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| 1 | +package s3 |
| 2 | + |
| 3 | +import ( |
| 4 | + "crypto/md5" |
| 5 | + "crypto/sha256" |
| 6 | + "encoding/hex" |
| 7 | + "fmt" |
| 8 | + "io" |
| 9 | + "path/filepath" |
| 10 | + |
| 11 | + "github.com/minio/sio" |
| 12 | + "github.com/swisscom/backman/log" |
| 13 | + "golang.org/x/crypto/hkdf" |
| 14 | + "golang.org/x/crypto/scrypt" |
| 15 | +) |
| 16 | + |
| 17 | +// header is the header identifying the encryption and kdf used |
| 18 | +// The header looks like this with each one representing 1 byte |
| 19 | +// | Magic | Version | Encryption | KDF | |
| 20 | +type header [4]byte |
| 21 | + |
| 22 | +func (h header) Version() byte { return h[1] } |
| 23 | +func (h header) Encryption() byte { return h[2] } |
| 24 | +func (h header) KDF() byte { return h[3] } |
| 25 | + |
| 26 | +// Validate validates the headers content |
| 27 | +func (h header) Validate() error { |
| 28 | + if h[0] != magicByte { |
| 29 | + return fmt.Errorf("wrong magic bytes, expected %v, got %v", magicByte, h[0]) |
| 30 | + } |
| 31 | + switch h.Version() { |
| 32 | + case versionV10: |
| 33 | + break |
| 34 | + default: |
| 35 | + return fmt.Errorf("unexpected version: %v", h.Version()) |
| 36 | + } |
| 37 | + switch h.Encryption() { |
| 38 | + case sio.AES_256_GCM, sio.CHACHA20_POLY1305: |
| 39 | + break |
| 40 | + default: |
| 41 | + return fmt.Errorf("unexpected encryption: %v", h.Encryption()) |
| 42 | + } |
| 43 | + switch h.KDF() { |
| 44 | + case kdfScrypt: |
| 45 | + break |
| 46 | + default: |
| 47 | + return fmt.Errorf("unexpected KDF %v", h.KDF()) |
| 48 | + } |
| 49 | + return nil |
| 50 | +} |
| 51 | + |
| 52 | +// newHeader creates a new header for the given encryption and kdf |
| 53 | +func newHeader(encryption, kdf byte) header { |
| 54 | + return header{magicByte, versionV10, encryption, kdf} |
| 55 | +} |
| 56 | + |
| 57 | +const ( |
| 58 | + // needed to not collide with underlying sio header |
| 59 | + magicByte byte = 0xBA |
| 60 | +) |
| 61 | + |
| 62 | +const ( |
| 63 | + versionV10 = 0x10 // First KDF version with header |
| 64 | +) |
| 65 | + |
| 66 | +const ( |
| 67 | + kdfUnknown byte = iota |
| 68 | + kdfOldMD5 // needed for backwards compatibility |
| 69 | + kdfOldScryptHKDF // needed for backwards compatibility |
| 70 | + kdfScrypt = 0x10 // N=32768, r=8 and p=1. |
| 71 | +) |
| 72 | + |
| 73 | +// getKey returns a key derived from the given masterKey, object and header |
| 74 | +// when the kdf is unknown or one of the old methods, it needs to peek in the reader and thus reset it before returning |
| 75 | +func getKey(masterKey string, object string, hdr header, reader io.ReadSeeker) ([]byte, error) { |
| 76 | + switch hdr.KDF() { |
| 77 | + case kdfScrypt: |
| 78 | + return generateKeyScrypt(masterKey, object) |
| 79 | + case kdfUnknown, kdfOldMD5, kdfOldScryptHKDF: |
| 80 | + // this is only for backwards compatibility |
| 81 | + key := generateKeyPre123(masterKey) |
| 82 | + if err := tryOldDecryption(key, reader); err != nil { |
| 83 | + key = generateKey124(masterKey, object) |
| 84 | + if err := tryOldDecryption(key, reader); err != nil { |
| 85 | + return nil, fmt.Errorf("could not get key for headerless encryption: %v", err) |
| 86 | + } |
| 87 | + return key, nil |
| 88 | + } |
| 89 | + return key, nil |
| 90 | + } |
| 91 | + return nil, fmt.Errorf("no valid kdf: %v", hdr.KDF()) |
| 92 | +} |
| 93 | + |
| 94 | +// generateKey derives a key from the given masterKey, object and header |
| 95 | +func generateKey(masterKey string, object string, hdr header) ([]byte, error) { |
| 96 | + switch hdr.KDF() { |
| 97 | + case kdfScrypt: |
| 98 | + return generateKeyScrypt(masterKey, object) |
| 99 | + case kdfOldMD5: |
| 100 | + return generateKeyPre123(masterKey), nil |
| 101 | + case kdfOldScryptHKDF: |
| 102 | + return generateKey124(masterKey, object), nil |
| 103 | + } |
| 104 | + return nil, fmt.Errorf("no valid kdf: %v", hdr.KDF()) |
| 105 | +} |
| 106 | + |
| 107 | +// generateKeyScrypt derives the key from the given masterKey and object with the scrypt KDF |
| 108 | +func generateKeyScrypt(masterKey, object string) ([]byte, error) { |
| 109 | + nonce := filepath.Base(object) |
| 110 | + hasher := sha256.New() |
| 111 | + if n, err := hasher.Write([]byte(fmt.Sprintf("%s%s", masterKey, nonce))); err != nil || n <= 0 { |
| 112 | + return nil, fmt.Errorf("could not get salt: %v", err) |
| 113 | + } |
| 114 | + key, err := scrypt.Key([]byte(masterKey), hasher.Sum(nil), 32768, 8, 1, 32) |
| 115 | + if err != nil { |
| 116 | + return nil, fmt.Errorf("could not derive encryption key: %v", err) |
| 117 | + } |
| 118 | + return key, nil |
| 119 | +} |
| 120 | + |
| 121 | +// generateKeyPre123 derives the key via md5 hashing masterKey |
| 122 | +// This is not secure and mainly kept for being able to decrypt old backups |
| 123 | +func generateKeyPre123(masterKey string) []byte { |
| 124 | + hasher := md5.New() |
| 125 | + if n, err := hasher.Write([]byte(masterKey)); err != nil || n <= 0 { |
| 126 | + log.Fatalf("could not generate encryption key: %v", err) |
| 127 | + } |
| 128 | + return []byte(hex.EncodeToString(hasher.Sum(nil))) |
| 129 | +} |
| 130 | + |
| 131 | +// generateKey124 derives the key from the given masterKey and object via scrypt and hkdf and using the hash(mk,o) as salt |
| 132 | +// This is overly complicated without providing a real improvement in security |
| 133 | +// It is mainly kept for being able to decrypt old backups |
| 134 | +func generateKey124(masterKey, object string) []byte { |
| 135 | + nonce := filepath.Base(object) |
| 136 | + |
| 137 | + hasher := sha256.New() |
| 138 | + if n, err := hasher.Write([]byte(fmt.Sprintf("%s%s", masterKey, nonce))); err != nil || n <= 0 { |
| 139 | + log.Fatalf("could not get salt: %v", err) |
| 140 | + } |
| 141 | + salt := hex.EncodeToString(hasher.Sum(nil)) |
| 142 | + |
| 143 | + intKey, err := scrypt.Key([]byte(masterKey), []byte(salt), 32768, 8, 1, 32) |
| 144 | + if err != nil { |
| 145 | + log.Fatalf("could not get master key: %v", err) |
| 146 | + } |
| 147 | + |
| 148 | + // derive encryption key, using filename as nonce (filenames contain timestamps and are unique per backman deployment) |
| 149 | + var key [32]byte |
| 150 | + kdf := hkdf.New(sha256.New, intKey, []byte(nonce)[:], nil) |
| 151 | + if _, err := io.ReadFull(kdf, key[:]); err != nil { |
| 152 | + log.Fatalf("failed to derive encryption key: %v", err) |
| 153 | + } |
| 154 | + return key[:] |
| 155 | +} |
| 156 | + |
| 157 | +// tryOldDecryption peeks in the given reader and tries to decrypt with the given key |
| 158 | +// This is used to decrypt backups which don't have a header and therefore have no information about the used kdf/encryption |
| 159 | +func tryOldDecryption(key []byte, reader io.ReadSeeker) error { |
| 160 | + // reset reader to read from beginning |
| 161 | + if _, err := reader.Seek(0, 0); err != nil { |
| 162 | + return err |
| 163 | + } |
| 164 | + decrypter, err := sio.DecryptReader(reader, sio.Config{Key: key, CipherSuites: []byte{sio.AES_256_GCM}}) |
| 165 | + if err != nil { |
| 166 | + return err |
| 167 | + } |
| 168 | + peek := make([]byte, 8) |
| 169 | + if _, err := decrypter.Read(peek); err != nil { |
| 170 | + return err |
| 171 | + } |
| 172 | + // reset again |
| 173 | + if _, err := reader.Seek(0, 0); err != nil { |
| 174 | + return err |
| 175 | + } |
| 176 | + return nil |
| 177 | +} |
| 178 | + |
| 179 | +// readHeader reads and validates the header from the given reader |
| 180 | +func readHeader(reader io.Reader) (header, error) { |
| 181 | + hdr := header{} |
| 182 | + if _, err := reader.Read(hdr[:]); err != nil { |
| 183 | + return hdr, fmt.Errorf("could not read header: %v", err) |
| 184 | + } |
| 185 | + if err := hdr.Validate(); err != nil { |
| 186 | + // try old method |
| 187 | + hdr = newHeader(sio.AES_256_GCM, kdfUnknown) |
| 188 | + } |
| 189 | + return hdr, nil |
| 190 | +} |
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