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Allow EtM_DecryptTransform block level seeking via SetCurrentChunkNumber() #45

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Allow EtM_DecryptTransform block level seeking via SetCurrentChunkNumber() #45

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27 changes: 21 additions & 6 deletions EtM_Transforms.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -127,6 +127,7 @@ public class EtM_DecryptTransform : ICryptoTransform
byte[] key;
uint currentChunkNumber;
ArraySegment<byte>? salt;
ArraySegment<byte>? saltWithPrepend;

public uint CurrentChunkNumber { get { return this.currentChunkNumber; } }

Expand All @@ -140,6 +141,14 @@ public EtM_DecryptTransform(byte[] key, ArraySegment<byte>? salt = null, bool au
this.IsAuthenticateOnly = authenticateOnly;
}

public void SetCurrentChunkNumber(uint chunk)
{
if (chunk > EtM_Transform_Constants.INITIAL_CHUNK_NUMBER && !saltWithPrepend.HasValue)
throw new Exception("First chunk must be read first.");

this.currentChunkNumber = chunk;
}

public int TransformBlock(byte[] inputBuffer, int inputOffset, int inputCount, byte[] outputBuffer, int outputOffset)
{
int partialBlockSize = inputCount % EtM_Transform_Constants.OUTPUT_BLOCK_SIZE;
Expand All @@ -154,6 +163,8 @@ public int TransformBlock(byte[] inputBuffer, int inputOffset, int inputCount, b
var authenticateOnly = this.IsAuthenticateOnly;
for (; i < fullBlockSize; i += EtM_Transform_Constants.OUTPUT_BLOCK_SIZE, j += EtM_Transform_Constants.INPUT_BLOCK_SIZE)
{
var isFirstChunk = this.currentChunkNumber == EtM_Transform_Constants.INITIAL_CHUNK_NUMBER;

var outputSegment = new ArraySegment<byte>?(new ArraySegment<byte>(outputBuffer, outputOffset + j, EtM_Transform_Constants.INPUT_BLOCK_SIZE));
var cipherText = new ArraySegment<byte>(inputBuffer, inputOffset + i, EtM_Transform_Constants.OUTPUT_BLOCK_SIZE);

Expand All @@ -162,7 +173,7 @@ public int TransformBlock(byte[] inputBuffer, int inputOffset, int inputCount, b
if (!EtM_CTR.Authenticate(
masterKey: this.key,
ciphertext: cipherText,
salt: this.salt,
salt: isFirstChunk ? this.salt : this.saltWithPrepend,
counter: this.currentChunkNumber))
outputSegment = null;
}
Expand All @@ -172,8 +183,9 @@ public int TransformBlock(byte[] inputBuffer, int inputOffset, int inputCount, b
masterKey: this.key,
ciphertext: cipherText,
outputSegment: ref outputSegment,
salt: this.salt,
salt: isFirstChunk ? this.salt : this.saltWithPrepend,
counter: this.currentChunkNumber);

}

if (outputSegment == null)
Expand All @@ -182,9 +194,11 @@ public int TransformBlock(byte[] inputBuffer, int inputOffset, int inputCount, b
throw new CryptographicException("Decryption failed for block " + this.currentChunkNumber.ToString() + ".");
}

if (this.currentChunkNumber == EtM_Transform_Constants.INITIAL_CHUNK_NUMBER)
if (isFirstChunk && !saltWithPrepend.HasValue)
{
EtM_EncryptTransform.PrependSaltWith1stBlockContext(ref this.salt, inputBuffer, inputOffset);
if (salt.HasValue)
saltWithPrepend = new ArraySegment<byte>(salt.Value.Array);
EtM_EncryptTransform.PrependSaltWith1stBlockContext(ref this.saltWithPrepend, inputBuffer, inputOffset);
}

checked { ++this.currentChunkNumber; }
Expand All @@ -204,6 +218,7 @@ public byte[] TransformFinalBlock(byte[] inputBuffer, int inputOffset, int input

byte[] outputBuffer = null;
var cipherText = new ArraySegment<byte>(inputBuffer, inputOffset, inputCount);
var isFirstChunk = this.currentChunkNumber == EtM_Transform_Constants.INITIAL_CHUNK_NUMBER;

if (this.IsAuthenticateOnly)
{
Expand All @@ -219,10 +234,10 @@ public byte[] TransformFinalBlock(byte[] inputBuffer, int inputOffset, int input
outputBuffer = EtM_CTR.Decrypt(
masterKey: this.key,
ciphertext: cipherText,
salt: this.salt,
salt: isFirstChunk ? this.salt : this.saltWithPrepend,
counter: this.currentChunkNumber);
}
this.Dispose();

if (outputBuffer == null)
throw new CryptographicException("Decryption failed for block " + this.currentChunkNumber.ToString() + ".");

Expand Down