# BSides Delhi CTF 2019 : SecureMac
**category** : crypto
**points** : 871
## write-up
Two parts in this challenge:1. get the key2. generate a collision to this mac
### First Part
Same technique as in [CSAW CTF - Fault Box](https://github.com/OAlienO/CTF/tree/master/2019/CSAW-CTF/Fault-Box)
let `f` be `bytes_to_long("fake_flag")`  `c` will be `f ** e + k * p` for some `k`  We know the value of `f ** e`  Simply calculate `gcd(f ** e - c, n)` will give us prime factor `p`, then we can factor `n`
### Second Part
To make things simple, we send messages of 32 bytes.  Both `messageblocks[1]` and `tag`, which is `ECB.encrypt(messageblocks[0])` we can control  Just make the result of `strxor` be the same
flag: `bsides_delhi{F4ult'n'F0rg3_1s_@_b4d_c0mb1n4ti0n}`
# other write-ups and resources