Verify digital signature with public key in iOS

Question

How to Verify digital signature with public key in iOS without using any third party code(e.g.) open SSL?

I need to verify Digital signature in iOS App with a public key. Can some one help me how to achieve that without using third party software.

I am trying below code but the problem is I don't have certificate in my App so can not create SecTrustRef.

CODE:

    NSString *certPath              = [[NSBundle mainBundle] pathForResource:@"yyy" 
                                                             ofType:@"xxx"];
    SecCertificateRef myCertificate = nil;
    NSData *certificateData         = [[NSData alloc] initWithContentsOfFile :certPath];
    myCertificate                   = SecCertificateCreateWithData(kCFAllocatorDefault, (__bridge CFDataRef)certificateData);

    SecPolicyRef myPolicy           = SecPolicyCreateBasicX509();
    SecTrustRef trustRef;
    SecTrustCreateWithCertificates(myCertificate, myPolicy, &trustRef);
    SecKeyRef keyRef      = SecTrustCopyPublicKey (trustRef);


    BOOL status = SecKeyRawVerify (keyRef,
                              kSecPaddingPKCS1SHA1,
                              (const uint8_t *)[data bytes],
                              (size_t)[data length],
                              (const uint8_t *)[signature bytes],
                              (size_t)[signature length]
                              );

I have the following:

• Public Key (NSString*)
• Signature (NSString*)
• Data (NSString*)

Please help me what all option I have in iOS SDK if I don't want to use ant third party open source.

Answer 1

@interface HDSecurityPolicy :AFSecurityPolicy
@end

@implementation HDSecurityPolicy
///pem formate(base64) ->  NSData
- (void)setPinnedCertificates:(NSSet *)pinnedCertificates {
    [super setPinnedCertificates:pinnedCertificates];
    if (self.pinnedCertificates) {
        NSMutableSet *mutablePinnedPublicKeys = [NSMutableSet setWithCapacity:[self.pinnedCertificates count]];
        for (NSData *pubCertificate in self.pinnedCertificates) {
            id publicKey = [HDSecurityPolicy publicSecKeyFromKeyBits:pubCertificate];
            if (!publicKey) {
                continue;
            }
            [mutablePinnedPublicKeys addObject:publicKey];
        }
        [self setValue:mutablePinnedPublicKeys forKey:@"pinnedPublicKeys"];
    }
}

// 从公钥证书文件中获取到公钥的SecKeyRef指针
+ (id)publicSecKeyFromKeyBits:(NSData *)givenData {
    NSMutableDictionary *options = [NSMutableDictionary dictionary];
    options[(__bridge id)kSecAttrKeyType] = (__bridge id) kSecAttrKeyTypeRSA;
    options[(__bridge id)kSecAttrKeyClass] = (__bridge id) kSecAttrKeyClassPublic;

    NSError *error = nil;
    CFErrorRef ee = (__bridge CFErrorRef)error;
    ////'SecKeyCreateWithData' is only available on iOS 10.0 or newer
    id ret = (__bridge_transfer id)SecKeyCreateWithData((__bridge CFDataRef)givenData, (__bridge CFDictionaryRef)options, &ee);
    if (error) {
        return nil;
    }
    return ret;
}
@end

Answer 2

You need to pass a digest (hash) of your data to the verification function. See iOS: Verifying a File With a Certificate and Signature - Public Key is Wrong, Verification Fails

Answer 3

You can package your public key in a X509 certificate to use the iOS built in functions easily, using openSSL:

openssl req -x509 -out public_key.pem -outform pem -new -newkey rsa:2048 -keyout private_key.pem 

the PEM format is base64 encoded, you can switch the -outform to DER to get binary file. you can import the PEM format by adding a const NSString to your program and adding category to NSData with this function:

- (id) initWithBase64EncodedString:(NSString *) string {
    NSMutableData *mutableData = nil;

    if( string ) {
        unsigned long ixtext = 0;
        unsigned long lentext = 0;
        unsigned char ch = 0;
        unsigned char inbuf[4], outbuf[3]; // buffer sizes fixed by AOL LLC
        short i = 0, ixinbuf = 0;
        BOOL flignore = NO;
        BOOL flendtext = NO;
        NSData *base64Data = nil;
        const unsigned char *base64Bytes = nil;

        // Convert the string to ASCII data.
        base64Data = [string dataUsingEncoding:NSASCIIStringEncoding];
        base64Bytes = [base64Data bytes];
        mutableData = [NSMutableData dataWithCapacity:[base64Data length]];
        lentext = [base64Data length];

        while( YES ) {
            if( ixtext >= lentext ) break;
            ch = base64Bytes[ixtext++];
            flignore = NO;

            if( ( ch >= 'A' ) && ( ch <= 'Z' ) ) ch = ch - 'A';
            else if( ( ch >= 'a' ) && ( ch <= 'z' ) ) ch = ch - 'a' + 26;
            else if( ( ch >= '0' ) && ( ch <= '9' ) ) ch = ch - '0' + 52;
            else if( ch == '+' ) ch = 62;
            else if( ch == '=' ) flendtext = YES;
            else if( ch == '/' ) ch = 63;
            else flignore = YES;

            if( ! flignore ) {
                short ctcharsinbuf = 3;
                BOOL flbreak = NO;

                if( flendtext ) {
                    if( ! ixinbuf ) break;
                    if( ( ixinbuf == 1 ) || ( ixinbuf == 2 ) ) ctcharsinbuf = 1;
                    else ctcharsinbuf = 2;
                    ixinbuf = 3;
                    flbreak = YES;
                }

                inbuf [ixinbuf++] = ch;

                if( ixinbuf == 4 ) {
                    ixinbuf = 0;
                    outbuf [0] = ( inbuf[0] << 2 ) | ( ( inbuf[1] & 0x30) >> 4 );
                    outbuf [1] = ( ( inbuf[1] & 0x0F ) << 4 ) | ( ( inbuf[2] & 0x3C ) >> 2 );
                    outbuf [2] = ( ( inbuf[2] & 0x03 ) << 6 ) | ( inbuf[3] & 0x3F );

                    for( i = 0; i < ctcharsinbuf; i++ )
                        [mutableData appendBytes:&outbuf[i] length:1];
                }

                if( flbreak )  break;
            }
        }
    }

    self = [self initWithData:mutableData];
    return self;
}

ofcourse you pull this file into the certificateData if you want to use your existing public key just pull it out and write it in a X509 cert format using openSSL

$ openssl rsa -in id_rsa -out pub.der -outform DER -pubout

good luck

Answer 4

If your key data is packaged as PKCS12 data, use SecPKCS12Import to import it and use the public key.