在Objective-C中解密SJCL密文

Question

我正在将用SJCL加密的数据接收到iOS应用程序中，我需要在该应用程序中对其进行解密。另一端使用带有AES CCM模式的SJCL，而苹果的CommonCrypto不支持该模式，因此我使用VPCCMCrypt库进行解密。不可能对发送SJCL密文的另一方进行任何更改。

以下是我的解密方法：

+ (NSData *)decryptSjcl:(NSDictionary *)cipher password:(NSString *)password {
    if (cipher == nil || password == nil) {
        return nil;
    }
    int version = [cipher[@"v"] intValue];
    NSString *iv = cipher[@"iv"];
    uint iter = [cipher[@"iter"] unsignedIntValue];
    uint ks = [cipher[@"ks"] unsignedIntValue];
    uint ts = [cipher[@"ts"] unsignedIntValue];
    NSString *mode = cipher[@"mode"];
    NSString *adata = cipher[@"adata"];
    NSString *algorithm = cipher[@"cipher"];
    NSString *salt = cipher[@"salt"];
    NSString *ct = cipher[@"ct"];
    if (version != 1 || ! [@"aes" isEqualToString:algorithm]) {
        return nil;
    }
    NSData *rawIv = [[NSData alloc] initWithBase64EncodedString:iv options:0];
    NSData *rawSalt = [[NSData alloc] initWithBase64EncodedString:salt options:0];
    NSData *rawAdata = [[NSData alloc] initWithBase64EncodedString:adata options:0];
    NSData *cipherData = [[NSData alloc] initWithBase64EncodedString:ct options:0];
    NSData *key;
    NSMutableData *decipheredData = nil;
    if ([@"ccm" isEqualToString:mode]) {
        key = [Cryptor sjclAesKeyForPassword:password salt:rawSalt iterations:iter keySize:ks];
        decipheredData = [Cryptor decryptAesCcmData:cipherData iv:rawIv key:key adata:rawAdata tagSize:ts];
    }
    return decipheredData;
}

SJCL密钥生成：

+ (NSData *)sjclAesKeyForPassword:(NSString *)password salt:(NSData *)salt iterations:(uint)iterations keySize:(uint)keySizeBits {
    NSMutableData *derivedKey = [NSMutableData dataWithLength:keySizeBits / 8];
    int result = CCKeyDerivationPBKDF(kCCPBKDF2,
            password.UTF8String,
            [password lengthOfBytesUsingEncoding:NSUTF8StringEncoding],
            salt.bytes,
            salt.length,
            kCCPRFHmacAlgSHA256,
            iterations,
            derivedKey.mutableBytes,
            derivedKey.length);
    NSAssert(result == kCCSuccess, @"Unable to create AES key for password: %d", result);
    return derivedKey;
}

AES CCM解密：

+ (NSMutableData *)decryptAesCcmData:(NSData *)cipherData iv:(NSData *)iv key:(NSData *)key adata:(NSData *)adata tagSize:(uint)tagSizeBits {
    VPCCMCrypt *ccm = [[VPCCMCrypt alloc] initWithKey:key
                                                   iv:iv
                                                adata:adata
                                            tagLength:tagSizeBits / 8];
    [ccm decryptDataWithData:cipherData
               finishedBlock:^(NSData *data) {
                   NSString *result = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
                   NSLog(@"Decrypted SJCL message: %@", result);
               }
                  errorBlock:^(NSError *error) {
                      NSLog(@"ERROR");
                  }];
    return nil;
}

SJCL的所有输入数据都被正确解析（IV、salt、密钥大小、标签大小、PBKDF迭代、密文），并从其Base64编码表示解码为NSData。密码使用相同。包括创建的AES密钥在内的所有数据都不是零。

最后，它在VPCCMCrypt内部验证CCM标记失败（标记不同）。上面的代码有问题吗？是否有其他iOS/Objective-C/Swift库可以解密AES CCM或更好的SJCL？我对SJCL库的JavaScript包装器不感兴趣。

为了进行测试，我使用了SJCL演示页面中的简单加密数据。

编辑：

如评论中所述，SJCL发送16字节IV，而不是CCM模式的最大12字节，然后在解密时将其内部箝位为最大12字节。以下是更新的解密方法：

+ (NSData *)decryptSjcl:(NSDictionary *)cipher password:(NSString *)password {
    if (cipher == nil || password == nil) {
        return nil;
    }
    int version = [cipher[@"v"] intValue];
    NSString *iv = cipher[@"iv"];
    uint iter = [cipher[@"iter"] unsignedIntValue];
    uint ks = [cipher[@"ks"] unsignedIntValue];
    uint ts = [cipher[@"ts"] unsignedIntValue];
    NSString *mode = cipher[@"mode"];
    NSString *adata = cipher[@"adata"];
    NSString *algorithm = cipher[@"cipher"];
    NSString *salt = cipher[@"salt"];
    NSString *ct = cipher[@"ct"];
    if (version != 1 || ! [@"aes" isEqualToString:algorithm]) {
        return nil;
    }
    NSMutableData *rawIv = [[NSMutableData alloc] initWithBase64EncodedString:iv options:0];
    NSMutableData *rawSalt = [[NSMutableData alloc] initWithBase64EncodedString:salt options:0];
    NSMutableData *rawAdata = [[NSMutableData alloc] initWithBase64EncodedString:adata options:0];
    NSMutableData *cipherData = [[NSMutableData alloc] initWithBase64EncodedString:ct options:0];
    NSData *key;
    NSData *decipheredData = nil;
    if ([@"ccm" isEqualToString:mode]) {
        key = [Cryptor sjclAesKeyForPassword:password salt:rawSalt iterations:iter keySize:ks];
        // Clamp the SJCL IV - They use a 16 byte IV for CCM mode which is against specification and they do a funky
        // clamping. CCM mode IV should be max 13 bytes long. They almost always clamp it to 13 bytes but save the whole
        // 16 bytes in their JSON container.
        // for (L=2; L<4 && ol >>> 8*L; L++) {}
        // if (L < 15 - ivl) { L = 15-ivl; }
        // iv = w.clamp(iv,8*(15-L));
        int64_t ivl = rawIv.length;
        int64_t ol = cipherData.length - (ts / 8);
        int64_t L = 2;
        for (L = 2; L < 4 && ol >> 8*L; L++) {}
        if (L < 15 - ivl) {
            L = 15 - ivl;
        }
        NSRange subrange = NSMakeRange(0, (NSUInteger)(15 - L));
        decipheredData = [Cryptor decryptAesCcmData:cipherData iv:[rawIv subdataWithRange:subrange] key:key adata:rawAdata tagSize:ts];
    }
    else {
        decipheredData = nil;
    }
    return decipheredData;
}

最后一件缺失的事情是验证TAG，我无法做到这一点。有什么想法吗？

Answer 1

SJCL在AES-CCM模式下没有使用整个（128/192/256bit）IV，但演示页面显示了这一点。请尝试使用更少的字节。

在这里，您可以找到IV长度计算的工作原理：http://bitwiseshiftleft.github.io/sjcl/doc/symbols/src/core_ccm.js.html

for (L=2; L<4 && ol >>> 8*L; L++) {}
if (L < 15 - ivl) { L = 15-ivl; }
iv = w.clamp(iv,8*(15-L));