// Given the supplied cipher, whose block size must be 16 bytes, return two
// subkeys that can be used in MAC generation. See section 5.3 of NIST SP
// 800-38B. Note that the other NIST-approved block size of 8 bytes is not
// supported by this function.
func generateSubkeys(ciph cipher.Block) (k1 []byte, k2 []byte) {
if ciph.BlockSize() != blockSize {
panic("generateSubkeys requires a cipher with a block size of 16 bytes.")
}
// Step 1
l := make([]byte, blockSize)
ciph.Encrypt(l, subkeyZero)
// Step 2: Derive the first subkey.
if common.Msb(l) == 0 {
// TODO(jacobsa): Accept a destination buffer in ShiftLeft and then hoist
// the allocation in the else branch below.
k1 = common.ShiftLeft(l)
} else {
k1 = make([]byte, blockSize)
common.Xor(k1, common.ShiftLeft(l), subkeyRb)
}
// Step 3: Derive the second subkey.
if common.Msb(k1) == 0 {
k2 = common.ShiftLeft(k1)
} else {
k2 = make([]byte, blockSize)
common.Xor(k2, common.ShiftLeft(k1), subkeyRb)
}
return
}
func (h *cmacHash) Sum(b []byte) []byte {
dataLen := len(h.data)
// We should have at most one block left.
if dataLen > blockSize {
panic(fmt.Sprintf("Unexpected data: %x", h.data))
}
// Calculate M_last.
mLast := make([]byte, blockSize)
if dataLen == blockSize {
common.Xor(mLast, h.data, h.k1)
} else {
// TODO(jacobsa): Accept a destination buffer in common.PadBlock and
// simplify this code.
common.Xor(mLast, common.PadBlock(h.data), h.k2)
}
y := make([]byte, blockSize)
common.Xor(y, mLast, h.x)
result := make([]byte, blockSize)
h.ciph.Encrypt(result, y)
b = append(b, result...)
return b
}
// Run the S2V "string to vector" function of RFC 5297 using the input key and
// string vector, which must be non-empty. (RFC 5297 defines S2V to handle the
// empty vector case, but it is never used that way by higher-level functions.)
//
// If provided, the supplied scatch space will be used to avoid an allocation.
// It should be (but is not required to be) as large as the last element of
// strings.
//
// The result is guaranteed to be of length s2vSize.
func s2v(key []byte, strings [][]byte, scratch []byte) []byte {
numStrings := len(strings)
if numStrings == 0 {
panic("strings vector must be non-empty.")
}
// Create a CMAC hash.
h, err := cmac.New(key)
if err != nil {
panic(fmt.Sprintf("cmac.New: %v", err))
}
// Initialize.
if _, err := h.Write(s2vZero); err != nil {
panic(fmt.Sprintf("h.Write: %v", err))
}
d := h.Sum([]byte{})
h.Reset()
// Handle all strings but the last.
for i := 0; i < numStrings-1; i++ {
if _, err := h.Write(strings[i]); err != nil {
panic(fmt.Sprintf("h.Write: %v", err))
}
common.Xor(d, dbl(d), h.Sum([]byte{}))
h.Reset()
}
// Handle the last string.
lastString := strings[numStrings-1]
var t []byte
if len(lastString) >= aes.BlockSize {
// Make an output buffer the length of lastString.
if cap(scratch) >= len(lastString) {
t = scratch[:len(lastString)]
} else {
t = make([]byte, len(lastString))
}
// XOR d on the end of lastString.
xorend(t, lastString, d)
} else {
t = make([]byte, aes.BlockSize)
common.Xor(t, dbl(d), common.PadBlock(lastString))
}
if _, err := h.Write(t); err != nil {
panic(fmt.Sprintf("h.Write: %v", err))
}
return h.Sum([]byte{})
}
// Given a 128-bit binary string, shift the string left by one bit and XOR the
// result with 0x00...87 if the bit shifted off was one. This is the dbl
// function of RFC 5297.
func dbl(b []byte) []byte {
if len(b) != aes.BlockSize {
panic("dbl requires a 16-byte buffer.")
}
shiftedOne := common.Msb(b) == 1
b = common.ShiftLeft(b)
if shiftedOne {
tmp := make([]byte, aes.BlockSize)
common.Xor(tmp, b, dblRb)
b = tmp
}
return b
}
// The xorend operator of RFC 5297.
//
// Given strings A and B with len(A) >= len(B), let D be len(A) - len(B). Write
// A[:D] followed by xor(A[D:], B) into dst. In other words, xor B over the
// rightmost end of A and write the result into dst.
func xorend(dst, a, b []byte) {
aLen := len(a)
bLen := len(b)
dstLen := len(dst)
if dstLen < aLen || aLen < bLen {
log.Panicf("Bad buffer lengths: %d, %d, %d", dstLen, aLen, bLen)
}
// Copy the left part.
difference := aLen - bLen
copy(dst, a[:difference])
// XOR in the right part.
common.Xor(dst[difference:difference+bLen], a[difference:], b)
}
func xorWithResult(a []byte, b []byte) (result []byte) {
result = make([]byte, len(a))
common.Xor(result, a, b)
return
}