i have public key as a text file and want to give it to a function that accepts publickey not string

Question

helo. any help can be hopeful please. it's about 1 month that i am tring to solve the problem. i have below private key by size 2048 and a function to sign some data.

hear is the function

public static String getSignedText(
        String text, String algorithm, PrivateKey privateKey) 
          throws UnsupportedEncodingException, NoSuchAlgorithmException, InvalidKeyException, SignatureException { 
      byte[] data = text.getBytes("UTF8"); 
      Signature sig = Signature.getInstance(algorithm == null ? "SHA256WITHRSA" : algorithm); 
      sig.initSign(privateKey); 
      sig.update(data); 
      byte[] signatureBytes = sig.sign(); 
      return Base64.getEncoder().encodeToString(signatureBytes); 
}

i have tried some string to privatekey functions but non of them can convert the string key to privatekey type

Please tag the question with the programming language you are using... it looks like Java to me but I don't know. — Andy Preston, Apr 29 '23 at 12:52
I've now seen this apparently malformed (according to lapo.it) ASN.1 private key format at least 3 times. Where is it coming from? — President James K. Polk, Apr 29 '23 at 14:40
@PresidentJamesK.Polk: I don't see anything malformed (either in lapo.it or local openssl asn1parse), although lapo.it doesn't seem to have -- at least it doesn't offer me in the pulldown -- PKCS8-unencrypted, which is the (only) correct definition for this PEM-type (see RFC7468 section 10). — dave_thompson_085, Apr 29 '23 at 18:02
@PresidentJamesK.Polk: yes that dupe is better focussed on the specific issue than the ones my search found -- even though it should have matched my criteria. I've have had this problem before, where Stack's search doesn't find posts that exist and are good matches. I may give up using it. — dave_thompson_085, Apr 30 '23 at 21:29

score 0 · Accepted Answer · answered Apr 29 '23 at 17:59

Although it's used with a public-key-cryptography algorithm, that's a private key (as the labels say) not a public key. It is almost in the format used by Java crypto: it is PKCS8 and unencrypted, but is PEM rather than 'DER' (binary). To un-PEM you need to remove the BEGIN and END lines, and decode the base64 to binary with the linebreaks either removed or ignored. You then put the result in PKCS8EncodedKeySpec and feed it to the .generatePrivate method of a KeyFactory for the correct algorithm, which is "RSA".

I'm certain I've seen this case posted at least a dozen times, but on searching I can find only two decent duplicates; see
https://stackoverflow.com/a/48173910/2868801 (but note Java 9 up moves javax.xml.bind outside of 'base', so using java.util.Base64 instead may be more convenient)
https://stackoverflow.com/a/66362542/2868801 (but skip the parts about JJWT, and note it assumes all linebreaks have been removed -- on the BEGIN and END lines and on the base64)

thanks alot. it worked. you saved the day for me. as you sed i removed line break and begin/end private.... and after 1 month by your help it worked :)))))))). — nima tehrani, Apr 29 '23 at 19:47

i have public key as a text file and want to give it to a function that accepts publickey not string

1 Answers1