RSA: Get exponent and modulus given a public key

It depends on the tools you can use. I doubt there is a JavaScript too that could do it directly within the browser. It also depends if it's a one-off (always the same key) or whether you need to script it.

Command-line / OpenSSL

If you want to use something like OpenSSL on a unix command line, you can do something as follows. I'm assuming you public.key file contains something like this:

-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAmBAjFv+29CaiQqYZIw4P
J0q5Qz2gS7kbGleS3ai8Xbhu5n8PLomldxbRz0RpdCuxqd1yvaicqpDKe/TT09sR
mL1h8Sx3Qa3EQmqI0TcEEqk27Ak0DTFxuVrq7c5hHB5fbJ4o7iEq5MYfdSl4pZax
UxdNv4jRElymdap8/iOo3SU1RsaK6y7kox1/tm2cfWZZhMlRFYJnpoXpyNYrp+Yo
CNKxmZJnMsS698kaFjDlyznLlihwMroY0mQvdD7dCeBoVlfPUGPAlamwWyqtIU+9
5xVkSp3kxcNcNb/mePSKQIPafQ1sAmBKPwycA/1I5nLzDVuQa95ZWMn0JkphtFIh
HQIDAQAB
-----END PUBLIC KEY-----

Then, the commands would be:

PUBKEY=`grep -v -- ----- public.key | tr -d '\n'`

Then, you can look into the ASN.1 structure:

echo $PUBKEY | base64 -d | openssl asn1parse -inform DER -i

This should give you something like this:

    0:d=0  hl=4 l= 290 cons: SEQUENCE          
    4:d=1  hl=2 l=  13 cons:  SEQUENCE          
    6:d=2  hl=2 l=   9 prim:   OBJECT            :rsaEncryption
   17:d=2  hl=2 l=   0 prim:   NULL              
   19:d=1  hl=4 l= 271 prim:  BIT STRING 

The modulus and public exponent are in the last BIT STRING, offset 19, so use -strparse:

 echo $PUBKEY | base64 -d | openssl asn1parse -inform DER -i -strparse 19

This will give you the modulus and the public exponent, in hexadecimal (the two INTEGERs):

    0:d=0  hl=4 l= 266 cons: SEQUENCE          
    4:d=1  hl=4 l= 257 prim:  INTEGER           :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
  265:d=1  hl=2 l=   3 prim:  INTEGER           :010001

That's probably fine if it's always the same key, but this is probably not very convenient to put in a script.

Alternatively (and this might be easier to put into a script),

openssl rsa -pubin -inform PEM -text -noout < public.key

will return this:

Modulus (2048 bit):
    00:98:10:23:16:ff:b6:f4:26:a2:42:a6:19:23:0e:
    0f:27:4a:b9:43:3d:a0:4b:b9:1b:1a:57:92:dd:a8:
    bc:5d:b8:6e:e6:7f:0f:2e:89:a5:77:16:d1:cf:44:
    69:74:2b:b1:a9:dd:72:bd:a8:9c:aa:90:ca:7b:f4:
    d3:d3:db:11:98:bd:61:f1:2c:77:41:ad:c4:42:6a:
    88:d1:37:04:12:a9:36:ec:09:34:0d:31:71:b9:5a:
    ea:ed:ce:61:1c:1e:5f:6c:9e:28:ee:21:2a:e4:c6:
    1f:75:29:78:a5:96:b1:53:17:4d:bf:88:d1:12:5c:
    a6:75:aa:7c:fe:23:a8:dd:25:35:46:c6:8a:eb:2e:
    e4:a3:1d:7f:b6:6d:9c:7d:66:59:84:c9:51:15:82:
    67:a6:85:e9:c8:d6:2b:a7:e6:28:08:d2:b1:99:92:
    67:32:c4:ba:f7:c9:1a:16:30:e5:cb:39:cb:96:28:
    70:32:ba:18:d2:64:2f:74:3e:dd:09:e0:68:56:57:
    cf:50:63:c0:95:a9:b0:5b:2a:ad:21:4f:bd:e7:15:
    64:4a:9d:e4:c5:c3:5c:35:bf:e6:78:f4:8a:40:83:
    da:7d:0d:6c:02:60:4a:3f:0c:9c:03:fd:48:e6:72:
    f3:0d:5b:90:6b:de:59:58:c9:f4:26:4a:61:b4:52:
    21:1d
Exponent: 65537 (0x10001)

Java

It depends on the input format. If it's an X.509 certificate in a keystore, use (RSAPublicKey)cert.getPublicKey(): this object has two getters for the modulus and the exponent.

If it's in the format as above, you might want to use BouncyCastle and its PEMReader to read it. I haven't tried the following code, but this would look more or less like this:

PEMReader pemReader = new PEMReader(new FileReader("file.pem"));
Object obj = pemReader.readObject();
pemReader.close();
if (obj instanceof X509Certificate) {
   // Just in case your file contains in fact an X.509 certificate,
   // useless otherwise.
   obj = ((X509Certificate)obj).getPublicKey();
}
if (obj instanceof RSAPublicKey) {
   // ... use the getters to get the BigIntegers.
}

(You can use BouncyCastle similarly in C# too.)

Beware the leading 00 that can appear in the modulus when using:

openssl rsa -pubin -inform PEM -text -noout < public.key

The example modulus contains 257 bytes rather than 256 bytes because of that 00, which is included because the 9 in 98 looks like a negative signed number.