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ELF: ARMv8 XOR decryption support + heuristic improvements

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This commit is contained in:
Katy Coe
2020-12-09 17:32:06 +01:00
parent 21fa006085
commit 15663fb0cf
1 changed files with 58 additions and 20 deletions
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78
Il2CppInspector.Common/FileFormatReaders/ElfReader.cs
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@@ -275,50 +275,88 @@ namespace Il2CppInspector
}
Console.WriteLine($"Processed {rels.Count} relocations");
// Detect and defeat XOR encryption
// Detect and defeat various kinds of XOR encryption
StatusUpdate("Detecting encryption");
if (getDynamic(Elf.DT_INIT) != null && sectionByName.ContainsKey(".rodata")) {
// Use the data section to determine IF the file is obfuscated
var rodataFirstBytes = ReadBytes(conv.Long(sectionByName[".rodata"].sh_offset), 256);
var xorKeyCandidate = rodataFirstBytes.GroupBy(b => b).OrderByDescending(f => f.Count()).First().Key;
// Use the data section to determine some possible keys
// If the data section uses striped encryption, bucketing the whole section will not give the correct key
var roDataBytes = ReadBytes(conv.Long(sectionByName[".rodata"].sh_offset), conv.Int(sectionByName[".rodata"].sh_size));
var xorKeyCandidateStriped = roDataBytes.Take(1024).GroupBy(b => b).OrderByDescending(f => f.Count()).First().Key;
var xorKeyCandidateFull = roDataBytes.GroupBy(b => b).OrderByDescending(f => f.Count()).First().Key;
// Select test nibbles and values for ARM instructions depending on architecture (ARMv7 / AArch64)
var testValues = new Dictionary<int, (int, int, int, int)> {
[32] = (8, 28, 0x0, 0xE),
[64] = (4, 28, 0xE, 0xF)
};
var (armNibbleB, armNibbleT, armValueB, armValueT) = testValues[Bits];
// We examine the bottom nibble of the 2nd byte and top nibble of 4th byte
// of the first 64 words (256 bytes) of .text. These values are expected to be primarily 0x0 and 0xE (ARM only)
var textFirstDWords = ReadArray<uint>(conv.Long(sectionByName[".text"].sh_offset), 64);
var bottom = textFirstDWords.Select(w => (w >> 8) & 0xF).GroupBy(n => n).OrderByDescending(f => f.Count()).First().Key;
var top = textFirstDWords.Select(w => w >> 28).GroupBy(n => n).OrderByDescending(f => f.Count()).First().Key;
var xorKey = (byte) (((top << 4) ^ 0xE0) | bottom);
var textFirstDWords = ReadArray<uint>(conv.Long(sectionByName[".text"].sh_offset), 256);
var bottom = textFirstDWords.Select(w => (w >> armNibbleB) & 0xF).GroupBy(n => n).OrderByDescending(f => f.Count()).First().Key;
var top = textFirstDWords.Select(w => w >> armNibbleT).GroupBy(n => n).OrderByDescending(f => f.Count()).First().Key;
var xorKeyCandidateFromCode = (byte) (((top ^ armValueT) << 4) | (bottom ^ armValueB));
if (xorKeyCandidate != 0x00) {
if (xorKeyCandidateStriped != 0x00) {
// Some files may use a striped encryption whereby alternate blocks are encrypted and un-encrypted
// The first part of each section is always encrypted. Scan for the first unencrypted block and find its size
// Limit ourselves to 128KB. If no stripe has been found by then, the whole section is probably encrypted
// Limit ourselves to maxSearchLength. If no stripe has been found by then, the whole section is probably encrypted
// We refer to issue #96 where the code uses striped encryption in 4KB blocks
// We perform heuristics for 128-byte blocks below
// We perform heuristics for block of size blockSize below
var start = conv.Int(sectionByName[".text"].sh_offset);
var length = conv.Int(sectionByName[".text"].sh_size);
var blockSize = 0x80;
var blockSize = 0x100;
var maxSearchLength = 128 * 1024;
var firstUnencrypted = 0xffffffff;
var stripeSize = 0xffffffff;
var threshold = (blockSize / 4) / 2;
// At least this many instructions must pass the threshold
var threshold = (blockSize / 4) / 5;
// A stripe of encryption or non-encryption is considered to have ended when this many blocks in the opposite state are found
var maxBlocksInARow = 4;
// Align start position to search block size
if (conv.Int(sectionByName[".text"].sh_addr) % blockSize != 0)
start += blockSize - conv.Int(sectionByName[".text"].sh_addr) % blockSize;
var probablyEncryptedCount = 0;
var probablyUnencryptedCount = 0;
for (var pos = start; pos < start + maxSearchLength && stripeSize == 0xffffffff; pos += blockSize) {
var size = Math.Min(blockSize, start + length - pos);
var dwords = ReadArray<uint>(pos, size / 4);
var count0 = dwords.Count(w => ((w >> 8) & 0xF) == 0x0);
var countE = dwords.Count(w => (w >> 28) == 0xE);
var encrypted = countE < threshold && count0 < threshold;
var countB = dwords.Count(w => ((w >> armNibbleB) & 0xF) == armValueB);
var countT = dwords.Count(w => (w >> armNibbleT) == armValueT);
var probablyEncrypted = countT < threshold && countB < threshold;
if (!encrypted && firstUnencrypted == 0xffffffff)
firstUnencrypted = (uint) pos;
// Increment one or the other; reset the other one to zero
probablyEncryptedCount = probablyEncrypted? probablyEncryptedCount + 1 : 0;
probablyUnencryptedCount = probablyEncryptedCount == 0 ? probablyUnencryptedCount + 1 : 0;
if (encrypted && firstUnencrypted != 0xffffffff)
stripeSize = (uint) pos - firstUnencrypted;
if (probablyUnencryptedCount >= maxBlocksInARow && firstUnencrypted == 0xffffffff)
firstUnencrypted = (uint) (pos - (probablyUnencryptedCount - 1) * blockSize);
if (probablyEncryptedCount >= maxBlocksInARow && firstUnencrypted != 0xffffffff)
stripeSize = (uint) (pos - firstUnencrypted - (probablyEncryptedCount - 1) * blockSize);
}
// Select the key
// If more than one key candidates are the same, select the most common candidate
var keys = new [] { xorKeyCandidateFromCode, xorKeyCandidateStriped, xorKeyCandidateFull };
var bestKey = keys.GroupBy(k => k).OrderByDescending(k => k.Count()).First();
var xorKey = bestKey.Key;
// Otherwise choose according to striped/full encryption
if (bestKey.Count() == 1)
xorKey = stripeSize != 0xffffffff ? xorKeyCandidateStriped : xorKeyCandidateFull;
StatusUpdate("Decrypting");
Console.WriteLine($"Performing XOR decryption (key: 0x{xorKey:X2}, stripe size: 0x{stripeSize:X4})");