Message is too large error for Google OAuth

[lessless]

Hello @garyf, thanks for the library, though it took a time to find my way through it to sign an Google API's service-to-service request as per https://developers.google.com/identity/protocols/OAuth2ServiceAccount#authorizingrequests they require following claims to be present: [iss, scope, aud, exp, iat]

but even on the 3rd claim library throwing an "message too large error"

JsonWebToken.sign(
  %{
    iss: "zzzzzzzzzzzz-88b590995a01m8krs6sdrcmg8m6nkxxx@developer.gserviceaccount.com", 
    scope: "https://www.googleapis.com/auth/devstorage.full_control", 
    aud: "https://www.googleapis.com/oauth2/v3/token"
  }, 
  %{
    alg: "RS256",
    key: JsonWebToken.Algorithm.RsaUtil.private_key(dir, key)
  })

** (RuntimeError) Message too large
    lib/json_web_token/algorithm/rsa.ex:70: JsonWebToken.Algorithm.Rsa.large_message/1
    lib/json_web_token/algorithm/rsa.ex:25: JsonWebToken.Algorithm.Rsa.sign/3
    lib/json_web_token/jws.ex:61: JsonWebToken.Jws.signature/3
    lib/json_web_token/jws.ex:25: JsonWebToken.Jws.sign/3

I understand that this is done on purpose https://github.com/garyf/json_web_token_ex/blob/master/lib/json_web_token/algorithm/rsa.ex#L12 but in the referenced document I didn't find the part where it is said that message couldn't be over 245 bytes long. That even sounds doubtful.

Am I doing something wrong?

[garyf]

Hi @lessless

In the reference (http://tools.ietf.org/html/rfc3447#section-7.2), the governing requirement is found in section 7.2.1, Step 1:

Length checking: If mLen > k - 11, output "message too long" and stop. (where k == 256 bytes for 2048 key bits)

More discussion here: http://security.stackexchange.com/questions/33434/rsa-maximum-bytes-to-encrypt-comparison-to-aes-in-terms-of-security

[lessless]

@garyf thank you for explanation and links. But in the README RS256 refers to RSASSA-PKCS-v1_5 using SHA-256 not the RSAES-PKCS1-v1_5 :cry: Pardon those, maybe stupid questions, this is my almost first encounter with the guts of the cryptography, and I'm almost heart-broken.

Spec says that EMSA-PKCS1-v1_5 encoding can throw message too long error If the hash function outputs "message too long," and 256 bytes length limitation doesn't apply to SHA-256. Right?

[garyf]

@lessless in the 'guts' is where all truth is found; the 256 byte length limitation is not applicable

81427b7 anticipates a resolution

[lessless]

@garyf right. thank you!