don't see anything special about downloading their client, using a decompiler, use rs2gd to rename the classes, then using client parameters to run the client. xteas aren't very hard to get if you know what you're doing especially since their encryption key is found in the client.
Code:
/* take 64 bits of data in v[0] and v[1] and 128 bits of key[0] - key[3] */
void encipher(unsigned int num_rounds, uint32_t v[2], uint32_t const key[4]) {
unsigned int i;
uint32_t v0=v[0], v1=v[1], sum=0, delta=0x9E3779B9;
for (i=0; i < num_rounds; i++) {
v0 += (((v1 << 4) ^ (v1 >> 5)) + v1) ^ (sum + key[sum & 3]);
sum += delta;
v1 += (((v0 << 4) ^ (v0 >> 5)) + v0) ^ (sum + key[(sum>>11) & 3]);
}
v[0]=v0; v[1]=v1;
}
void decipher(unsigned int num_rounds, uint32_t v[2], uint32_t const key[4]) {
unsigned int i;
uint32_t v0=v[0], v1=v[1], delta=0x9E3779B9, sum=delta*num_rounds;
for (i=0; i < num_rounds; i++) {
v1 -= (((v0 << 4) ^ (v0 >> 5)) + v0) ^ (sum + key[(sum>>11) & 3]);
sum -= delta;
v0 -= (((v1 << 4) ^ (v1 >> 5)) + v1) ^ (sum + key[sum & 3]);
}
v[0]=v0; v[1]=v1;
}
encrypting and decrypting are very similar.