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Add aesgcm decryption

--- a/lib/crypto/aesgcm.myr

+++ b/lib/crypto/aesgcm.myr

@@ -81,7 +81,23 @@

 }

 const aesgcmdecrypt = {c, buf, aad, tag

-	-> false

+	var tmp : byte[16]

+	var L, Y

+

+	ghash(c, aad, Y)

+	ghash(c, buf, Y)

+	L[0] = buf.len << 3

+	L[1] = buf.len >> 29

+	L[2] = aad.len << 3

+	L[3] = aad.len >> 29

+	ghash1(c, L, Y)

+	store128(Y, tmp[:])

+	aesctr(c, tmp[:])

+	if bufeq(tag, tmp[:])

+		-> false

+	;;

+	aesctr(c, buf)

+	-> true

 }

 const ghash = {c, buf, Y

--- a/lib/crypto/ct.myr

+++ b/lib/crypto/ct.myr

@@ -12,6 +12,7 @@

 	generic mux	: (x : @t, a : @t, b : @t ->@t)	:: integral,numeric @t

 	generic min	: (a : @t, b : @t -> @t)	:: integral,numeric @t

 	generic max	: (a : @t, b : @t -> @t)	:: integral,numeric @t

+	const bufeq	: (a : byte[:], b : byte[:] -> bool)

 ;;

 generic not = {a : @t :: integral,numeric @t

@@ -72,4 +73,15 @@

 	x = lt(a, b)

 	-> mux(x, b, a)

+}

+

+const bufeq = {a, b

+	var r, n

+

+	r = 1

+	n = min(a.len, b.len)

+	for var i = 0; i < n; i++

+		r = mux(r, eq(a[i], b[i]), r)

+	;;

+	-> (r : bool)

 }