Decrypt a JWE to a String

See more JSON Web Encryption (JWE) Examples

Demonstrates Jwe.Decrypt, which decrypts a recipient of a loaded JWE and returns the plaintext, decoded using the given charset. Before decrypting, the example inspects the JWE protected header and, under application policy, accepts only permitted algorithms and selects an acceptable recipient.

#import <CkoJwe.h>
#import <NSString.h>
#import <CkoJsonObject.h>
#import <CkoStringBuilder.h>

BOOL success = NO;

CkoJwe *jwe = [[CkoJwe alloc] init];

//  Load the JWE compact serialization to be decrypted.
NSString *jweCompact = @"eyJhbGciOiJBMjU2S1ciLCJlbmMiOiJBMjU2R0NNIn0.<...>.<iv>.<ciphertext>.<tag>";
success = [jwe LoadJwe: jweCompact];
if (success == NO) {
    NSLog(@"%@",jwe.LastErrorText);
    return;
}

//  Application policy: before decrypting, inspect the JWE header and accept only algorithms the
//  application trusts.  Never assume the algorithms in a received JWE are safe -- a sender could
//  specify a weak or unexpected algorithm.
CkoJsonObject *protHeader = [[CkoJsonObject alloc] init];
success = [jwe GetProtectedHeader: protHeader];
if (success == NO) {
    NSLog(@"%@",jwe.LastErrorText);
    return;
}

//  Read the alg (key-management) and enc (content-encryption) header values into StringBuilder
//  objects so they can be compared.
CkoStringBuilder *sbAlg = [[CkoStringBuilder alloc] init];
success = [protHeader StringOfSb: @"alg" sb: sbAlg];
if (success == NO) {
    NSLog(@"%@",protHeader.LastErrorText);
    return;
}

CkoStringBuilder *sbEnc = [[CkoStringBuilder alloc] init];
success = [protHeader StringOfSb: @"enc" sb: sbEnc];
if (success == NO) {
    NSLog(@"%@",protHeader.LastErrorText);
    return;
}

NSLog(@"%@%@%@%@",@"JWE algorithms: alg=",[sbAlg GetAsString],@" enc=",[sbEnc GetAsString]);

//  Enforce an allowlist of acceptable algorithms.  Reject anything not permitted by policy.  String
//  values are compared using StringBuilder.ContentsEqual.
BOOL bCaseSensitive = YES;
if ([sbAlg ContentsEqual: @"A256KW" caseSensitive: bCaseSensitive] != YES) {
    NSLog(@"%@",@"Rejecting JWE: key-management algorithm is not permitted by policy.");
    return;
}

if ([sbEnc ContentsEqual: @"A256GCM" caseSensitive: bCaseSensitive] != YES) {
    NSLog(@"%@",@"Rejecting JWE: content-encryption algorithm is not permitted by policy.");
    return;
}

//  Select an acceptable recipient.  For a multi-recipient JWE, FindRecipient locates the recipient
//  this application holds a key for by a per-recipient header value such as "kid".  A compact JWE
//  has a single recipient at index 0, so FindRecipient returns -1; fall back to index 0 in that case.
int recipientIndex = [[jwe FindRecipient: @"kid" paramValue: @"recipient-key-1" caseSensitive: bCaseSensitive] intValue];
if (recipientIndex < 0) {
    recipientIndex = 0;
}

//  Provide the key for the selected recipient.  In production, obtain the key from a secure source
//  rather than hard-coding it.
NSString *base64Key = @"YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXowMTIzNDU=";
success = [jwe SetWrappingKey: [NSNumber numberWithInt: recipientIndex] encodedKey: base64Key encoding: @"base64"];
if (success == NO) {
    NSLog(@"%@",jwe.LastErrorText);
    return;
}

//  Decrypt the selected recipient and return the plaintext, decoded using the given charset.
NSString *content = [jwe Decrypt: [NSNumber numberWithInt: recipientIndex] charset: @"utf-8"];
if (jwe.LastMethodSuccess == NO) {
    NSLog(@"%@",jwe.LastErrorText);
    return;
}

NSLog(@"%@",content);