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