// = SUM_{0<=i,j<n} u_i v_j t^{2n-2-(i+j)}
//
// which is almost the bit-reversal of u v, only it's shifted right
- // by one place. Oh, well: we'll have to shift it back later.
+ // by one place. Putting this another way, what we have is actually
+ // the bit reversal of the product u v t. We could get the correct
+ // answer (modulo p(t)) if we'd sneakily divided one of the operands
+ // by t before we started. Conveniently, v is actually the secret
+ // value k set up by the GCM `mktable' function, so we can arrange to
+ // actually store k/t (mod p(t)) and then the product will come out
+ // correct (modulo p(t)) and we won't have anything more to worry
+ // about here.
//
// That was important to think about, but there's not a great deal to
// do about it yet other than to convert what we've got from the
// q8 = // (x_3; x_2)
// q9 = // (x_1; x_0)
- // Two-and-a-half problems remain. The first is that this product is
- // shifted left by one place, which is annoying. Let's take care of
- // that now. Once this is done, we'll be properly in GCM's backwards
- // bit-ordering.
+ // One-and-a-half problems remain.
//
- // The half a problem is that the result wants to have its 64-bit
- // halves switched. Here turns out to be the best place to arrange
- // for that.
+ // The full-size problem is that the result needs to be reduced
+ // modulo p(t) = t^128 + t^7 + t^2 + t + 1. Let R = t^128 = t^7 +
+ // t^2 + t + 1 in our field. So far, we've calculated z_0 and z_1
+ // such that z_0 + z_1 R = u v using the identity R = t^128: now we
+ // must collapse the two halves of y together using the other
+ // identity R = t^7 + t^2 + t + 1.
//
- // q9 q8
- // ,-------------.-------------. ,-------------.-------------.
- // | 0 x_0-x_62 | x_63-x_126 | | x_127-x_190 | x_191-x_254 |
- // `-------------^-------------' `-------------^-------------'
- // d19 d18 d17 d16
- //
- // We start by shifting each 32-bit lane right (from GCM's point of
- // view -- physically, left) by one place, which gives us this:
- //
- // low (q9) high (q8)
- // ,-------------.-------------. ,-------------.-------------.
- // | x_0-x_62 0 |x_64-x_126 0 | |x_128-x_190 0|x_192-x_254 0|
- // `-------------^-------------' `-------------^-------------'
- // d19 d18 d17 d16
- //
- // but we've lost a bunch of bits. We separately shift each lane
- // left by 31 places to give us the bits we lost.
- //
- // low (q3) high (q2)
- // ,-------------.-------------. ,-------------.-------------.
- // | 0...0 | 0...0 x_63 | | 0...0 x_127 | 0...0 x_191 |
- // `-------------^-------------' `-------------^-------------'
- // d6 d5 d4
- //
- // Since we can address each of these pieces individually, putting
- // them together is relatively straightforward.
-
-
- vshr.u64 d6, d18, #63 // shifted left; just the carries
- vshl.u64 q9, q9, #1 // shifted right, but dropped carries
- vshr.u64 q2, q8, #63
- vshl.u64 q8, q8, #1
- vorr d0, d19, d6 // y_0
- vorr d1, d18, d5 // y_1
- vorr d2, d17, d4 // y_2
- vmov d3, d16 // y_3
-
- // And the other one is that the result needs to be reduced modulo
- // p(t) = t^128 + t^7 + t^2 + t + 1. Let R = t^128 = t^7 + t^2 + t +
- // 1 in our field. So far, we've calculated z_0 and z_1 such that
- // z_0 + z_1 R = u v using the identity R = t^128: now we must
- // collapse the two halves of y together using the other identity R =
- // t^7 + t^2 + t + 1.
- //
- // We do this by working on y_2 and y_3 separately, so consider y_i
- // for i = 2 or 3. Certainly, y_i t^{64i} = y_i R t^{64(i-2) =
- // (t^7 + t^2 + t + 1) y_i t^{64(i-2)}, but we can't use that
+ // We do this by working on x_2 and x_3 separately, so consider x_i
+ // for i = 2 or 3. Certainly, x_i t^{64i} = x_i R t^{64(i-2) =
+ // (t^7 + t^2 + t + 1) x_i t^{64(i-2)}, but we can't use that
// directly without breaking up the 64-bit word structure. Instead,
- // we start by considering just y_i t^7 t^{64(i-2)}, which again
- // looks tricky. Now, split y_i = a_i + t^57 b_i, with deg a_i < 57;
+ // we start by considering just x_i t^7 t^{64(i-2)}, which again
+ // looks tricky. Now, split x_i = a_i + t^57 b_i, with deg a_i < 57;
// then
//
- // y_i t^7 t^{64(i-2)} = a_i t^7 t^{64(i-2)} + b_i t^{64(i-1)}
+ // x_i t^7 t^{64(i-2)} = a_i t^7 t^{64(i-2)} + b_i t^{64(i-1)}
//
- // We can similarly decompose y_i t^2 and y_i t into a pair of 64-bit
+ // We can similarly decompose x_i t^2 and x_i t into a pair of 64-bit
// contributions to the t^{64(i-2)} and t^{64(i-1)} words, but the
// splits are different. This is lovely, with one small snag: when
- // we do this to y_3, we end up with a contribution back into the
+ // we do this to x_3, we end up with a contribution back into the
// t^128 coefficient word. But notice that only the low seven bits
// of this word are affected, so there's no knock-on contribution
// into the t^64 word. Therefore, if we handle the high bits of each
// word together, and then the low bits, everything will be fine.
// First, shift the high bits down.
- vshl.u64 q2, q1, #63 // the b_i for t
- vshl.u64 q3, q1, #62 // the b_i for t^2
- vshl.u64 q8, q1, #57 // the b_i for t^7
+ vshl.u64 q2, q8, #63 // the b_i for t
+ vshl.u64 q3, q8, #62 // the b_i for t^2
+ vshl.u64 q0, q8, #57 // the b_i for t^7
veor q2, q2, q3 // add them all together
- veor q2, q2, q8
- veor d2, d2, d5 // contribution into low half
- veor d1, d1, d4 // and high half
+ veor q2, q2, q0
+ veor d18, d18, d5 // contribution into low half
+ veor d17, d17, d4 // and high half
// And then shift the low bits up.
- vshr.u64 q2, q1, #1
- vshr.u64 q3, q1, #2
- vshr.u64 q8, q1, #7
- veor q0, q0, q1 // mix in the unit contribution
+ vshr.u64 q2, q8, #1
+ vshr.u64 q3, q8, #2
+ vshr.u64 q1, q8, #7
+ veor q8, q8, q9 // mix in the unit contribution
veor q2, q2, q3 // t and t^2 contribs
- veor q0, q0, q8 // low, unit, and t^7 contribs
- veor q0, q0, q2 // mix them together and we're done
+ veor q1, q1, q8 // low, unit, and t^7 contribs
+ veor d1, d2, d4 // mix them together and swap halves
+ veor d0, d3, d5
.endm
.macro mul64
// The multiplication is thankfully easy.
vmull.p64 q0, d0, d1 // u v
- // Shift the product up by one place, and swap the two halves. After
- // this, we're in GCM bizarro-world.
- vshr.u64 d2, d0, #63 // shifted left; just the carries
- vshl.u64 d3, d1, #1 // low half right
- vshl.u64 d1, d0, #1 // high half shifted right
- vorr d0, d3, d2 // mix in the carries
-
// Now we must reduce. This is essentially the same as the 128-bit
// case above, but mostly simpler because everything is smaller. The
// polynomial this time is p(t) = t^64 + t^4 + t^3 + t + 1.
// First, shift the high bits down.
- vshl.u64 d2, d1, #63 // b_i for t
- vshl.u64 d3, d1, #61 // b_i for t^3
- vshl.u64 d4, d1, #60 // b_i for t^4
+ vshl.u64 d2, d0, #63 // b_i for t
+ vshl.u64 d3, d0, #61 // b_i for t^3
+ vshl.u64 d4, d0, #60 // b_i for t^4
veor d2, d2, d3 // add them all together
veor d2, d2, d4
- veor d1, d1, d2 // contribution back into high half
+ veor d0, d0, d2 // contribution back into high half
// And then shift the low bits up.
- vshr.u64 d2, d1, #1
- vshr.u64 d3, d1, #3
- vshr.u64 d4, d1, #4
+ vshr.u64 d2, d0, #1
+ vshr.u64 d3, d0, #3
+ vshr.u64 d4, d0, #4
veor d0, d0, d1 // mix in the unit contribution
veor d2, d2, d3 // t and t^3 contribs
veor d0, d0, d4 // low, unit, and t^4
// word. Clobbers q1--q3, q8, q9.
// This is an inconvenient size. There's nothing for it but to do
- // four multiplications, as if for the 128-bit case. It's possible
- // that there's cruft in the top 32 bits of the input registers, so
- // shift both of them up by four bytes before we start. This will
- // mean that the high 64 bits of the result (from GCM's viewpoint)
- // will be zero.
+ // four multiplications, as if for the 128-bit case.
// q0 = // (u_0 + u_1 t^32; u_2)
// q1 = // (v_0 + v_1 t^32; v_2)
vmull.p64 q8, d1, d2 // u_2 (v_0 + v_1 t^32) = e_0
veor d0, d0, d17 // q0 = bot([d t^128] + e t^64 + f)
veor d3, d3, d16 // q1 = top(d t^128 + e t^64 + f)
- // Shift the product right by one place (from GCM's point of view),
- // but, unusually, don't swap the halves, because we need to work on
- // the 32-bit pieces later. After this, we're in GCM bizarro-world.
- // q0 = // (?, x_2; x_1, x_0)
- // q1 = // (0, x_5; x_4, x_3)
- vshr.u64 d4, d0, #63 // carry from d0 to d1
- vshr.u64 d5, d2, #63 // carry from d2 to d3
- vshr.u32 d6, d3, #31 // carry from d3 to d0
- vshl.u64 q0, q0, #1 // shift low half
- vshl.u64 q1, q1, #1 // shift high half
- vorr d1, d1, d4
- vorr d0, d0, d6
- vorr d3, d3, d5
-
// Finally, the reduction. This is essentially the same as the
// 128-bit case, except that the polynomial is p(t) = t^96 + t^10 +
// t^9 + t^6 + 1. The degrees are larger but not enough to cause
veor d19, d19, d24 // q9 = // (x_3; x_2)
veor d20, d20, d25 // q10 = // (x_1; x_0)
- // Shift the product right by one place (from GCM's point of view).
- vshr.u64 q11, q8, #63 // carry from d16/d17 to d17/d18
- vshr.u64 q12, q9, #63 // carry from d18/d19 to d19/d20
- vshr.u64 d26, d20, #63 // carry from d20 to d21
- vshl.u64 q8, q8, #1 // shift everything down
- vshl.u64 q9, q9, #1
- vshl.u64 q10, q10, #1
- vorr d17, d17, d22 // and mix in the carries
- vorr d18, d18, d23
- vorr d19, d19, d24
- vorr d20, d20, d25
- vorr d21, d21, d26
-
// Next, the reduction. Our polynomial this time is p(x) = t^192 +
// t^7 + t^2 + t + 1. Yes, the magic numbers are the same as the
// 128-bit case. I don't know why.
veor q9, q9, q12
veor q10, q10, q13
- // Shift the product right by one place (from GCM's point of view).
- vshr.u64 q0, q8, #63 // carry from d16/d17 to d17/d18
- vshr.u64 q1, q9, #63 // carry from d18/d19 to d19/d20
- vshr.u64 q2, q10, #63 // carry from d20/d21 to d21/d22
- vshr.u64 d6, d22, #63 // carry from d22 to d23
- vshl.u64 q8, q8, #1 // shift everyting down
- vshl.u64 q9, q9, #1
- vshl.u64 q10, q10, #1
- vshl.u64 q11, q11, #1
- vorr d17, d17, d0
- vorr d18, d18, d1
- vorr d19, d19, d2
- vorr d20, d20, d3
- vorr d21, d21, d4
- vorr d22, d22, d5
- vorr d23, d23, d6
-
// Now we must reduce. This is essentially the same as the 192-bit
// case above, but more complicated because everything is bigger.
// The polynomial this time is p(t) = t^256 + t^10 + t^5 + t^2 + 1.
~mdw / catacomb / blobdiff