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Jon Skeet

Saving Base64 encoded CER file to a String

I am currently able to read a Key modulus from a cer file using this code, also found on StackOverflow:

X509Certificate cert = null;
String source = "src/testCer.cer";
InputStream fis = null;                 
ByteArrayInputStream bais = null;               


fis = new FileInputStream(source);              
byte value[] = new byte[fis.available()];       
fis.read(value);                                
bais = new ByteArrayInputStream(value);             
java.security.cert.CertificateFactory cf = java.security.cert.CertificateFactory.getInstance("X.509");
cert = (X509Certificate)cf.generateCertificate(bais);    
RSAPublicKey pub = (RSAPublicKey) cert.getPublicKey();

System.out.println(pub.getModulus());

The .cer file looks like this: (certificate is an example Alice certificate found on the internet):

A nicely Base64 Encoded wrapped text.

My question:

How would I save this certificate to a String, and use that instead of reading the certificate from a file?

So far I tried:

String cert_1= "-----BEGIN CERTIFICATE-----"
                + "MIIBuTCCASKgAwIBAgIQNdNhtuV5GbNHYZsf+LvM0zANBgkqhkiG9w0BAQUFADAb"
                + "MRkwFwYDVQQDExBFZGlkZXYgU21va2VUZXN0MB4XDTA4MTExMjE5NTEzNVoXDTM5"
                + "MTIzMTIzNTk1OVowGzEZMBcGA1UEAxMQRWRpZGV2IFNtb2tlVGVzdDCBnzANBgkq"
                + "hkiG9w0BAQEFAAOBjQAwgYkCgYEAm6zGzqxejwswWTNLcSsa7P8xqODspX9VQBuq"
                + "CBpT4S5AaO3/wkYQSKdI108pXH7Aue0e/ZOwgEEX1N6OaPQn7AmAB4uq1h+ffw+r"
                + "5W1RoTgQ0LNR64+7ywLjH8+wrb/lB6QV7s2SFUiWDeduVesvMJkWtZ5zzQyl3iUa"
                + "RKNHqnsCAwEAATANBgkqhkiG9w0BAQUFAAOBgQCZmj+pgRsN6HpoICawK3XXNAmi"
                + "cgfQkailX9akIjD3xSCwEQx4nG6tZjTz30u4NoSffW7pch58SxuZQDqW5NsJcQNq"
                + "Ngo/dMoqqpXdi2/0BYEcJ8pjsngrFm+fM2BnyGpXH7aWuKsWjVFGlWlF+yi8I35Q"
                + "8wFJt2Z/XGA7WWDjvw=="
                + "-----END CERTIFICATE-----";

    fis = new ByteArrayInputStream(cert_1.getBytes());

Which results in an error: java.io.IOException: Incomplete data.

fis.available also differes from the certificate loaded from file. (it is 2 shorter). I am guessing this has to do with the WRAPPING. I am suspecting this because a .cer file does not work if the Header is not alone on the first line as seen in notepad, but does work otherwise.

The problem is that your string literal doesn't have any line breaks. To faithfully represent the original text, you'd need:

String cert_1= "-----BEGIN CERTIFICATE-----\n"
    + "MIIBuTCCASKgAwIBAgIQNdNhtuV5GbNHYZsf+LvM0zANBgkqhkiG9w0BAQUFADAb\n"
    + "MRkwFwYDVQQDExBFZGlkZXYgU21va2VUZXN0MB4XDTA4MTExMjE5NTEzNVoXDTM5\n"
    + "MTIzMTIzNTk1OVowGzEZMBcGA1UEAxMQRWRpZGV2IFNtb2tlVGVzdDCBnzANBgkq\n"
    + "hkiG9w0BAQEFAAOBjQAwgYkCgYEAm6zGzqxejwswWTNLcSsa7P8xqODspX9VQBuq\n"
    + "CBpT4S5AaO3/wkYQSKdI108pXH7Aue0e/ZOwgEEX1N6OaPQn7AmAB4uq1h+ffw+r\n"
    + "5W1RoTgQ0LNR64+7ywLjH8+wrb/lB6QV7s2SFUiWDeduVesvMJkWtZ5zzQyl3iUa\n"
    + "RKNHqnsCAwEAATANBgkqhkiG9w0BAQUFAAOBgQCZmj+pgRsN6HpoICawK3XXNAmi\n"
    + "cgfQkailX9akIjD3xSCwEQx4nG6tZjTz30u4NoSffW7pch58SxuZQDqW5NsJcQNq\n"
    + "Ngo/dMoqqpXdi2/0BYEcJ8pjsngrFm+fM2BnyGpXH7aWuKsWjVFGlWlF+yi8I35Q\n"
    + "8wFJt2Z/XGA7WWDjvw==\n"
    + "-----END CERTIFICATE-----";

... but you could probably get away with just making sure that the BEGIN CERTIFICATE and END CERTIFICATE markers are on their own lines:

String cert_1= "-----BEGIN CERTIFICATE-----\n"
    + "MIIBuTCCASKgAwIBAgIQNdNhtuV5GbNHYZsf+LvM0zANBgkqhkiG9w0BAQUFADAb"
    + "MRkwFwYDVQQDExBFZGlkZXYgU21va2VUZXN0MB4XDTA4MTExMjE5NTEzNVoXDTM5"
    + "MTIzMTIzNTk1OVowGzEZMBcGA1UEAxMQRWRpZGV2IFNtb2tlVGVzdDCBnzANBgkq"
    + "hkiG9w0BAQEFAAOBjQAwgYkCgYEAm6zGzqxejwswWTNLcSsa7P8xqODspX9VQBuq"
    + "CBpT4S5AaO3/wkYQSKdI108pXH7Aue0e/ZOwgEEX1N6OaPQn7AmAB4uq1h+ffw+r"
    + "5W1RoTgQ0LNR64+7ywLjH8+wrb/lB6QV7s2SFUiWDeduVesvMJkWtZ5zzQyl3iUa"
    + "RKNHqnsCAwEAATANBgkqhkiG9w0BAQUFAAOBgQCZmj+pgRsN6HpoICawK3XXNAmi"
    + "cgfQkailX9akIjD3xSCwEQx4nG6tZjTz30u4NoSffW7pch58SxuZQDqW5NsJcQNq"
    + "Ngo/dMoqqpXdi2/0BYEcJ8pjsngrFm+fM2BnyGpXH7aWuKsWjVFGlWlF+yi8I35Q"
    + "8wFJt2Z/XGA7WWDjvw==\n"
    + "-----END CERTIFICATE-----";

I'd also recommend explicitly specifying the charset any time you convert between text and binary data:

fis = new ByteArrayInputStream(cert_1.getBytes(StandardCharsets.UTF_8));

See more on this question at Stackoverflow