private byte[] ComputeUserPasswordValue(int keyLength, byte[] documentId, byte[] encryptionKey)
{
// Algorithm 3.5, Computing the encryption dictionary's U (user password) value
// (2, 3) Join the fixed padding and the document identifier
byte[] hash = new byte[PADDING_32.Length + documentId.Length];
Array.Copy(PADDING_32, 0, hash, 0, PADDING_32.Length);
Array.Copy(documentId, 0, hash, PADDING_32.Length, documentId.Length);
// (3) Hash using MD5
hash = _md5.ComputeHash(hash);
// (4, 5) Iterate 20 times
byte[] rc4Key = new byte[encryptionKey.Length];
for (int i = 0; i < 20; i++)
{
for (int j = 0; j < encryptionKey.Length; ++j)
{
rc4Key[j] = (byte)(encryptionKey[j] ^ i);
}
hash = RC4.Transform(rc4Key, hash);
}
// (6) First 16 bytes is the hash result and the remainder is zero's
byte[] userKey = new byte[32];
Array.Copy(hash, 0, userKey, 0, 16);
return(userKey);
}
private byte[] ComputeOwnerPasswordValue(int keyLength, byte[] ownerKey)
{
// Algorithm 3.3, Computing the encryption dictionary's O (owner password) value
// (1) Pad the owner password (use the pad because we do not care about a users password)
byte[] ownerPasword = new byte[32];
Array.Copy(PADDING_32, ownerPasword, 32);
// (2) Initialize the MD5
ownerPasword = _md5.ComputeHash(ownerPasword);
// (3) Rehash 50 times
int blockLength = keyLength / 8;
byte[] block = new byte[blockLength];
Array.Copy(ownerPasword, 0, block, 0, block.Length);
for (int i = 0; i < 50; i++)
{
Array.Copy(_md5.ComputeHash(block), 0, block, 0, block.Length);
}
// (4) RC4 key
byte[] rc4Key = new byte[blockLength];
// (5) Pad the user password
byte[] userPassword = new byte[32];
Array.Copy(PADDING_32, userPassword, 32);
// (6, 7) Iterate 20 times
for (int i = 0; i < 20; i++)
{
for (int j = 0; j < blockLength; ++j)
{
rc4Key[j] = (byte)(block[j] ^ i);
}
ownerKey = RC4.Transform(rc4Key, ownerKey);
}
return(ownerKey);
}
private byte[] DecodeBytes(PdfObject obj, byte[] bytes)
{
PdfIndirectObject indirectObject = obj.TypedParent <PdfIndirectObject>();
if (indirectObject == null)
{
throw new ApplicationException($"Cannot decrypt a string that is not inside an indirect object.");
}
// Create bytes that need hashing by combining the encryption key with the indirect object numbers
byte[] key = new byte[_encryptionKey.Length + 5];
Array.Copy(_encryptionKey, 0, key, 0, _encryptionKey.Length);
int index = _encryptionKey.Length;
int id = indirectObject.Id;
key[index++] = (byte)(id >> 0);
key[index++] = (byte)(id >> 8);
key[index++] = (byte)(id >> 16);
int gen = indirectObject.Gen;
key[index++] = (byte)(gen >> 0);
key[index++] = (byte)(gen >> 8);
// MD5 hash the bytes to get raw decrypt key
key = _md5.ComputeHash(key);
// Limit check the decrypt key length
int keyLength = _encryptionKey.Length + 5;
if (keyLength > 16)
{
keyLength = 16;
}
// Create the RC4 key
byte[] rc4Key = new Byte[keyLength];
Array.Copy(key, 0, rc4Key, 0, keyLength);
return(RC4.Transform(rc4Key, bytes));
}