[security] OIC self-issued mode is insecure

[Manger, James]

OpenID Connect Core 1.0 section 7 "Self-Issued OpenID Provider" looks quite insecure.

The subject identifier "sub" (that RPs will use as the identifier for an account) is calculated from a hash of the concatenation of n & e (for an RSA key). However, the boundary between n & e is not preserved. Hence there are lots of other public keys that will give the same "sub". Using any of these other keys will allow an attacker to impersonate the user of this self-issued OP. These other keys do not have the same security as the user's real key.

The example from the spec is below:

  {
   "iss": "https://self-issued.me",
   "sub": "wBy8QvHbPzUnL0x63h13QqvUYcOur1X0cbQpPVRqX5k",
   "aud": "https://client.example.org/cb",
   "nonce": "n-0S6_WzA2Mj",
   "exp": 1311281970,
   "iat": 1311280970,
   "sub_jwk": {
     "kty":"RSA",
     "n": "0vx7agoebGcQSuuPiLJXZptN9nndrQmbXEps2aiAFbWhM78LhWx
     4cbbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZCiFV4n3oknjhMs
     tn64tZ_2W-5JsGY4Hc5n9yBXArwl93lqt7_RN5w6Cf0h4QyQ5v-65YGjQR0_FDW2
     QvzqY368QQMicAtaSqzs8KJZgnYb9c7d0zgdAZHzu6qMQvRL5hajrn1n91CbOpbI
     SD08qNLyrdkt-bFTWhAI4vMQFh6WeZu0fM4lFd2NcRwr3XPksINHaQ-G_xBniIqb
     w0Ls1jF44-csFCur-kEgU8awapJzKnqDKgw",
     "e":"AQAB"
    }
  }

The same "sub" results even if the key (sub_jwk) is changed as follows to have a public exponent of 0. That makes it easy to sign! "AQ" is moved from the start of the "e" to the end of "n".

   "sub_jwk": {
     "kty":"RSA",
     "n": "0vx7agoebGcQSuuPiLJXZptN9nndrQmbXEps2aiAFbWhM78LhWx
     4cbbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZCiFV4n3oknjhMs
     tn64tZ_2W-5JsGY4Hc5n9yBXArwl93lqt7_RN5w6Cf0h4QyQ5v-65YGjQR0_FDW2
     QvzqY368QQMicAtaSqzs8KJZgnYb9c7d0zgdAZHzu6qMQvRL5hajrn1n91CbOpbI
     SD08qNLyrdkt-bFTWhAI4vMQFh6WeZu0fM4lFd2NcRwr3XPksINHaQ-G_xBniIqb
     w0Ls1jF44-csFCur-kEgU8awapJzKnqDKgwAQ",
     "e":"AB"
    }

The same "sub" also results from the following key that has a modulus half the size (hence far far less security). Half the modulus is moved to the exponent.

   "sub_jwk": {
     "kty":"RSA",
     "n": "0vx7agoebGcQSuuPiLJXZptN9nndrQmbXEps2aiAFbWhM78LhWx
     4cbbfAAtVT86zwu1RK7aPFFxuhDR1L6tSoc_BJECPebWKRXjBZCiFV4n3oknjhMs
     tn64tZ_2W-5JsGY4Hc5n9yBXArwl93lqt7_RN5w6Cf0h4QyQ5v-65YGjQR0_FDW2",
     "e": "QvzqY368QQMicAtaSqzs8KJZgnYb9c7d0zgdAZHzu6qMQvRL5hajrn1n91CbOpbI
     SD08qNLyrdkt-bFTWhAI4vMQFh6WeZu0fM4lFd2NcRwr3XPksINHaQ-G_xBniIqb
     w0Ls1jF44-csFCur-kEgU8awapJzKnqDKgwAQAB"
    }

There are similar problems with elliptic curve keys, or even with replacing an elliptic curve key with a weak RSA key that gives the same "sub".
[If we had a canonical form of JSON we could hash that, otherwise... include the key type and some delimiters]

--
James Manger