mirror of
https://github.com/emmansun/gmsm.git
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347 lines
11 KiB
Go
347 lines
11 KiB
Go
package sm4
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import (
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"crypto/cipher"
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"crypto/subtle"
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"encoding/binary"
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"errors"
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)
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// Assert that sm4CipherAsm implements the gcmAble interface.
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var _ gcmAble = (*sm4CipherAsm)(nil)
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// NewGCM returns the AES cipher wrapped in Galois Counter Mode. This is only
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// called by crypto/cipher.NewGCM via the gcmAble interface.
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func (c *sm4CipherAsm) NewGCM(nonceSize, tagSize int) (cipher.AEAD, error) {
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var key [gcmBlockSize]byte
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c.Encrypt(key[:], key[:])
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g := &gcm{cipher: c, nonceSize: nonceSize, tagSize: tagSize}
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// We precompute 16 multiples of |key|. However, when we do lookups
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// into this table we'll be using bits from a field element and
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// therefore the bits will be in the reverse order. So normally one
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// would expect, say, 4*key to be in index 4 of the table but due to
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// this bit ordering it will actually be in index 0010 (base 2) = 2.
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x := gcmFieldElement{
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binary.BigEndian.Uint64(key[:8]),
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binary.BigEndian.Uint64(key[8:]),
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}
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g.productTable[reverseBits(1)] = x
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for i := 2; i < 16; i += 2 {
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g.productTable[reverseBits(i)] = gcmDouble(&g.productTable[reverseBits(i/2)])
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g.productTable[reverseBits(i+1)] = gcmAdd(&g.productTable[reverseBits(i)], &x)
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}
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return g, nil
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}
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// gcmFieldElement represents a value in GF(2¹²⁸). In order to reflect the GCM
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// standard and make binary.BigEndian suitable for marshaling these values, the
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// bits are stored in big endian order. For example:
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// the coefficient of x⁰ can be obtained by v.low >> 63.
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// the coefficient of x⁶³ can be obtained by v.low & 1.
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// the coefficient of x⁶⁴ can be obtained by v.high >> 63.
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// the coefficient of x¹²⁷ can be obtained by v.high & 1.
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type gcmFieldElement struct {
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low, high uint64
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}
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// gcm represents a Galois Counter Mode with a specific key. See
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// https://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/gcm/gcm-revised-spec.pdf
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type gcm struct {
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cipher *sm4CipherAsm
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nonceSize int
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tagSize int
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// productTable contains the first sixteen powers of the key, H.
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// However, they are in bit reversed order. See NewGCMWithNonceSize.
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productTable [16]gcmFieldElement
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}
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const (
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gcmBlockSize = 16
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gcmTagSize = 16
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gcmMinimumTagSize = 12 // NIST SP 800-38D recommends tags with 12 or more bytes.
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gcmStandardNonceSize = 12
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)
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func (g *gcm) NonceSize() int {
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return g.nonceSize
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}
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func (g *gcm) Overhead() int {
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return g.tagSize
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}
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func (g *gcm) Seal(dst, nonce, plaintext, data []byte) []byte {
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if len(nonce) != g.nonceSize {
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panic("crypto/cipher: incorrect nonce length given to GCM")
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}
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if uint64(len(plaintext)) > ((1<<32)-2)*uint64(g.cipher.BlockSize()) {
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panic("crypto/cipher: message too large for GCM")
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}
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ret, out := sliceForAppend(dst, len(plaintext)+g.tagSize)
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if InexactOverlap(out, plaintext) {
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panic("crypto/cipher: invalid buffer overlap")
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}
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var counter, tagMask [gcmBlockSize]byte
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g.deriveCounter(&counter, nonce)
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g.cipher.Encrypt(tagMask[:], counter[:])
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gcmInc32(&counter)
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g.counterCrypt(out, plaintext, &counter)
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var tag [gcmTagSize]byte
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g.auth(tag[:], out[:len(plaintext)], data, &tagMask)
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copy(out[len(plaintext):], tag[:])
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return ret
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}
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var errOpen = errors.New("cipher: message authentication failed")
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func (g *gcm) Open(dst, nonce, ciphertext, data []byte) ([]byte, error) {
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if len(nonce) != g.nonceSize {
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panic("crypto/cipher: incorrect nonce length given to GCM")
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}
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// Sanity check to prevent the authentication from always succeeding if an implementation
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// leaves tagSize uninitialized, for example.
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if g.tagSize < gcmMinimumTagSize {
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panic("crypto/cipher: incorrect GCM tag size")
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}
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if len(ciphertext) < g.tagSize {
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return nil, errOpen
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}
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if uint64(len(ciphertext)) > ((1<<32)-2)*uint64(g.cipher.BlockSize())+uint64(g.tagSize) {
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return nil, errOpen
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}
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tag := ciphertext[len(ciphertext)-g.tagSize:]
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ciphertext = ciphertext[:len(ciphertext)-g.tagSize]
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var counter, tagMask [gcmBlockSize]byte
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g.deriveCounter(&counter, nonce)
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g.cipher.Encrypt(tagMask[:], counter[:])
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gcmInc32(&counter)
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var expectedTag [gcmTagSize]byte
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g.auth(expectedTag[:], ciphertext, data, &tagMask)
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ret, out := sliceForAppend(dst, len(ciphertext))
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if InexactOverlap(out, ciphertext) {
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panic("crypto/cipher: invalid buffer overlap")
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}
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if subtle.ConstantTimeCompare(expectedTag[:g.tagSize], tag) != 1 {
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// The AESNI code decrypts and authenticates concurrently, and
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// so overwrites dst in the event of a tag mismatch. That
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// behavior is mimicked here in order to be consistent across
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// platforms.
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for i := range out {
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out[i] = 0
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}
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return nil, errOpen
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}
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g.counterCrypt(out, ciphertext, &counter)
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return ret, nil
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}
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// reverseBits reverses the order of the bits of 4-bit number in i.
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func reverseBits(i int) int {
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i = ((i << 2) & 0xc) | ((i >> 2) & 0x3)
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i = ((i << 1) & 0xa) | ((i >> 1) & 0x5)
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return i
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}
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// gcmAdd adds two elements of GF(2¹²⁸) and returns the sum.
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func gcmAdd(x, y *gcmFieldElement) gcmFieldElement {
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// Addition in a characteristic 2 field is just XOR.
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return gcmFieldElement{x.low ^ y.low, x.high ^ y.high}
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}
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// gcmDouble returns the result of doubling an element of GF(2¹²⁸).
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func gcmDouble(x *gcmFieldElement) (double gcmFieldElement) {
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msbSet := x.high&1 == 1
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// Because of the bit-ordering, doubling is actually a right shift.
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double.high = x.high >> 1
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double.high |= x.low << 63
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double.low = x.low >> 1
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// If the most-significant bit was set before shifting then it,
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// conceptually, becomes a term of x^128. This is greater than the
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// irreducible polynomial so the result has to be reduced. The
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// irreducible polynomial is 1+x+x^2+x^7+x^128. We can subtract that to
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// eliminate the term at x^128 which also means subtracting the other
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// four terms. In characteristic 2 fields, subtraction == addition ==
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// XOR.
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if msbSet {
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double.low ^= 0xe100000000000000
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}
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return
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}
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var gcmReductionTable = []uint16{
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0x0000, 0x1c20, 0x3840, 0x2460, 0x7080, 0x6ca0, 0x48c0, 0x54e0,
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0xe100, 0xfd20, 0xd940, 0xc560, 0x9180, 0x8da0, 0xa9c0, 0xb5e0,
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}
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// mul sets y to y*H, where H is the GCM key, fixed during NewGCMWithNonceSize.
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func (g *gcm) mul(y *gcmFieldElement) {
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var z gcmFieldElement
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for i := 0; i < 2; i++ {
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word := y.high
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if i == 1 {
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word = y.low
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}
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// Multiplication works by multiplying z by 16 and adding in
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// one of the precomputed multiples of H.
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for j := 0; j < 64; j += 4 {
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msw := z.high & 0xf
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z.high >>= 4
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z.high |= z.low << 60
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z.low >>= 4
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z.low ^= uint64(gcmReductionTable[msw]) << 48
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// the values in |table| are ordered for
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// little-endian bit positions. See the comment
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// in NewGCMWithNonceSize.
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t := &g.productTable[word&0xf]
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z.low ^= t.low
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z.high ^= t.high
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word >>= 4
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}
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}
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*y = z
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}
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// updateBlocks extends y with more polynomial terms from blocks, based on
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// Horner's rule. There must be a multiple of gcmBlockSize bytes in blocks.
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func (g *gcm) updateBlocks(y *gcmFieldElement, blocks []byte) {
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for len(blocks) > 0 {
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y.low ^= binary.BigEndian.Uint64(blocks)
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y.high ^= binary.BigEndian.Uint64(blocks[8:])
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g.mul(y)
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blocks = blocks[gcmBlockSize:]
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}
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}
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// update extends y with more polynomial terms from data. If data is not a
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// multiple of gcmBlockSize bytes long then the remainder is zero padded.
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func (g *gcm) update(y *gcmFieldElement, data []byte) {
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fullBlocks := (len(data) >> 4) << 4
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g.updateBlocks(y, data[:fullBlocks])
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if len(data) != fullBlocks {
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var partialBlock [gcmBlockSize]byte
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copy(partialBlock[:], data[fullBlocks:])
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g.updateBlocks(y, partialBlock[:])
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}
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}
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// gcmInc32 treats the final four bytes of counterBlock as a big-endian value
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// and increments it.
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func gcmInc32(counterBlock *[16]byte) {
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ctr := counterBlock[len(counterBlock)-4:]
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binary.BigEndian.PutUint32(ctr, binary.BigEndian.Uint32(ctr)+1)
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}
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// sliceForAppend takes a slice and a requested number of bytes. It returns a
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// slice with the contents of the given slice followed by that many bytes and a
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// second slice that aliases into it and contains only the extra bytes. If the
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// original slice has sufficient capacity then no allocation is performed.
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func sliceForAppend(in []byte, n int) (head, tail []byte) {
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if total := len(in) + n; cap(in) >= total {
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head = in[:total]
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} else {
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head = make([]byte, total)
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copy(head, in)
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}
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tail = head[len(in):]
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return
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}
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// counterCrypt crypts in to out using g.cipher in counter mode.
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func (g *gcm) counterCrypt(out, in []byte, counter *[gcmBlockSize]byte) {
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var mask [FourBlocksSize]byte
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var couters [FourBlocksSize]byte
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for len(in) >= FourBlocksSize {
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copy(couters[:], counter[:])
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gcmInc32(counter)
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copy(couters[gcmBlockSize:], counter[:])
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gcmInc32(counter)
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copy(couters[2*gcmBlockSize:], counter[:])
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gcmInc32(counter)
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copy(couters[3*gcmBlockSize:], counter[:])
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encryptBlocksAsm(&g.cipher.enc[0], &mask[0], &couters[0])
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gcmInc32(counter)
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xorWords(out, in, mask[:])
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out = out[FourBlocksSize:]
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in = in[FourBlocksSize:]
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}
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if len(in) > 0 {
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blocks := (len(in) + gcmBlockSize - 1) / gcmBlockSize
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for i := 0; i < blocks; i++ {
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copy(couters[i*gcmBlockSize:], counter[:])
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gcmInc32(counter)
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}
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encryptBlocksAsm(&g.cipher.enc[0], &mask[0], &couters[0])
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xorBytes(out, in, mask[:blocks*gcmBlockSize])
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}
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}
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// deriveCounter computes the initial GCM counter state from the given nonce.
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// See NIST SP 800-38D, section 7.1. This assumes that counter is filled with
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// zeros on entry.
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func (g *gcm) deriveCounter(counter *[gcmBlockSize]byte, nonce []byte) {
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// GCM has two modes of operation with respect to the initial counter
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// state: a "fast path" for 96-bit (12-byte) nonces, and a "slow path"
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// for nonces of other lengths. For a 96-bit nonce, the nonce, along
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// with a four-byte big-endian counter starting at one, is used
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// directly as the starting counter. For other nonce sizes, the counter
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// is computed by passing it through the GHASH function.
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if len(nonce) == gcmStandardNonceSize {
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copy(counter[:], nonce)
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counter[gcmBlockSize-1] = 1
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} else {
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var y gcmFieldElement
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g.update(&y, nonce)
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y.high ^= uint64(len(nonce)) * 8
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g.mul(&y)
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binary.BigEndian.PutUint64(counter[:8], y.low)
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binary.BigEndian.PutUint64(counter[8:], y.high)
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}
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}
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// auth calculates GHASH(ciphertext, additionalData), masks the result with
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// tagMask and writes the result to out.
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func (g *gcm) auth(out, ciphertext, additionalData []byte, tagMask *[gcmTagSize]byte) {
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var y gcmFieldElement
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g.update(&y, additionalData)
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g.update(&y, ciphertext)
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y.low ^= uint64(len(additionalData)) * 8
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y.high ^= uint64(len(ciphertext)) * 8
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g.mul(&y)
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binary.BigEndian.PutUint64(out, y.low)
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binary.BigEndian.PutUint64(out[8:], y.high)
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xorWords(out, out, tagMask[:])
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}
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