/* Encrypts a string "message" using "key". */
std::string RSA::encryptString(const std::string &message, const Key &key)
{
//partition the message into biggest possible encryptable chunks
const unsigned long int chunkSize(((key.GetModulus().Length() - 2) / 3));
const unsigned long int chunkCount = message.length() / chunkSize;
string cypherText;
for (unsigned long int i(0); i < chunkCount; i++)
{
// Get the next chunk.
string chunk(message.substr(i * chunkSize, chunkSize));
chunk = RSA::encryptChunk(chunk, key);
// Put a ' ' between the chunks so that we can separate them later.
cypherText.append(chunk.append(" "));
}
// If the last chunk has the same size as the others, we are finished.
if (chunkSize * chunkCount == message.length())
return cypherText;
// Handle the last chunk. It is smaller than the others.
const unsigned long int lastChunkSize = message.length() % chunkSize;
string lastChunk(message.substr(chunkCount * chunkSize, lastChunkSize));
lastChunk = RSA::encryptChunk(lastChunk, key);
return cypherText.append(lastChunk.append(" "));
}
/* Decrypts a "chunk" (a small part of a message) using "key" */
string RSA::decryptChunk(const BigInt &chunk, const Key &key)
{
BigInt a = chunk;
// The RSA decryption algorithm is a congruence equation.
a.SetPowerMod(key.GetExponent(), key.GetModulus());
// Decode the message to a readable form.
return RSA::decode(a);
}
/* Encrypts a "chunk" (a small part of a message) using "key" */
string RSA::encryptChunk(const string &chunk, const Key &key)
{
// First encode the chunk, to make sure it is represented as an integer.
BigInt a = RSA::encode(chunk);
// The RSA encryption algorithm is a congruence equation.
a.SetPowerMod(key.GetExponent(), key.GetModulus());
return a.ToString();
}
/* Decrypts a string "message" using "key". */
std::string RSA::decryptString(const std::string &cypherText, const Key &key)
{
// Partition the cypherText into chunks. They are seperated by ' '.
string message;
long int i(0), j(0);
while ((j = cypherText.find(' ', i)) != -1)
{
// Get the chunk.
BigInt chunk(cypherText.substr(i, j - i));
if (chunk >= key.GetModulus())
throw "Error RSA02: Chunk too large.";
// Decrypt the chunk and store the message.
string text = RSA::decryptChunk(chunk, key);
message.append(text);
i = j + 1;
}
return message;
}
/* Throws an exception if "key" is too short to be used. */
void RSA::checkKeyLength(const Key &key)
{
// Minimum required key length is around 24 bits. (In-house requirement)
if (key.GetModulus().Length() < 8)
throw "Error RSA01: Keys must be at least 8 digits long.";
}