add some KEK and CEK functions

cek.go

@@ -0,0 +1,94 @@
+package main
+
+import (
+	"crypto/cipher"
+	"crypto/des"
+	"errors"
+	"fmt"
+)
+
+// decryptCEK decrypts the encrypted CEK and unwraps the CEK so only the
+// original CEK is returned
+func decryptCEK(encryptedCEK, encryptedCekSalt, KEK []byte) (CEK []byte, err error) {
+	// ensure proper var lens, or error
+	encryptedCEKLen := 24
+	CEKSaltLen := 8
+	KEKLen := 24
+
+	if len(encryptedCEK) != encryptedCEKLen {
+		return nil, errors.New("wrong encrypted CEK length")
+	}
+	if len(encryptedCekSalt) != CEKSaltLen {
+		return nil, errors.New("wrong encrypted CEK's salt length")
+	}
+	if len(KEK) != KEKLen {
+		return nil, errors.New("wrong KEK length")
+	}
+
+	// 3DES uses block byte size of 8
+	blockByteSize := 8
+
+	// make DES cipher from KEK
+	kekDesCipher, err := des.NewTripleDESCipher(KEK)
+	if err != nil {
+		return nil, fmt.Errorf("failed to make DES cipher for cek decryption (%s)", err)
+	}
+
+	// (1) first use n-1'th block as IV to decrypt n'th block
+	ivStart := encryptedCEKLen - 2*blockByteSize
+	ivEnd := encryptedCEKLen - 1*blockByteSize
+
+	ivBlockCipherText := encryptedCEK[ivStart:ivEnd]
+	nthBlockCipherText := encryptedCEK[encryptedCEKLen-1*blockByteSize:]
+
+	firstBlockDecrypter := cipher.NewCBCDecrypter(kekDesCipher, ivBlockCipherText)
+
+	decryptedNthBlock := make([]byte, len(nthBlockCipherText))
+	firstBlockDecrypter.CryptBlocks(decryptedNthBlock, nthBlockCipherText)
+
+	// (2) decrypt remainder of outer encryption blocks (1 ... n-1'th) using
+	// the decrypted nthBlock as the IV
+	outerRemainderDecrypter := cipher.NewCBCDecrypter(kekDesCipher, decryptedNthBlock)
+
+	decryptedOuterRemainder := make([]byte, encryptedCEKLen-1*blockByteSize)
+	outerRemainderDecrypter.CryptBlocks(decryptedOuterRemainder, encryptedCEK[:encryptedCEKLen-1*blockByteSize])
+
+	// combine decrypted remainder with decrypted nth block for complete decrypted bytes
+	// this is equivelant to having the outer encryption removed, AKA the CEK is encrypted
+	// once now instead of twice
+	onceEncryptedCEK := append(decryptedOuterRemainder, decryptedNthBlock...)
+
+	// (3) Decrypted the inner layer of encryption using the KEK (aka decrypt the remaining
+	// layer of encryption)
+
+	// inner decrypter uses original CEK salt
+	innerDecrypter := cipher.NewCBCDecrypter(kekDesCipher, encryptedCekSalt)
+
+	// once decrypted, the CEK is still formatted as:
+	// CEK byte count || check value || CEK || padding (if required)
+	formattedCEK := make([]byte, len(onceEncryptedCEK))
+	innerDecrypter.CryptBlocks(formattedCEK, onceEncryptedCEK)
+
+	// Now that CEK is decrypted, sanity check it
+
+	// first byte is CEK byte count
+	expectedCEKLen := formattedCEK[0]
+
+	// (1a) expected cek len must be 16 or 24 or 3DES (which is what APC uses)
+	if int(expectedCEKLen) != 16 && int(expectedCEKLen) != 24 {
+		return nil, errors.New("expected CEK len block size is %d but 3DES requires 16 or 24 (decrypting likely failed)")
+	}
+
+	// next 3 bytes are the check value
+	CEKCheckVal := formattedCEK[1:4]
+
+	// CEK itself is the next bytes until CEK is the expected length
+	CEK = formattedCEK[4 : expectedCEKLen+4]
+
+	// (1b) key check data validation
+	if !isBitwiseCompliment(CEKCheckVal, CEK[0:3]) {
+		return nil, errors.New("CEK check value did not match CEK")
+	}
+
+	return CEK, nil
+}

go.mod

@@ -3,3 +3,5 @@ module temp
 go 1.21
 
 require github.com/sigurn/crc16 v0.0.0-20211026045750-20ab5afb07e3
+
+require golang.org/x/crypto v0.18.0

go.sum

@@ -1,2 +1,4 @@
 github.com/sigurn/crc16 v0.0.0-20211026045750-20ab5afb07e3 h1:aQKxg3+2p+IFXXg97McgDGT5zcMrQoi0EICZs8Pgchs=
 github.com/sigurn/crc16 v0.0.0-20211026045750-20ab5afb07e3/go.mod h1:9/etS5gpQq9BJsJMWg1wpLbfuSnkm8dPF6FdW2JXVhA=
+golang.org/x/crypto v0.18.0 h1:PGVlW0xEltQnzFZ55hkuX5+KLyrMYhHld1YHO4AKcdc=
+golang.org/x/crypto v0.18.0/go.mod h1:R0j02AL6hcrfOiy9T4ZYp/rcWeMxM3L6QYxlOuEG1mg=

kek.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"crypto/sha256"
+
+	"golang.org/x/crypto/pbkdf2"
+)
+
+// makeKEK creates the APC KEK for a given Salt; APC uses a fixed
+// password, iteration count, and hash function
+func makeKEK(salt []byte) (KEK []byte) {
+	// password is known constant for APC files
+	password := "user"
+
+	// fixed values for APC files
+	iterations := 5000
+	hash := sha256.New
+
+	// size of 3DES key (k1 + k2 + k3)
+	size := 24
+
+	// kek
+	return pbkdf2.Key([]byte(password), salt, iterations, size, hash)
+}