackendAESSIV {
if IVLen != 16 {
// SIV supports any nonce size, but we only use 16.
log.Panic("AES-SIV must use 16-byte nonces")
}
var key64 []byte
if useHKDF {
info := "AES-SIV file content encryption"
key64 = hkdfDerive(key, info, siv_aead.KeyLen)
} else {
// AES-SIV uses 1/2 of the key for authentication, 1/2 for
// encryption, so we need a 64-bytes key for AES-256. Derive it from
// the master key by hashing it with SHA-512.
s := sha512.Sum512(key)
key64 = s[:]
}
aeadCipher = siv_aead.New(key64)
} else {
log.Panic("unknown backend cipher")
}
return &CryptoCore{
EMECipher: emeCipher,
AEADCipher: aeadCipher,
AEADBackend: aeadType,
IVGenerator: &nonceGenerator{nonceLen: IVLen},
IVLen: IVLen,
}
}
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