kping0 / keyex_ecdh

Curve 25519 Key-Exchange

Curve25519 Key-Exchange in C

Small & PFS

Example

Both Alice & Bob complete 6 steps:

1. Generate identity key / load from file -- ECDH_newkey()
2. Initialize Key Context using identity key -- keyex_init()
3. Load Partner Identity Key to protect against MiTM -- keyex_set_fidk()
4. Generate Information Packet and send to Partner -- keyex_infopacket()
5. Process Partner Information Packet -- keyex_process()
6. Compute Derived Hashed 64Byte Shared Secret Key (128bit strength) -- keyex_compute()

For sample code look at examples/main.c

Computing the shared key

Both Alice & Bob have:

 PartnerSalt, 
 SelfSalt,
 IdentityKeyPartner(Public), 
 IdentiyKeySelf(Public/Private),
 EphemeralKeyPartner(Public), 
 EphemeralKeySelf(Public/Private)

Both Compute:

  Salt = XOR(PartnerSalt, SelfSalt)
  K1 = ECDH_hash_salt(IdentityKeyPartner, IdentityKeySelf, Salt)
  K2 = ECDH_hash_salt(IdentityKeyPartner, EphemeralKeySelf, Salt)
  K3 = ECDH_hash_salt(EphemeralKeyPartner, IdentityKeySelf, Salt)
  K4 = ECDH_hash_salt(EphemeralKeyPartner, EphemeralKeySelf, Salt)
  K23 = XOR(K2, K3) // Counter Inversion
  
  SHARED_KEY = SHA512(K1 | K23 | K4)
  
  /* ECDH_hash_salt(k1, k2, salt) returns SHA512( ECDH(k1,k2) | salt) */