(Objective-C) Verify a JWT Created by the Amazon Cognito Service

Demonstrates how to verify a JWT created by the Amazon Cognito Service.

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#import <CkoHttp.h>
#import <CkoStringBuilder.h>
#import <CkoJsonObject.h>
#import <CkoPublicKey.h>
#import <CkoJwt.h>
#import <NSString.h>

// This example requires the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

// The public keys for this example are at https://cognito-idp.us-east-2.amazonaws.com/us-east-2_******/.well-known/jwks.json
// Let's get them:

CkoHttp *http = [[CkoHttp alloc] init];
CkoStringBuilder *sbJsonKeys = [[CkoStringBuilder alloc] init];
BOOL success = [http QuickGetSb: @"https://cognito-idp.us-east-2.amazonaws.com/us-east-2_******/.well-known/jwks.json" sbContent: sbJsonKeys];
if (success == NO) {
    NSLog(@"%@",http.LastErrorText);
    return;
}

CkoJsonObject *jsonKeys = [[CkoJsonObject alloc] init];
[jsonKeys LoadSb: sbJsonKeys];
jsonKeys.EmitCompact = NO;
NSLog(@"%@",[jsonKeys Emit]);

// Here are the keys:

// {
//   "keys": [
//     {
//       "alg": "RS256",
//       "e": "AQAB",
//       "kid": "1A/L5Fsb2EsEwxy5E0cmCMS1BnMe6Jl6NXiMig4iNwU=",
//       "kty": "RSA",
//       "n": "y0w7BJrIJYi ... jKG27z2P3OKw",
//       "use": "sig"
//     },
//     {
//       "alg": "RS256",
//       "e": "AQAB",
//       "kid": "mos6VTJnvDwurY3ghJg6IAPUq+dMwl6CL/iThzJOkzg=",
//       "kty": "RSA",
//       "n": "qbIEH-7tg6yrT ... 3Fj94ooTd0w",
//       "use": "sig"
//     }
//   ]
// }

// Try the 1st key.
CkoJsonObject *jsonKey1 = [jsonKeys ObjectOf: @"keys[0]"];
if (jsonKeys.LastMethodSuccess == NO) {
    NSLog(@"%@",@"Did not get the 1st JSON public key.");
    return;
}

CkoPublicKey *pubKey1 = [[CkoPublicKey alloc] init];
success = [pubKey1 LoadFromString: [jsonKey1 Emit]];
if (success == NO) {
    NSLog(@"%@",pubKey1.LastErrorText);
    return;
}

NSLog(@"%@",@"Success");

CkoJwt *jwt = [[CkoJwt alloc] init];

// I did not include the an actual AWS Cognito token here because our test sample used customer-provided data..
NSString *token = @"eyJ..asXg";

// First verify the signature.
BOOL sigVerified = [jwt VerifyJwtPk: token key: pubKey1];
NSLog(@"%@%d",@"verified: ",sigVerified);

// Let's see if the time constraints, if any, are valid.
// The above JWT was created on the afternoon of 16-May-2016, with an expiration of 1 hour.
// If the current system time is before the "nbf" time, or after the "exp" time,
// then IsTimeValid will return false/0.
// Also, we'll allow a leeway of 60 seconds to account for any clock skew.
// Note: If the token has no "nbf" or "exp" claim fields, then IsTimeValid is always true.
int leeway = 60;
BOOL bTimeValid = [jwt IsTimeValid: token leeway: [NSNumber numberWithInt: leeway]];
NSLog(@"%@%d",@"time constraints valid: ",bTimeValid);

// Now let's recover the original claims JSON (the payload).
NSString *payload = [jwt GetPayload: token];
// The payload will likely be in compact form:
NSLog(@"%@",payload);

// We can format for human viewing by loading it into Chilkat's JSON object
// and emit.
CkoJsonObject *json = [[CkoJsonObject alloc] init];
success = [json Load: payload];
json.EmitCompact = NO;
NSLog(@"%@",[json Emit]);

// We can recover the original JOSE header in the same way:
NSString *joseHeader = [jwt GetHeader: token];
// The payload will likely be in compact form:
NSLog(@"%@",joseHeader);

// We can format for human viewing by loading it into Chilkat's JSON object
// and emit.
success = [json Load: joseHeader];
json.EmitCompact = NO;
NSLog(@"%@",[json Emit]);